Category Archives: Math.NT

Smooth Sums to Sharp Sums 1

In this note, I describe a combination of two smoothed integral transforms that has been very useful in my collaborations with Alex Walker, Chan Ieong Kuan, and Tom Hulse. I suspect that this particular technique was once very well-known. But we were not familiar with it, and so I describe it here.

In application, this is somewhat more complicated. But to show the technique, I apply it to reprove some classic bounds on $\text{GL}(2)$ $L$-functions.

This note is also available as a pdf. This was first written as a LaTeX document, and then modified to fit into wordpress through latex2jax.


Consider a Dirichlet series
D(s) = \sum_{n \geq 1} \frac{a(n)}{n^s}. \notag
Suppose that this Dirichlet series converges absolutely for $\Re s > 1$, has meromorphic continuation to the complex plane, and satisfies a functional equation of shape
\Lambda(s) := G(s) D(s) = \epsilon \Lambda(1-s), \notag
where $\lvert \epsilon \rvert = 1$ and $G(s)$ is a product of Gamma factors.

Dirichlet series are often used as a tool to study number theoretic functions with multiplicative properties. By studying the analytic properties of the Dirichlet series, one hopes to extract information about the coefficients $a(n)$. Some of the most common interesting information within Dirichlet series comes from partial sums
S(n) = \sum_{m \leq n} a(m). \notag
For example, the Gauss Circle and Dirichlet Divisor problems can both be stated as problems concerning sums of coefficients of Dirichlet series.

One can try to understand the partial sum directly by understanding the integral transform
S(n) = \frac{1}{2\pi i} \int_{(2)} D(s) \frac{X^s}{s} ds, \notag
a Perron integral. However, it is often challenging to understand this integral, as delicate properties concerning the convergence of the integral often come into play.

Instead, one often tries to understand a smoothed sum of the form
\sum_{m \geq 1} a(m) v(m) \notag
where $v(m)$ is a smooth function that vanishes or decays extremely quickly for values of $m$ larger than $n$. A large class of smoothed sums can be obtained by starting with a very nicely behaved weight function $v(m)$ and take its Mellin transform
V(s) = \int_0^\infty v(x) x^s \frac{dx}{x}. \notag
Then Mellin inversion gives that
\sum_{m \geq 1} a(m) v(m/X) = \frac{1}{2\pi i} \int_{(2)} D(s) X^s V(s) ds, \notag
as long as $v$ and $V$ are nice enough functions.

In this note, we will use two smoothing integral transforms and corresponding smoothed sums. We will use one smooth function $v_1$ (which depends on another parameter $Y$) with the property that
\sum_{m \geq 1} a(m) v_1(m/X) \approx \sum_{\lvert m – X \rvert < X/Y} a(m). \notag
And we will use another smooth function $v_2$ (which also depends on $Y$) with the property that
\sum_{m \geq 1} a(m) v_2(m/X) = \sum_{m \leq X} a(m) + \sum_{X < m < X + X/Y} a(m) v_2(m/X). \notag
Further, as long as the coefficients $a(m)$ are nonnegative, it will be true that
\sum_{X < m < X + X/Y} a(m) v_2(m/X) \ll \sum_{\lvert m – X \rvert < X/Y} a(m), \notag
which is exactly what $\sum a(m) v_1(m/X)$ estimates. Therefore
\sum_{m \leq X} a(m) = \sum_{m \geq 1} a(m) v_2(m/X) + O\Big(\sum_{m \geq 1} a(m) v_1(m/X) \Big).

Hence sufficient understanding of $\sum a(m) v_1(m/X)$ and $\sum a(m) v_2(m/X)$ allows one to understand the sharp sum
\sum_{m \leq X} a(m). \notag

Two Smooth Cutoff Functions

Let us now introduce the two cutoff functions that we will use.

Concentrating Integral

We use the Mellin transform
\frac{1}{2\pi i} \int_{(2)} \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds = \frac{1}{2\pi} \exp \Big( – \frac{Y^2 \log^2 X}{4\pi} \Big). \notag
\frac{1}{2\pi i} \int_{(2)} D(s) \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds = \frac{1}{2\pi} \sum_{n \geq 1} a(n) \exp \Big( – \frac{Y^2 \log^2 (X/n)}{4\pi} \Big). \notag
For $n \in [X – X/Y, X + X/Y]$, the exponential damping term is essentially constant. However for $n$ with $\lvert n – X \rvert > X/Y$, this quickly exponential decay. Therefore this integral is very nearly the sum over those $n$ with $\lvert n – X \rvert < X/Y$.

For this reason we sometimes call this transform a concetrating integral transform. All of the mass of the integral is concentrated in a small interval of width $X/Y$ around the point $X$.

Note that if $a(n)$ is nonnegative, then we have the trivial bound
\sum_{\lvert n – X \rvert < X/Y} a(n) \ll \sum_{n \geq 1} a(n) \exp \Big( – \frac{Y^2 \log^2 (X/n)}{4\pi} \Big). \notag

As this is a bit less known, we include a brief proof of this transform.

Write $X^s = e^{s\log X}$ and complete the square in the exponents. Since the integrand is entire and the integral is absolutely convergent, we may perform a change of variables $s \mapsto s-Y^2 \log X/2\pi$ and shift the line of integration back to the imaginary axis. This yields
\frac{1}{2\pi i} \exp\left( – \frac{Y^2 \log^2 X}{4\pi}\right) \int_{(0)} e^{\pi s^2/Y^2} \frac{ds}{Y}. \notag
The change of variables $s \mapsto isY$ transforms the integral into the standard Gaussian, completing the proof.

Bump and Decay Integral

For $X, Y > 0$, let $v_Y(X)$ denote a smooth non-negative function with maximum value $1$ satisfying

  1. $v_Y(X) = 1$ for $X \leq 1$,
  2. $v_Y(X) = 0$ for $X \geq 1 + \frac{1}{Y}$.

Let $V(s)$ denote the Mellin transform of $v_Y(X)$, given by
V(s)=\int_0^\infty t^s v_Y(t) \frac{dt}{t}. \notag
when $\Re(s) > 0$. Through repeated applications of integration by parts, one can show that $V(s)$ satisfies the following properties:

  1. $V(s) = \frac{1}{s} + O_s(\frac{1}{Y})$.
  2. $V(s) = -\frac{1}{s}\int_1^{1 + \frac{1}{Y}}v'(t)t^s dt$.
  3. For all positive integers $m$, and with $s$ constrained to within a vertical strip where $\lvert s\rvert >\epsilon$, we have
    $$\begin{equation} \label{vbound}
    V(s) \ll_\epsilon \frac{1}{Y}\left(\frac{Y}{1 + \lvert s \rvert}\right)^m.

Property $(3)$ above can be extended to real $m > 1$ through the Phragmén-Lindelőf principle.

Then we have that
\frac{1}{2\pi i} \int_{(2)} D(s) V(s) X^s ds = \sum_{n \leq X} a(n) + \sum_{X < n < X + X/Y} a(n) v_Y(n/X). \notag

In other words, the sharp sum $\sum_{n \leq X} a(n)$ is captured perfectly, and then there is an amount of smooth fuzz for an additional $X/Y$ terms. As long as the short sum of length $X/Y$ isn’t as large as the sum over the first $X$ terms, then this transform gives a good way of understanding the sharp sum.

When $a(n)$ is nonnegative, we have the trivial bound that
\sum_{X < n < X + X/Y} a(n) v_Y(n/X) \ll \sum_{\lvert n – X \rvert < X/Y} a(n). \notag

In Combination

We have the equality
\sum_{n \geq 1} a(n) v_Y(n/X) &= \sum_{n \leq X} a(n) + \sum_{X < n < X + X/Y} a(n) v_Y(n/X) \notag \\
&= \sum_{n \leq X} a(n) + O\Big( \sum_{\lvert n – X \rvert < X/Y} a(n) \Big) \notag \\
&= \sum_{n \leq X} a(n) + O\bigg( \sum_{n \geq 1} a(n) \exp \Big( – \frac{Y^2 \log^2 (X/n)}{4\pi} \Big)\bigg).\notag
Rearranging, we have
\sum_{n \leq X} a(n) = \sum_{n \geq 1} a(n) v_Y(n/X) + O\bigg( \sum_{n \geq 1} a(n) \exp \Big( – \frac{Y^2 \log^2 (X/n)}{4\pi} \Big)\bigg). \notag
In terms of integral transforms, we then have that
\sum_{n \leq X} a(n) &= \frac{1}{2\pi i} \int_{(2)} D(s) V(s) X^s ds \notag \\
&\quad + O \bigg( \frac{1}{2\pi i} \int_{(2)} D(s) \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds \bigg). \notag

Fortunately, the process of understanding these two integral transforms often boils down to the same fundamental task: determine how quickly Dirichlet series grow in vertical strips.

Application: Sums of Coefficients of $\text{GL}(2)$ Cusp Forms

Suppose that $f(z) = \sum_{n \geq 1} a(n) e(nz)$ is a $\text{GL}(2)$ holomorphic cusp form of weight $k$. We do not restrict $k$ to be an integer, and in fact $k$ might be any rational number as long as $k > 2$. Then the Rankin-Selberg convolution
L(s, f \otimes \overline{f}) = \zeta(2s) \sum_{n \geq 1} \frac{\lvert a(n) \rvert^2}{n^{s + k – 1}} \notag
is an $L$-function satisfying a functional equation of shape
\Lambda(s, f \otimes \overline{f}) := (2\pi)^{-2s} L(s, f \otimes \overline{f}) \Gamma(s) \Gamma(s + k – 1) = \epsilon \Lambda(s, f\otimes \overline{f}), \notag
where $\lvert \epsilon \rvert = 1$ (and in fact the right hand side $L$-function may actually correspond to a related pair of forms $\widetilde{f} \otimes \overline{\widetilde{f}}$, though this does not affect the computations done here).

It is a classically interesting question to consider the sizes of the coefficients $a(n)$. The Ramanujan-Petersson conjecture states that $a(n) \ll n^{\frac{k-1}{2} + \epsilon}$. The Ramanujan-Petersson conjecture is known for full-integral forms on $\text{GL}(2)$, but this is a very deep and very technical result. In general, this type of question is very deep, and very hard.

Using nothing more than the functional equation and the pair of integral transforms, let us analyze the sizes of
\sum_{n \leq X} \frac{\lvert a(n) \rvert^2}{n^{k-1}}. \notag
Note that the power $n^{k-1}$ serves to normalize the sum to be $1$ on average (at least conjecturally).

As described above, it is now apparent that
\sum_{n \leq X} \frac{\lvert a(n) \rvert^2}{n^{k-1}} &= \frac{1}{2\pi i} \int_{(2)} \frac{L(s, f \otimes \overline{f})}{\zeta(2s)} V(s) X^s ds \notag \\
&\quad + O \bigg( \frac{1}{2\pi i} \int_{(2)} \frac{L(s, f \otimes \overline{f})}{\zeta(2s)} \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds \bigg). \notag

We now seek to understand the two integral transforms. Due to the $\zeta(2s)^{-1}$ in the denominator, and due to the mysterious nature of the zeroes of the zeta function, it will only be possible to shift each line of integration to $\Re s = \frac{1}{2}$. Note that $L(s, f\otimes \overline{f})$ has a simple pole at $s = 1$ with a residue that I denote by $R$.

By the Phragmén-Lindelőf Convexity principle, it is known from the functional equation that
L(\frac{1}{2} + it, f \otimes \overline{f}) \ll (1 + \lvert t \rvert)^{3/4}. \notag
Then we have by Cauchy’s Theorem that
&\frac{1}{2\pi i} \int_{(2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds \notag \\
&\quad = \frac{RX e^{1/Y^2}}{Y\zeta(2)} + \frac{1}{2\pi i} \int_{(1/2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds. \notag
The shifted integral can be written
\int_{-\infty}^\infty \frac{L(\frac{1}{2} + it, f \otimes \overline{f})}{\zeta(1 + 2it)} \exp \Big( \frac{\pi (\frac{1}{4} – t^2 + it)}{Y^2}\Big) \frac{X^{\frac{1}{2} + it}}{Y}dt.
It is known that
\zeta(1 + 2it)^{-1} \ll \log (1 + \lvert t \rvert). \notag
Therefore, bounding by absolute values shows that~\eqref{eq:exp_shift1} is bounded by
\int_{-\infty}^\infty (1 + \lvert t \rvert)^{\frac{3}{4} + \epsilon} e^{-t^2/Y^2} \frac{X^{\frac{1}{2}}}{Y}dt. \notag

Heuristically, the exponential decay causes this to be an integral over $t \in [-Y, Y]$, as outside this interval there is exponential decay. We can recognize this more formally by performing the change of variables $t \mapsto tY$. Then we have
\int_{-\infty}^\infty (1 + \lvert tY \rvert)^{\frac{3}{4} + \epsilon} e^{-t^2} X^{\frac{1}{2}} dt \ll X^{\frac{1}{2}} Y^{\frac{3}{4}+\epsilon}. \notag
In total, this means that
\frac{1}{2\pi i} \int_{(2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} \exp \Big( \frac{\pi s^2}{Y^2} \Big) \frac{X^s}{Y} ds = \frac{RX e^{1/Y^2}}{Y\zeta(2)} + O(X^{\frac{1}{2}}Y^{\frac{3}{4}+\epsilon}). \notag

Working now with the other integral transform, Cauchy’s theorem gives
&\frac{1}{2\pi i} \int_{(2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} V(s) X^s ds \notag \\
&\quad = \frac{RX V(1)}{\zeta(2)} + \frac{1}{2\pi i} \int_{(1/2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} V(s)X^s ds. \notag
The shifted integral can again be written
\int_{-\infty}^\infty \frac{L(\frac{1}{2} + it, f \otimes \overline{f})}{\zeta(1 + 2it)} V(\tfrac{1}{2} + it) X^{\frac{1}{2} + it} dt,
and, bounding~\eqref{eq:exp_shift2} by absolute values as above, we get
\int_{-\infty}^\infty (1 + \lvert t \rvert)^{\frac{3}{4} + \epsilon} \lvert V(\tfrac{1}{2} + it) \rvert X^{\frac{1}{2}} dt \ll \int_{-\infty}^\infty (1 + \lvert t \rvert)^{\frac{3}{4} + \epsilon} \frac{1}{Y} \bigg(\frac{Y}{1 + \lvert t \rvert}\bigg)^m X^{\frac{1}{2}} dt \notag
for any $m \geq 0$. In order to make the integral converge, we choose $m = \frac{7}{4} + 2\epsilon$, which shows that
\int_{-\infty}^\infty (1 + \lvert t \rvert)^{\frac{3}{4} + \epsilon} \lvert V(\tfrac{1}{2} + it) \rvert X^{\frac{1}{2}} dt \ll X^{\frac{1}{2}}Y^{\frac{3}{4} + \epsilon}. \notag
Therefore, we have in total that
\frac{1}{2\pi i} \int_{(2)} \frac{L(s, f\otimes \overline{f})}{\zeta(2s)} V(s) X^s ds = \frac{RX V(1)}{\zeta(2)} + O(X^{\frac{1}{2}}Y^{\frac{3}{4} + \epsilon}). \notag

Notice that the $X$ and $Y$ bounds are the exact same for the two separate integral bounds, and that the bounding process was essentially identical. Heuristically, this should generally be true (although in practice there may be some advantage to one over the other).

Now that we have estimated these two integrals, we can say that
\sum_{n \leq X} \frac{\lvert a(n) \rvert^2}{n^{k-1}} = cX + O\big(\frac{X}{Y}\big) + O(X^{\frac{1}{2}}Y^{\frac{3}{4}+\epsilon}) \notag
for some computable constant $c$. This is optimized when
X^{\frac{1}{2}} = Y^{\frac{7}{4} + \epsilon} \implies Y \approx X^{\frac{2}{7}}, \notag
leading to
\sum_{n \leq X} \frac{\lvert a(n) \rvert^2}{n^{k-1}} = cX + O(X^{\frac{5}{7} + \epsilon}). \notag

This isn’t the best possible or best-known result, but it came for almost free! (So one can’t complain too much). Smooth cutoffs and understood polynomial growth allow sharp cutoffs with polynomial-savings error term.

In the next note

In a forthcoming note, we will revisit this example and be slighly more clever in our application of this technique of comparing two smooth integral transforms together. We will discuss an improved (almost still free) result, and some of the limitations of the technique.

Posted in Expository, Math.NT, Mathematics | Tagged , , , , | 3 Comments

Revealing zero in fully homomorphic encryption is a Bad Thing

When I was first learning number theory, cryptography seemed really fun and really practical. I thought elementary number theory was elegant, and that cryptography was an elegant application. As I continued to learn more about mathematics, and in particular modern mathematics, I began to realize that decades of instruction and improvement (and perhaps of more useful points of view) have simplified the presentation of elementary number theory, and that modern mathematics is less elegant in presentation.

Similarly, as I learned more about cryptography, I learned that though the basic ideas are very simple, their application is often very inelegant. For example, the basis of RSA follows immediately from Euler’s Theorem as learned while studying elementary number theory, or alternately from Lagrange’s Theorem as learned while studying group theory or abstract algebra. And further, these are very early topics in these two areas of study!

But a naive implementation of RSA is doomed (For that matter, many professional implementations have their flaws too). Every now and then, a very clever expert comes up with a new attack on popular cryptosystems, generating new guidelines and recommendations. Some guidelines make intuitive sense [e.g. don’t use too small of an exponent for either the public or secret keys in RSA], but many are more complicated or designed to prevent more sophisticated attacks [especially side-channel attacks].

In the summer of 2013, I participated in the ICERM IdeaLab working towards more efficient homomorphic encryption. We were playing with existing homomorphic encryption schemes and trying to come up with new methods. One guideline that we followed is that an attacker should not be able to recognize an encryption of zero. This seems like a reasonable guideline, but I didn’t really understand why, until I was chatting with others at the 2017 Joint Mathematics Meetings in Atlanta.

It turns out that revealing zero isn’t just against generally sound advice. Revealing zero is a capital B capital T Bad Thing.

Basic Setup

For the rest of this note, I’ll try to identify some of this reasoning.

In a typical cryptosystem, the basic setup is as follows. Andrew has a message that he wants to send to Beatrice. So Andrew converts the message into a list of numbers $M$, and uses some sort of encryption function $E(\cdot)$ to encrypt $M$, forming a ciphertext $C$. We can represent this as $C = E(M)$. Andrew transmits $C$ to Beatrice. If an eavesdropper Eve happens to intercept $C$, it should be very hard for Eve to recover any information about the original message from $C$. But when Beatrice receives $C$, she uses a corresponding decryption function $D(\cdot)$ to decrypt $C$, $M = d(C)$.

Often, the encryption and decryption techniques are based on number theoretic or combinatorial primitives. Some of these have extra structure (or at least they do with basic implementation). For instance, the RSA cryptosystem involves a public exponent $e$, a public mod $N$, and a private exponent $d$. Andrew encrypts the message $M$ by computing $C = E(M) \equiv M^e \bmod N$. Beatrice decrypts the message by computing $M = C^d \equiv M^{ed} \bmod N$.

Notice that in the RSA system, given two messages $M_1, M_2$ and corresponding ciphertexts $C_1, C_2$, we have that
E(M_1 M_2) \equiv (M_1 M_2)^e \equiv M_1^e M_2^e \equiv E(M_1) E(M_2) \pmod N. \notag
The encryption function $E(\cdot)$ is a group homomorphism. This is an example of extra structure.

A fully homomorphic cryptosystem has an encryption function $E(\cdot)$ satisfying both $E(M_1 + M_2) = E(M_1) + E(M_2)$ and $E(M_1M_2) = E(M_1)E(M_2)$ (or more generally an analogous pair of operations). That is, $E(\cdot)$ is a ring homomorphism.

This extra structure allows for (a lot of) extra utility. A fully homomorphic $E(\cdot)$ would allow one to perform meaningful operations on encrypted data, even though you can’t read the data itself. For example, a clinic could store (encrypted) medical information on an external server. A doctor or nurse could pull out a cellphone or tablet with relatively little computing power or memory and securely query the medical data. Fully homomorphic encryption would allow one to securely outsource data infrastructure.

A different usage model suggests that we use a different mental model. So suppose Alice has sensitive data that she wants to store for use on EveCorp’s servers. Alice knows an encryption method $E(\cdot)$ and a decryption method $D(\cdot)$, while EveCorp only ever has mountains of ciphertexts, and cannot read the data [even though they have it].

Why revealing zero is a Bad Thing

Let us now consider some basic cryptographic attacks. We should assume that EveCorp has access to a long list of plaintext messages $M_i$ and their corresponding ciphertexts $C_i$. Not everything, but perhaps from small leaks or other avenues. Among the messages $M_i$ it is very likely that there are two messages $M_1, M_2$ which are relatively prime. Then an application of the Euclidean Algorithm gives a linear combination of $M_1$ and $M_2$ such that
M_1 x + M_2 y = 1 \notag
for some integers $x,y$. Even though EveCorp doesn’t know the encryption method $E(\cdot)$, since we are assuming that they have access to the corresponding ciphertexts $C_1$ and $C_2$, EveCorp has access to an encryption of $1$ using the ring homomorphism properties:
E(1) = E(M_1 x + M_2 y) = x E(M_1) + y E(M_2) = x C_1 + y C_2.
By multiplying $E(1)$ by $m$, EveCorp has access to a plaintext and encryption of $m$ for any message $m$.

Now suppose that EveCorp can always recognize an encryption of $0$. Then EveCorp can mount a variety of attacks exposing information about the data it holds.

For example, EveCorp can test whether a particular message $m$ is contained in the encrypted dataset. First, EveCorp generates a ciphertext $C_m$ for $m$ by multiplying $E(1)$ by $m$, as in \eqref{eq:encryption_of_one}. Then for each ciphertext $C$ in the dataset, EveCorp computes $C – C_m$. If $m$ is contained in the dataset, then $C – C_m$ will be an encryption of $0$ for the $C$ corresponding to $m$. EveCorp recognizes this, and now knows that $m$ is in the data. To be more specific, perhaps a list of encrypted names of medical patients appears in the data, and EveCorp wants to see if JohnDoe is in that list. If they can recognize encryptions of $0$, then EveCorp can access this information.

And thus it is unacceptable for external entities to be able to consistently recognize encryptions of $0$.

Up to now, I’ve been a bit loose by saying “an encryption of zero” or “an encryption of $m$”. The reason for this is that to protect against recognition of encryptions of $0$, some entropy is added to the encryption function $E(\cdot)$, making it multivalued. So if we have a message $M$ and we encrypt it once to get $E(M)$, and we encrypt $M$ later and get $E'(M)$, it is often not true that $E(M) = E'(M)$, even though they are both encryptions of the same message. But these systems are designed so that it is true that $C(E(M)) = C(E'(M)) = M$, so that the entropy doesn’t matter.

This is a separate matter, and something that I will probably return to later.

Posted in Crypto, Math.NT, Mathematics, Programming | Tagged , | Leave a comment

A Notebook Preparing for a Talk at Quebec-Maine

This is a notebook containing a representative sample of the code I used to  generate the results and pictures presented at the Quebec-Maine Number Theory Conference on 9 October 2016. It was written in a Jupyter Notebook using Sage 7.3, and later converted for presentation on this site.
There is a version of the notebook available on github. Alternately, a static html version without WordPress formatting is available here. Finally, this notebook is also available in pdf form.
The slides for my talk are available here.

Testing for a Generalized Conjecture on Iterated Sums of Coefficients of Cusp Forms

Let $f$ be a weight $k$ cusp form with Fourier expansion

$$ f(z) = \sum_{n \geq 1} a(n) e(nz). $$

Deligne has shown that $a(n) \ll n^{\frac{k-1}{2} + \epsilon}$. It is conjectured that

$$ S_f^1(n) := \sum_{m \leq X} a(m) \ll X^{\frac{k-1}{2} + \frac{1}{4} + \epsilon}. $$

It is known that this holds on average, and we recently showed that this holds on average in short intervals.
(See HKLDW1, HKLDW2, and HKLDW3 for details and an overview of work in this area).
This is particularly notable, as the resulting exponent is only 1/4 higher than that of a single coefficient.
This indicates extreme cancellation, far more than what is implied merely by the signs of $a(n)$ being random.

It seems that we also have

$$ \sum_{m \leq X} S_f^1(m) \ll X^{\frac{k-1}{2} + \frac{2}{4} + \epsilon}. $$

That is, the sum of sums seems to add in only an additional 1/4 exponent.
This is unexpected and a bit mysterious.

The purpose of this notebook is to explore this and higher conjectures.
Define the $j$th iterated sum as

$$ S_f^j(X) := \sum_{m \leq X} S_f^{j-1} (m).$$

Then we numerically estimate bounds on the exponent $\delta(j)$ such that

$$ S_f^j(X) \ll X^{\frac{k-1}{2} + \delta(j) + \epsilon}. $$

In [1]:
# This was written in SageMath 7.3 through a Jupyter Notebook.
# Jupyter interfaces to sage by loading it as an extension
%load_ext sage

# sage plays strangely with ipython. This re-allows inline plotting
from IPython.display import display, Image

We first need a list of coefficients of one (or more) cusp forms.
For initial investigation, we begin with a list of 50,000 coefficients of the weight $12$ cusp form on $\text{SL}(2, \mathbb{Z})$, $\Delta(z)$, i.e. Ramanujan’s delta function.
We will use the data associated to the 50,000 coefficients for pictoral investigation as well.

We will be performing some numerical investigation as well.
For this, we will use the first 2.5 million coefficients of $\Delta(z)$

In [2]:
# Gather 10 coefficients for simple checking
check_10 = delta_qexp(11).coefficients()
print check_10

fiftyk_coeffs = delta_qexp(50000).coefficients()
print fiftyk_coeffs[:10] # these match expected

twomil_coeffs = delta_qexp(2500000).coefficients()
print twomil_coeffs[:10] # these also match expected
[1, -24, 252, -1472, 4830, -6048, -16744, 84480, -113643, -115920]
[1, -24, 252, -1472, 4830, -6048, -16744, 84480, -113643, -115920]
[1, -24, 252, -1472, 4830, -6048, -16744, 84480, -113643, -115920]
In [3]:
# Function which iterates partial sums from a list of coefficients

def partial_sum(baselist):
    ret_list = [baselist[0]]
    for b in baselist[1:]:
        ret_list.append(ret_list[-1] + b)
    return ret_list

print check_10
print partial_sum(check_10) # Should be the partial sums
[1, -24, 252, -1472, 4830, -6048, -16744, 84480, -113643, -115920]
[1, -23, 229, -1243, 3587, -2461, -19205, 65275, -48368, -164288]
In [4]:
# Calculate the first 10 iterated partial sums
# We store them in a single list list, `sums_list`
# the zeroth elelemnt of the list is the array of initial coefficients
# the first element is the array of first partial sums, S_f(n)
# the second element is the array of second iterated partial sums, S_f^2(n)

fiftyk_sums_list = []
fiftyk_sums_list.append(fiftyk_coeffs) # zeroth index contains coefficients
for j in range(10):                    # jth index contains jth iterate
print partial_sum(check_10)
print fiftyk_sums_list[1][:10]         # should match above
twomil_sums_list = []
twomil_sums_list.append(twomil_coeffs) # zeroth index contains coefficients
for j in range(10):                    # jth index contains jth iterate
print twomil_sums_list[1][:10]         # should match above
[1, -23, 229, -1243, 3587, -2461, -19205, 65275, -48368, -164288]
[1, -23, 229, -1243, 3587, -2461, -19205, 65275, -48368, -164288]
[1, -23, 229, -1243, 3587, -2461, -19205, 65275, -48368, -164288]

As is easily visible, the sums alternate in sign very rapidly.
For instance, we believe tha the first partial sums should change sign about once every $X^{1/4}$ terms in the interval $[X, 2X]$.
In this exploration, we are interested in the sizes of the coefficients.
But in HKLDW3, we investigated some of the sign changes of the partial sums.

Now seems like a nice time to briefly look at the data we currently have.
What do the first 50 thousand coefficients look like?
So we normalize them, getting $A(n) = a(n)/n^{5.5}$ and plot these coefficients.

In [5]:
norm_list = []
for n,e in enumerate(fiftyk_coeffs, 1):
    normalized_element = 1.0 * e / (1.0 * n**(5.5))
print norm_list[:10]
[1.00000000000000, -0.530330085889911, 0.598733612492945, -0.718750000000000, 0.691213333204735, -0.317526448138560, -0.376547696558964, 0.911504835123284, -0.641518061271148, -0.366571226366719]
In [6]:
# Make a quick display
normed_coeffs_plot = scatter_plot(zip(range(1,60000), norm_list), markersize=.02)"normed_coeffs_plot.png")

Since some figures will be featuring prominently in the talk I’m giving at Quebec-Maine, let us make high-quality figures now.



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Paper: Sign Changes of Coefficients and Sums of Coefficients of Cusp Forms

This is joint work with Thomas Hulse, Chan Ieong Kuan, and Alex Walker, and is a another sequel to our previous work. This is the third in a trio of papers, and completes an answer to a question posed by our advisor Jeff Hoffstein two years ago.

We have just uploaded a preprint to the arXiv giving conditions that guarantee that a sequence of numbers contains infinitely many sign changes. More generally, if the sequence consists of complex numbers, then we give conditions that guarantee sign changes in a generalized sense.

Let $\mathcal{W}(\theta_1, \theta_2) := { re^{i\theta} : r \geq 0, \theta \in [\theta_1, \theta_2]}$ denote a wedge of complex plane.

Suppose ${a(n)}$ is a sequence of complex numbers satisfying the following conditions:

  1. $a(n) \ll n^\alpha$,
  2. $\sum_{n \leq X} a(n) \ll X^\beta$,
  3. $\sum_{n \leq X} \lvert a(n) \rvert^2 = c_1 X^{\gamma_1} + O(X^{\eta_1})$,

where $\alpha, \beta, c_1, \gamma_1$, and $\eta_1$ are all real numbers $\geq 0$. Then for any $r$ satisfying $\max(\alpha+\beta, \eta_1) – (\gamma_1 – 1) < r < 1$, the sequence ${a(n)}$ has at least one term outside any wedge $\mathcal{W}(\theta_1, \theta_2)$ with $0 \theta_2 – \theta_1 < \pi$ for some $n \in [X, X+X^r)$ for all sufficiently large $X$.

These wedges can be thought of as just slightly smaller than a half-plane. For a complex number to escape a half plane is analogous to a real number changing sign. So we should think of this result as guaranteeing a sort of sign change in intervals of width $X^r$ for all sufficiently large $X$.

The intuition behind this result is very straightforward. If the sum of coefficients is small while the sum of the squares of the coefficients are large, then the sum of coefficients must experience a lot of cancellation. The fact that we can get quantitative results on the number of sign changes is merely a task of bookkeeping.

Both the statement and proof are based on very similar criteria for sign changes when ${a(n)}$ is a sequence of real numbers first noticed by Ram Murty and Jaban Meher. However, if in addition it is known that

\sum_{n \leq X} (a(n))^2 = c_2 X^{\gamma_2} + O(X^{\eta_2}),

and that $\max(\alpha+\beta, \eta_1, \eta_2) – (\max(\gamma_1, \gamma_2) – 1) < r < 1$, then generically both sequences ${\text{Re} (a(n)) }$ and $latex{ \text{Im} (a(n)) }$ contain at least one sign change for some $n$ in $[X , X + X^r)$ for all sufficiently large $X$. In other words, we can detect sign changes for both the real and imaginary parts in intervals, which is a bit more special.

It is natural to ask for even more specific detection of sign changes. For instance, knowing specific information about the distribution of the arguments of $a(n)$ would be interesting, and very closely reltated to the Sato-Tate Conjectures. But we do not yet know how to investigate this distribution.

In practice, we often understand the various criteria for the application of these two sign changes results by investigating the Dirichlet series
&\sum_{n \geq 1} \frac{a(n)}{n^s} \\
&\sum_{n \geq 1} \frac{S_f(n)}{n^s} \\
&\sum_{n \geq 1} \frac{\lvert S_f(n) \rvert^2}{n^s} \\
&\sum_{n \geq 1} \frac{S_f(n)^2}{n^s},
S_f(n) = \sum_{m \leq n} a(n).

In the case of holomorphic cusp forms, the two previous joint projects with this group investigated exactly the Dirichlet series above. In the paper, we formulate some slightly more general criteria guaranteeing sign changes based directly on the analytic properties of the Dirichlet series involved.

In this paper, we apply our sign change results to our previous work to show that $S_f(n)$ changes sign in each interval $[X, X + X^{\frac{2}{3} + \epsilon})$ for sufficiently large $X$. Further, if there are coefficients with $\text{Im} a(n) \neq 0$, then the real and imaginary parts each change signs in those intervals.

We apply our sign change results to single coefficients of $\text{GL}(2)$ cusp forms (and specifically full integral weight holomorphic cusp forms, half-integral weight holomorphic cusp forms, and Maass forms). In large part these are minor improvements over folklore and what is known, except for the extension to complex coefficients.

We also apply our sign change results to single isolated coefficients $A(1,m)$ of $\text{GL}(3)$ Maass forms. This seems to be a novel result, and adds to the very sparse literature on sign changes of sequences associated to $\text{GL}(3)$ objects. Murty and Meher recently proved a general sign change result for $\text{GL}(n)$ objects which is similar in feel.

As a final application, we also consider sign changes of partial sums of $\nu$-normalized coefficients. Let
S_f^\nu(X) := \sum_{n \leq X} \frac{a(n)}{n^{\nu}}.
As $\nu$ gets larger, the individual coefficients $a(n)n^{-\nu}$ become smaller. So one should expect that sign changes in ${S_f^\nu(n)}$ to change based on $\nu$. And in particular, as $\nu$ gets very large, the number of sign changes of $S_f^\nu$ should decrease.

Interestingly, in the case of holomorphic cusp forms of weight $k$, we are able to show that there are sign changes of $S_f^\nu(n)$ in intervals even for normalizations $\nu$ a bit above $\nu = \frac{k-1}{2}$. This is particularly interesting as $a(n) \ll n^{\frac{k-1}{2} + \epsilon}$, so for $\nu > \frac{k-1}{2}$ the coefficients are \emph{decreasing} with $n$. We are able to show that when $\nu = \frac{k-1}{2} + \frac{1}{6} – \epsilon$, the sequence ${S_f^\nu(n)}$ has at least one sign change for $n$ in $[X, 2X)$ for all sufficiently large $X$.

It may help to consider a simpler example to understand why this is surprising. Consider the classic example of a sequence of $b(n)$, where $b(n) = 1$ or $b(n) = -1$, randomly, with equal probability. Then the expected size of the sums of $b(n)$ is about $\sqrt n$. This is an example of \emph{square-root cancellation}, and such behaviour is a common point of comparison. Similarly, the number of sign changes of the partial sums of $b(n)$ is also expected to be about $\sqrt n$.

Suppose now that $b(n) = \frac{\pm 1}{\sqrt n}$. If the first term is $1$, then it takes more then the second term being negative to make the overall sum negative. And if the first two terms are positive, then it would take more then the following three terms being negative to make the overall sum negative. So sign changes of the partial sums are much rarer. In fact, they’re exceedingly rare, and one might barely detect more than a dozen through computational experiment (although one should still expect infinitely many).

This regularity, in spite of the decreasing size of the individual coefficients $a(n)n^{-\nu}$, suggests an interesting regularity in the sign changes of the individual $a(n)$. We do not know how to understand or measure this effect or its regularity, and for now it remains an entirely qualitative observation.

For more details and specific references, see the paper on the arXiv.

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Math 420: Supplement on Gaussian Integers

This is a brief supplemental note on the Gaussian integers, written for my Spring 2016 Elementary Number Class at Brown University. With respect to the book, the nearest material is the material in Chapters 35 and 36, but we take a very different approach.

A pdf of this note can be found here. I’m sure there are typos, so feel free to ask me or correct me if you think something is amiss.

In this note, we cover the following topics.

  1. What are the Gaussian integers?
  2. Unique factorization within the Gaussian integers.
  3. An application of the Gaussian integers to the Diophantine equation $latex {y^2 = x^3 – 1}$.
  4. Other integer-like sets: general rings.
  5. Specific examples within $latex {\mathbb{Z}[\sqrt{2}]}$ and $latex {\mathbb{Z}[\sqrt{-5}]}$.

1. What are the Gaussian Integers?

The Gaussian Integers are the set of numbers of the form $latex {a + bi}$, where $latex {a}$ and $latex {b}$ are normal integers and $latex {i}$ is a number satisfying $latex {i^2 = -1}$. As a collection, the Gaussian Integers are represented by the symbol $latex {\mathbb{Z}[i]}$, or sometimes $latex {\mathbb{Z}[\sqrt{-1}]}$. These might be pronounced either as The Gaussian Integers or as Z append i.

In many ways, the Gaussian integers behave very much like the regular integers. We’ve been studying the qualities of the integers, but we should ask — which properties are really properties of the integers, and which properties hold in greater generality? Is it the integers themselves that are special, or is there something bigger and deeper going on?

These are the main questions that we ask and make some progress towards in these notes. But first, we need to describe some properties of Gaussian integers.

We will usually use the symbols $latex {z = a + bi}$ to represent our typical Gaussian integer. One adds and multiples two Gaussian integers just as you would add and multiply two complex numbers. Informally, you treat $latex {i}$ like a polynomial indeterminate $latex {X}$, except that it satisfies the relation $latex {X^2 = -1}$.

Definition 1 For each complex number $latex {z = a + bi}$, we define the conjugate of $latex {z}$, written as $latex {\overline{z}}$, by
\overline{z} = a – bi.
We also define the norm of $latex {z}$, written as $latex {N(z)}$, by
N(z) = a^2 + b^2.

You can check that $latex {N(z) = z \overline{z}}$ (and in fact this is one of your assigned problems). You can also chack that $latex {N(zw) = N(z)N(w)}$, or rather that the norm is multiplicative (this is also one of your assigned problems).

Even from our notation, it’s intuitive that $latex {z = a + bi}$ has two parts, the part corresponding to $latex {a}$ and the part corresponding to $latex {b}$. We call $latex {a}$ the real part of $latex {z}$, written as $latex {\Re z = a}$, and we call $latex {b}$ the imaginary part of $latex {z}$, written as $latex {\Im z = b}$. I should add that the name ”imaginary number” is a poor name that reflects historical reluctance to view complex numbers as acceptable. For that matter, the name ”complex number” is also a poor name.

As a brief example, consider the Gaussian integer $latex {z = 2 + 5i}$. Then $latex {N(z) = 4 + 25 = 29}$, $latex {\Re z = 2}$, $latex {\Im z = 5}$, and $latex {\overline{z} = 2 – 5i}$.

We can ask similar questions to those we asked about the regular integers. What does it mean for $latex {z \mid w}$ in the complex case?

Definition 2 We say that a Gaussian integer $latex {z}$ divides another Gaussian integer $latex {w}$ if there is some Gaussian integer $latex {k}$ so that $latex {zk = w}$. In this case, we write $latex {z \mid w}$, just as we write for regular integers.

For the integers, we immediately began to study the properties of the primes, which in many ways were the building blocks of the integers. Recall that for the regular integers, we said $latex {p}$ was a prime if its only divisors were $latex {\pm 1}$ and $latex {\pm p}$. In the Gaussian integers, the four numbers $latex {\pm 1, \pm i}$ play the same role as $latex {\pm 1}$ in the usual integers. These four numbers are distinguished as being the only four Gaussian integers with norm equal to $latex {1}$.

That is, the only solutions to $latex {N(z) = 1}$ where $latex {z}$ is a Gaussian integer are $latex {z = \pm 1, \pm i}$. We call these four numbers the Gaussian units.

With this in mind, we are ready to define the notion of a prime for the Gaussian integers.

Definition 3 We say that a Gaussian integer $latex {z}$ with $latex {N(z) > 1}$ is a Gaussian prime if the only divisors of $latex {z}$ are $latex {u}$ and $latex {uz}$, where $latex {u = \pm 1, \pm i}$ is a Gaussian unit.

Remark 1 When we look at other integer-like sets, we will actually use a different definition of a prime.

It’s natural to ask whether the normal primes in $latex {\mathbb{Z}}$ are also primes in $latex {\mathbb{Z}[i]}$. And the answer is no. For instance, $latex {5}$ is a prime in $latex {\mathbb{Z}}$, but
5 = (1 + 2i)(1 – 2i)
in the Gaussian integers. However, the two Gaussian integers $latex {1 + 4i}$ and $latex {1 – 4i}$ are prime. It also happens to be that $latex {3}$ is a Gaussian prime. We will continue to investigate which numbers are Gaussian primes over the next few lectures.

With a concept of a prime, it’s also natural to ask whether or not the primes form the building blocks for the Gaussian integers like they form the building blocks for the regular integers. We take up this in our next topic.

2. Unique Factorization in the Gaussian Integers

Let us review the steps that we followed to prove unique factorization for $latex {\mathbb{Z}}$.

  1. We proved that for $latex {a,b}$ in $latex {\mathbb{Z}}$ with $latex {b \neq 0}$, there exist unique $latex {q}$ and $latex {r}$ such that $latex {a = bq + r}$ with $latex {0 \leq r < b}$. This is called the Division Algorithm.
  2. By repeatedly applying the Division Algorithm, we proved the Euclidean Algorithm. In particular, we showed that the last nonzero remainder was the GCD of our initial numbers.
  3. By performing reverse substition on the steps of the Euclidean Algorithm, we showed that there are integer solutions in $latex {x,y}$ to the Diophantine equation $latex {ax + by = \gcd(a,b)}$. This is often called Bezout’s Theorem or Bezout’s Lemma, although we never called it by that name in class.
  4. With Bezout’s Theorem, we showed that if a prime $latex {p}$ divides $latex {ab}$, then $latex {p \mid a}$ or $latex {p \mid b}$. This is the crucial step towards proving Unique Factorization.
  5. We then proved Unique Factorization.

Each step of this process can be repeated for the Gaussian integers, with a few notable differences. Remarkably, once we have the division algorithm, each proof is almost identical for $latex {\mathbb{Z}[i]}$ as it is for $latex {\mathbb{Z}}$. So we will prove the division algorithm, and then give sketches of the remaining ideas, highlighting the differences that come up along the way.

In the division algorithm, we require the remainder $latex {r}$ to ”be less than what we are dividing by.” A big problem in translating this to the Gaussian integers is that the Gaussian integers are not ordered. That is, we don’t have a concept of being greater than or less than for $latex {\mathbb{Z}[i]}$.

When this sort of problem emerges, we will get around this by taking norms. Since the norm of a Gaussian integer is a typical integer, we will be able to use the ordering of the integers to order our norms.

Theorem 4 For $latex {z,w}$ in $latex {\mathbb{Z}[i]}$ with $latex {w \neq 0}$, there exist $latex {q}$ and $latex {r}$ in $latex {\mathbb{Z}[i]}$ such that $latex {z = qw + r}$ with $latex {N(r) < N(w)}$.

Proof: Here, we will cheat a little bit and use properties about general complex numbers and the rationals to perform this proof. One can give an entirely intrinsic proof, but I like the approach I give as it also informs how to actually compute the $latex {q}$ and $latex {r}$.

The entire proof boils down to the idea of writing $latex {z/w}$ as a fraction and approximating the real and imaginary parts by the nearest integers.

Let us now transcribe that idea. We will need to introduce some additional symbols. Let $latex {z = a_1 + b_1 i}$ and $latex {w = a_2 + b_2 i}$.

\frac{z}{w} &= \frac{a_1 + b_1 i}{a_2 + b_2 i} = \frac{a_1 + b_1 i}{a_2 + b_2 i} \frac{a_2 – b_2 i}{a_2 – b_2 i} \\
&= \frac{a_1a_2 + b_1 b_2}{a_2^2 + b_2^2} + i \frac{b_1 a_2 – a_1 b_2}{a_2^2 + b_2 ^2} \\
&= u + iv.
By rationalizing the denominator by multiplying by $latex {\overline{w}/ \overline{w}}$, we are able to separate out the real and imaginary parts. In this final expression, we have named $latex {u}$ to be the real part and $latex {v}$ to be the imaginary part. Notice that $latex {u}$ and $latex {v}$ are normal rational numbers.

We know that for any rational number $latex {u}$, there is an integer $latex {u’}$ such that $latex {\lvert u – u’ \rvert \leq \frac{1}{2}}$. Let $latex {u’}$ and $latex {v’}$ be integers within $latex {1/2}$ of $latex {u}$ and $latex {v}$ above, respectively.

Then we claim that we can choose $latex {q = u’ + i v’}$ to be the $latex {q}$ in the theorem statement, and let $latex {r}$ be the resulting remainder, $latex {r = z – qw}$. We need to check that $latex {N(r) < N(w)}$. We will check that explicitly.

We compute
N(r) &= N(z – qw) = N\left(w \left(\frac{z}{w} – q\right)\right) = N(w) N\left(\frac{z}{w} – q\right).
Note that we have used that $latex {N(ab) = N(a)N(b)}$. In this final expression, we have already come across $latex {\frac{z}{w}}$ before — it’s exactly what we called $latex {u + iv}$. And we called $latex {q = u’ + i v’}$. So our final expression is the same as
N(r) = N(w) N(u + iv – u’ – i v’) = N(w) N\left( (u – u’) + i (v – v’)\right).
How large can the real and imaginary parts of $latex {(u-u’) + i (v – v’)}$ be? By our choice of $latex {u’}$ and $latex {v’}$, they can be at most $latex {1/2}$.

So we have that
N(r) \leq N(w) N\left( (\tfrac{1}{2})^2 + (\tfrac{1}{2})^2\right) = \frac{1}{2} N(w).
And so in particular, we have that $latex {N(r) < N(w)}$ as we needed. $latex \Box$

Note that in this proof, we did not actually show that $latex {q}$ or $latex {r}$ are unique. In fact, unlike the case in the regular integers, it is not true that $latex {q}$ and $latex {r}$ are unique.

Example 1 Consider $latex {3+5i, 1 + 2i}$. Then we compute
\frac{3+5i}{1+2i} = \frac{3+5i}{1+2i}\frac{1-2i}{1-2i} = \frac{13}{5} + i \frac{-1}{5}.
The closest integer to $latex {13/5}$ is $latex {3}$, and the closest integer to $latex {-1/5}$ is $latex {0}$. So we take $latex {q = 3}$. Then $latex {r = (3+5i) – (1+2i)3 = -i}$, and we see in total that
3+5i = (1+2i) 3 – i.
Note that $latex {N(-i) = 1}$ and $latex {N(1 + 2i) = 5}$, so this choice of $latex {q}$ and $latex {r}$ works.

As $latex {13/5}$ is sort of close to $latex {2}$, what if we chose $latex {q = 2}$ instead? Then $latex {r = (3 + 5i) – (1 + 2i)2 = 1 + i}$, leading to the overall expression
3_5i = (1 + 2i) 2 + (1 + i).
Note that $latex {N(1+i) = 2 < N(1+2i) = 5}$, so that this choice of $latex {q}$ and $latex {r}$ also works.

This is an example of how the choice of $latex {q}$ and $latex {r}$ is not well-defined for the Gaussian integers. In fact, even if one decides to choose $latex {q}$ to that $latex {N(r)}$ is minimal, the resulting choices are still not necessarily unique.

This may come as a surprise. The letters $latex {q}$ and $latex {r}$ come from our tendency to call those numbers the quotient and remainder after division. We have shown that the quotient and remainder are not well-defined, so it does not make sense to talk about ”the remainder” or ”the quotient.” This is a bit strange!

Are we able to prove unique factorization when the process of division itself seems to lead to ambiguities? Let us proceed forwards and try to see.

Our next goal is to prove the Euclidean Algorithm. By this, we mean that by repeatedly performing the division algorithm starting with two Gaussian integers $latex {z}$ and $latex {w}$, we hope to get a sequence of remainders with the last nonzero remainder giving a greatest common divisor of $latex {z}$ and $latex {w}$.

Before we can do that, we need to ask a much more basic question. What do we mean by a greatest common divisor? In particular, the Gaussian integers are not ordered, so it does not make sense to say whether one Gaussian integer is bigger than another.

For instance, is it true that $latex {i > 1}$? If so, then certainly $latex {i}$ is positive. We know that multiplying both sides of an inequality by a positive number doesn’t change that inequality. So multiplying $latex {i > 1}$ by $latex {i}$ leads to $latex {-1 > i}$, which is absurd if $latex {i}$ was supposed to be positive!

To remedy this problem, we will choose a common divisor of $latex {z}$ and $latex {w}$ with the greatest norm (which makes sense, as the norm is a regular integer and thus is well-ordered). But the problem here, just as with the division algorithm, is that there may or may not be multiple such numbers. So we cannot talk about ”the greatest common divisor” and instead talk about ”a greatest common divisor.” To paraphrase Lewis Carroll’s\footnote{Carroll was also a mathematician, and hid some nice mathematics inside some of his works.} Alice, things are getting curiouser and curiouser!

Definition 5 For nonzero $latex {z,w}$ in $latex {\mathbb{Z}[i]}$, a greatest common divisor of $latex {z}$ and $latex {w}$, denoted by $latex {\gcd(z,w)}$, is a common divisor with largest norm. That is, if $latex {c}$ is another common divisor of $latex {z}$ and $latex {w}$, then $latex {N(c) \leq N(\gcd(z,w))}$.

If $latex {N(\gcd(z,w)) = 1}$, then we say that $latex {z}$ and $latex {w}$ are relatively prime. Said differently, if $latex {1}$ is a greatest common divisor of $latex {z}$ and $latex {w}$, then we say that $latex {z}$ and $latex {w}$ are relatively prime.

Remark 2 Note that $latex {\gcd(z,w)}$ as we’re writing it is not actually well-defined, and may stand for any greatest common divisor of $latex {z}$ and $latex {w}$.

With this definition in mind, the proof of the Euclidean Algorithm is almost identical to the proof of the Euclidean Algorithm for the regular integers. As with the regular integers, we need the following result, which we will use over and over again.

Lemma 6 Suppose that $latex {z \mid w_1}$ and $latex {z \mid w_2}$. Then for any $latex {x,y}$ in $latex {\mathbb{Z}[i]}$, we have that $latex {z \mid (x w_1 + y w_2)}$.

Proof: As $latex {z \mid w_1}$, there is some Gaussian integer $latex {k_1}$ such that $latex {z k_1 = w_1}$. Similarly, there is some Gaussian integer $latex {k_2}$ such that $latex {z k_2 = w_2}$.

Then $latex {xw_1 + yw_2 = zxk_1 + zyk_2 = z(xk_1 + yk_2)}$, which is divisible by $latex {z}$ as this is the definition of divisibility. $latex \Box$

Notice that this proof is identical to the analogous statement in the integers, except with differently chosen symbols. That is how the proof of the Euclidean Algorithm goes as well.

Theorem 7 let $latex {z,w}$ be nonzero Gaussian integers. Recursively apply the division algorithm, starting with the pair $latex {z, w}$ and then choosing the quotient and remainder in one equation the new pair for the next. The last nonzero remainder is divisible by all common divisors of $latex {z,w}$, is itself a common divisor, and so the last nonzero remainder is a greatest common divisor of $latex {z}$ and $latex {w}$.

Symbolically, this looks like
z &= q_1 w + r_1, \quad N(r_1) < N(w) \\\\
w &= q_2 r_1 + r_2, \quad N(r_2) < N(r_1) \\\\
r_1 &= q_3 r_2 + r_3, \quad N(r_3) < N(r_2) \\\\
\cdots &= \cdots \\\\
r_k &= q_{k+2} r_{k+1} + r_{k+2}, \quad N(r_{k+2}) < N(r_{k+1}) \\\\
r_{k+1} &= q_{k+3} r_{k+2} + 0,
where $latex {r_{k+2}}$ is the last nonzero remainder, which we claim is a greatest common divisor of $latex {z}$ and $latex {w}$.

Proof: We are claiming several thing. Firstly, we should prove our implicit claim that this algorithm terminates at all. Is it obvious that we should eventually reach a zero remainder?

In order to see this, we look at the norms of the remainders. After each step in the algorithm, the norm of the remainder is smaller than the previous step. As the norms are always nonnegative integers, and we know there does not exist an infinite list of decreasing positive integers, we see that the list of nonzero remainders is finite. So the algorithm terminates.

We now want to prove that the last nonzero remainder is a common divisor and is in fact a greatest common divisor. The proof is actually identical to the proof in the integer case, merely with a different choice of symbols.

Here, we only sketch the argument. Then the rest of the argument can be found by comparing with the proof of the Euclidean Algorithm for $latex {\mathbb{Z}}$ as found in the course textbook.

For ease of exposition, suppose that the algorithm terminated in exatly 3 steps, so that we have
z &= q_1 w + r_1, \\
w &= q_2 r_1 + r_2 \\
r_1 &= q_3 r_2 + 0.

On the one hand, suppose that $latex {d}$ is a common divisor of $latex {z}$ and $latex {w}$. Then by our previous lemma, $latex {d \mid z – q_1 w = r_1}$, so that we see that $latex {d}$ is a divisor of $latex {r_1}$ as well. Applying to the next line, we have that $latex {d \mid w}$ and $latex {d \mid r_1}$, so that $latex {d \mid w – q_2 r_1 = r_2}$. So every common divisor of $latex {z}$ and $latex {w}$ is a divisor of the last nonzero remainder $latex {r_2}$.

On the other hand, $latex {r_2 \mid r_1}$ by the last line of the algorithm. Then as $latex {r_2 \mid r_1}$ and $latex {r_2 \mid r_1}$, we know that $latex {r_2 \mid q_2 r_1 + r_2 = w}$. Applying this to the first line, as $latex {r_2 \mid r_1}$ and $latex {r_2 \mid w}$, we know that $latex {r_2 \mid q_1 w + r_1 = z}$. So $latex {r_2}$ is a common divisor.

We have shown that $latex {r_2}$ is a common divisor of $latex {z}$ and $latex {w}$, and that every common divisor of $latex {z}$ and $latex {w}$ divides $latex {r_2}$. How do we show that $latex {r_2}$ is a greatest common divisor?

Suppose that $latex {d}$ is a common divisor of $latex {z}$ and $latex {w}$, so that we know that $latex {d \mid r_2}$. In particular, this means that there is some nonzero $latex {k}$ so that $latex {dk = r_2}$. Taking norms, this means that $latex {N(dk) = N(d)N(k) = N(r_2)}$. As $latex {N(d)}$ and $latex {N(k)}$ are both at least $latex {1}$, this means that $latex {N(d) \leq N(r_2)}$.

This is true for every common divisor $latex {d}$, and so $latex {N(r_2)}$ is at least as large as the norm of any common divisor of $latex {z}$ and $latex {w}$. Thus $latex {r_2}$ is a greatest common divisor.

The argument carries on in the same way for when there are more steps in the algorithm. $latex \Box$

Theorem 8 The greatest common divisor of $latex {z}$ and $latex {w}$ is well-defined, up to multiplication by $latex {\pm 1, \pm i}$. In other words, if $latex {\gcd(z,w)}$ is a greatest common divisor of $latex {z}$ and $latex {w}$, then all greatest common divisors of $latex {z}$ and $latex {w}$ are given by $latex {\pm \gcd(z,w), \pm i \gcd(z,w)}$.

Proof: Suppose $latex {d}$ is a greatest common divisor, and let $latex {\gcd(z,w)}$ denote a greatest common divisor resulting from an application of the Euclidean Algorithm. Then we know that $latex {d \mid \gcd(z,w)}$, so that there is some $latex {k}$ so that $latex {dk = \gcd(z,w)}$. Taking norms, we see that $latex {N(d)N(k) = N(\gcd(z,w)}$.

But as both $latex {d}$ and $latex {\gcd(z,w)}$ are greatest common divisors, we must have that $latex {N(d) = N(\gcd(z,w))}$. So $latex {N(k) = 1}$. The only Gaussian integers with norm one are $latex {\pm 1, \pm i}$, so we have that $latex {du = \gcd(z,w)}$ where $latex {u}$ is one of the four Gaussian units, $latex {\pm 1, \pm i}$.

Conversely, it’s clear that the four numbers $latex {\pm \gcd(z,w), \pm i \gcd(z,w)}$ are all greatest common divisors. $latex \Box$

Now that we have the Euclidean Algorithm, we can go towards unique factorization in $latex {\mathbb{Z}[i]}$. Let $latex {g}$ denote a greatest common divisor of $latex {z}$ and $latex {w}$. Reverse substitution in the Euclidean Algorithm shows that we can find Gaussian integer solutions $latex {x,y}$ to the (complex) linear Diophantine equation
zx + wy = g.
Let’s see an example.

Example 2 Consider $latex {32 + 9i}$ and $latex {4 + 11i}$. The Euclidean Algorithm looks like
32 + 9i &= (4 + 11i)(2 – 2i) + 2 – 5i, \\\\
4 + 11i &= (2 – 5i)(-2 + i) + 3 – i, \\\\
2 – 5i &= (3-i)(1-i) – i, \\\\
3 – i &= -i (1 + 3i) + 0.
So we know that $latex {-i}$ is a greatest common divisor of $latex {32 + 9i}$ and $latex {4 + 11i}$, and so we know that $latex {32+9i}$ and $latex {4 + 11i}$ are relatively prime. Let us try to find a solution to the Diophantine equation
x(32 + 9i) + y(4 + 11i) = 1.
Performing reverse substition, we see that
-i &= (2 – 5i) – (3-i)(1-i) \\\\
&= (2 – 5i) – (4 + 11i – (2-5i)(-2 + i))(1-i) \\\\
&= (2 – 5i) – (4 + 11i)(1 – i) + (2 – 5i)(-2 + 1)(1 – i) \\\\
&= (2 – 5i)(3i) – (4 + 11i)(1 – i) \\\\
&= (32 + 9i – (4 + 11i)(2 – 2i))(3i) – (4 + 11i)(1 – i) \\\\
&= (32 + 9i) 3i – (4 + 11i)(2 – 2i)(3i) – (4 + 11i)(1-i) \\\\
&= (32 + 9i) 3i – (4 + 11i)(7 + 5i).
Multiplying this through by $latex {i}$, we have that
1 = (32 + 9i) (-3) + (4 + 11i)(5 – 7i).
So one solution is $latex {(x,y) = (-3, 5 – 7i)}$.

Although this looks more complicated, the process is the same as in the case over the regular integers. The apparent higher difficulty comes mostly from our lack of familiarity with basic arithmetic in $latex {\mathbb{Z}[i]}$.

The rest of the argument is now exactly as in the integers.

Theorem 9 Suppose that $latex {z, w}$ are relatively prime, and that $latex {z \mid wv}$. Then $latex {z \mid v}$.

Proof: This is left as an exercise (and will appear on the next midterm in some form — cheers to you if you’ve read this far in these notes). But it’s now the almost the same as in the regular integers. $latex \Box$

Theorem 10 Let $latex {z}$ be a Gaussian integer with $latex {N(z) > 1}$. Then $latex {z}$ can be written uniquely as a product of Gaussian primes, up to multiplication by one of the Gaussian units $latex {\pm 1, \pm i}$.

Proof: We only sketch part of the proof. There are multiple ways of doing this, but we present the one most similar to what we’ve done for the integers. If there are Gaussian integers without unique factorization, then there are some (maybe they tie) with minimal norm. So let $latex {z}$ be a Gaussian integer of minimal norm without unique factorization. Then we can write
p_1 p_2 \cdots p_k = z = q_1 q_2 \cdots q_\ell,
where the $latex {p}$ and $latex {q}$ are all primes. As $latex {p_1 \mid z = q_1 q_2 \cdots q_\ell}$, we know that $latex {p_1}$ divides one of the $latex {q}$ (by Theorem~9), and so (up to units) we can say that $latex {p_1}$ is one of the $latex {q}$ primes. We can divide each side by $latex {p_1}$ and we get two supposedly different factorizations of a Gaussian integer of norm $latex {N(z)/N(p_1) < N(z)}$, which is less than the least norm of an integer without unique factorization (by what we supposed). This is a contradiction, and we can conclude that there are no Gaussian integers without unique factorization. $latex \Box$

If this seems unclear, I recommend reviewing this proof and the proof of unique factroziation for the regular integers. I should also mention that one can modify the proof of unique factorization for $latex {\mathbb{Z}}$ as given in the course textbook as well (since it is a bit different than what we have done). Further, the course textbook does proof of unique factorization for $latex {\mathbb{Z}[i]}$ in Chapter 36, which is very similar to the proof sketched above (although the proof of Theorem~9 is very different.)

3. An application to $latex {y^2 = x^3 – 1}$.

We now consider the nonlinear Diophantine equation $latex {y^2 = x^3 – 1}$, where $latex {x,y}$ are in $latex {\mathbb{Z}}$. This is hard to solve over the integers, but by going up to $latex {\mathbb{Z}[i]}$, we can determine all solutions.

In $latex {\mathbb{Z}[i]}$, we can rewrite $$ y^2 + 1 = (y + i)(y – i) = x^3. \tag{1}$$
We claim that $latex {y+i}$ and $latex {y-i}$ are relatively prime. To see this, suppose that $latex {d}$ is a common divisor of $latex {y+i}$ and $latex {y-i}$. Then $latex {d \mid (y + i) – (y – i) = 2i}$. It happens to be that $latex {2i = (1 + i)^2}$, and that $latex {(1 + i)}$ is prime. To see this, we show the following.

Lemma 11 Suppose $latex {z}$ is a Gaussian integer, and $latex {N(z) = p}$ is a regular prime. Then $latex {z}$ is a Gaussian prime.

Proof: Suppose that $latex {z}$ factors nontrivially as $latex {z = ab}$. Then taking norms, $latex {N(z) = N(a)N(b)}$, and so we get a nontrivial factorization of $latex {N(z)}$. When $latex {N(z)}$ is a prime, then there are no nontrivial factorizations of $latex {N(z)}$, and so $latex {z}$ must have no nontrivial factorization. $latex \Box$

As $latex {N(1+i) = 2}$, which is a prime, we see that $latex {(1 + i)}$ is a Gaussian prime. So $latex {d \mid (1 + i)^2}$, which means that $latex {d}$ is either $latex {1, (1 + i)}$, or $latex {(1+i)^2}$ (up to multiplication by a Gaussian unit).

Suppose we are in the case of the latter two, so that $latex {(1+i) \mid d}$. Then as $latex {d \mid (y + i)}$, we know that $latex {(1 + i) \mid x^3}$. Taking norms, we have that $latex {2 \mid x^6}$.

By unique factorization in $latex {\mathbb{Z}}$, we know that $latex {2 \mid x}$. This means that $latex {4 \mid x^2}$, which allows us to conclude that $latex {x^3 \equiv 0 \pmod 4}$. Going back to the original equation $latex {y^2 + 1 = x^3}$, we see that $latex {y^2 + 1 \equiv 0 \pmod 4}$, which means that $latex {y^2 \equiv 3 \pmod 4}$. A quick check shows that $latex {y^2 \equiv 3 \pmod 4}$ has no solutions $latex {y}$ in $latex {\mathbb{Z}/4\mathbb{Z}}$.

So we rule out the case then $latex {(1 + i) \mid d}$, and we are left with $latex {d}$ being a unit. This es exactly the case that $latex {y+i}$ and $latex {y-i}$ are relatively prime.

Recall that $latex {(y+i)(y-i) = x^3}$. As $latex {y+i}$ and $latex {y-i}$ are relatively prime and their product is a cube, by unique factorization in $latex {\mathbb{Z}[i]}$ we know that $latex {y+i}$ and $latex {y-i}$ much each be Gaussian cubes. Then we can write $latex {y+i = (m + ni)^3}$ for some Gaussian integer $latex {m + ni}$. Expanding, we see that
y+i = m^3 – 3mn^2 + i(3m^2n – n^3).
Equating real and imaginary parts, we have that
y &= m(m^2 – 3n^2) \\
1 &= n(3m^2 – n^2).
This second line shows that $latex {n \mid 1}$. As $latex {n}$ is a regular integer, we see that $latex {n = 1}$ or $latex {-1}$.

If $latex {n = 1}$, then that line becomes $latex {1 = (3m^2 – 1)}$, or after rearranging $latex {2 = 3m^2}$. This has no solutions.

If $latex {n = -1}$, then that line becomes $latex {1 = -(3m^2 – 1)}$, or after rearranging $latex {0 = 3m^2}$. This has the solution $latex {m = 0}$, so that $latex {y+i = (-i)^3 = i}$, which means that $latex {y = 0}$. Then from $latex {y^2 + 1 = x^3}$, we see that $latex {x = 1}$.

And so the only solution is $latex {(x,y) = (1,0)}$, and there are no other solutions.

4. Other Rings

The Gaussian integers have many of the same properties as the regular integers, even though there are some differences. We could go further. For example, we might consider the following integer-like sets,
\mathbb{Z}(\sqrt{d}) = { a + b \sqrt{d} : a,b \in \mathbb{Z} }.
One can add, subtract, and multiply these together in similar ways to how we can add, subtract, and multiply together integers, or Gaussian integers.

We might ask what properties these other integer-like sets have. For instance, do they have unique factorization?

More generally, there is a better name than ”integer-like set” for this sort of construction.

Suppose $latex {R}$ is a collection of elements, and it makes sense to add, subtract, and multiply these elements together. Further, we want addition and multiplication to behave similarly to how they behave for the regular integers. In particular, if $latex {r}$ and $latex {s}$ are elements in $latex {R}$, then we want $latex {r + s = s + r}$ to be in $latex {R}$; we want something that behaves like $latex {0}$ in the sense that $latex {r + 0 = r}$; for each $latex {r}$, want another element $latex {-r}$ so that $latex {r + (-r) = 0}$; we want $latex {r \cdot s = s \cdot r}$; we want something that behaves like $latex {1}$ in the sense that $latex {r \cdot 1 = r}$ for all $latex {r \neq 0}$; and we want $latex {r(s_1 + s_2) = r s_1 + r s_2}$. Such a collection is called a ring. (More completely, this is called a commutative unital ring, but that’s not important.)

It is not important that you explicitly remember exactly what the definition of a ring is. The idea is that there is a name for things that are ”integer-like” and that we might wonder what properties we have been thinking of as properties of the integers are actually properties of rings.

As a total aside: there are very many more rings too, things that look much more different than the integers. This is one of the fundamental questions that leads to the area of mathematics called Abstract Algebra. With an understanding of abstract algebra, one could then focus on these general number theoretic problems in an area of math called Algebraic Number Theory.

5. The rings $latex {\mathbb{Z}[\sqrt{d}]}$

We can describe some of the specific properties of $latex {\mathbb{Z}[\sqrt{d}]}$, and suggest how some of the ideas we’ve been considering do (or don’t) generalize. For a general element $latex {n = a + b \sqrt{d}}$, we can define the conjugate $latex {\overline{n} = a – b\sqrt {d}}$ and the norm $latex {N(n) = n \cdot \overline{n} = a^2 – d b^2}$. We call those elements $latex {u}$ with $latex {N(u) = 1}$ the units in $latex {\mathbb{Z}[\sqrt{d}]}$.

Some of the definitions we’ve been using turn out to not generalize so easily, or in quite the ways we expect. If $latex {n}$ doesn’t have a nontrivial factoriation (meaning that we cannot write $latex {n = ab}$ with $latex {N(a), N(b) \neq 1}$), then we call $latex {n}$ an irreducible. In the cases of $latex {\mathbb{Z}}$ and $latex {\mathbb{Z}[i]}$, we would have called these elements prime.

In general, we call a number $latex {p}$ in $latex {\mathbb{Z}{\sqrt{d}}}$ a prime if $latex {p}$ has the property that $latex {p \mid ab}$ means that $latex {p \mid a}$ or $latex {p \mid b}$. Of course, in the cases of $latex {\mathbb{Z}}$ and $latex {\mathbb{Z}[i]}$, we showed that irreducibles are primes. But it turns out that this is not usually the case.

Let us look at $latex {\mathbb{Z}{\sqrt{-5}}}$ for a moment. In particular, we can write $latex {6}$ in two ways as
6 = 2 \cdot 3 = (1 + \sqrt{-5})(1 – \sqrt{-5}).
Although it’s a bit challenging to show, these are the only two fundamentally different factorizations of $latex {6}$ in $latex {\mathbb{Z}[\sqrt{-5}]}$. One can show (it’s not very hard, but it’s not particularly illuminating to do here) that neither $latex {2}$ or $latex {3}$ divides $latex {(1 + \sqrt{-5})}$ or $latex {(1 – \sqrt{-5})}$ (and vice versa), which means that none of these four numbers are primes in our more general definition. One can also show that all four numbers are irreducible.

What does this mean? This means that $latex {6}$ can be factored into irreducibles in fundamentally different ways, and that $latex {\mathbb{Z}[\sqrt{-5}]}$ does not have unique factorization.

It’s a good thought exercise to think about what is really different between $latex {\mathbb{Z}[\sqrt{-5}]}$ and $latex {\mathbb{Z}}$. At the beginning of this course, it seemed extremely obvious that $latex {\mathbb{Z}}$ had unique factorization. But in hindsight, is it really so obvious?

Understanding when there is and is not unique factorization in $latex {\mathbb{Z}[\sqrt{d}]}$ is something that people are still trying to understand today. The fact is that we don’t know! In particular, we really don’t know very much when $latex {d}$ is positive.

One reason why can be seen in $latex {\mathbb{Z}[\sqrt{2}]}$. If $latex {n = a + b \sqrt{2}}$, then $latex {N(n) = a^2 – 2 b^2}$. A very basic question that we can ask is what are the units? That is, which $latex {n}$ have $latex {N(n) = 1}$?

Here, that means trying to solve the equation $$ a^2 – 2 b^2 = 1. \tag{2}$$
We have seen this equation a few times before. On the second homework assignment, I asked you to show that there were infinitely many solutions to this equation by finding lines and intersecting them with hyperbolas. We began to investigate this Diophantine equation because each solution leads to another square-triangular number.

So there are infinitely many units in $latex {\mathbb{Z}[\sqrt{2}]}$. This is strange! For instance, $latex {3 + 2 \sqrt{2}}$ is a unit, which means that it behaves just like $latex {\pm 1}$ in $latex {\mathbb{Z}}$, or like $latex {\pm 1, \pm i}$ in $latex {\mathbb{Z}[i]}$. Very often, the statements we’ve been looking at and proving are true ”up to multiplication by units.” Since there are infinitely many in $latex {\mathbb{Z}[\sqrt 2]}$, it can mean that it’s annoying to determine even if two numbers are actually the same up to multiplication by units.

As you look further, there are many more strange and interesting behaviours. It is really interesting to see what properties are very general, and what properties vary a lot. It is also interesting to see the different ways in which properties we’re used to, like unique factorization, can fail.

For instance, we have seen that $latex {\mathbb{Z}[\sqrt -5]}$ does not have unique factorization. We showed this by seeing that $latex {6}$ factors in two fundamentally different ways. In fact, some numbers in $latex {\mathbb{Z}[\sqrt -5]}$ do factor uniquely, and others do not. But if one does not, then it factors in at most two fundamentally different ways.

In other rings, you can have numbers which factor in more fundamentally different ways. The actual behaviour here is also really poorly understood, and there are mathematicians who are actively pursuing these topics.

It’s a very large playground out there.

Posted in Brown University, Expository, Math 420, Math.NT, Mathematics, Teaching | Tagged , , , , , | 2 Comments

A Brief Notebook on Cryptography

This is a post written for my Spring 2016 Number Theory class at Brown University. It was written and originally presented from a Jupyter Notebook, and changed for presentation on this site. There is a version of the notebook available at github.


Recall the basic setup of cryptography. We have two people, Anabel and Bartolo. Anabel wants to send Bartolo a secure message. What do we mean by “secure?” We mean that even though that dastardly Eve might intercept and read the transmitted message, Eve won’t learn anything about the actual message Anabel wants to send to Bartolo.

Before the 1970s, the only way for Anabel to send Bartolo a secure message required Anabel and Bartolo to get together beforehand and agree on a secret method of communicating. To communicate, Anabel first decides on a message. The original message is sometimes called the plaintext. She then encrypts the message, producing a ciphertext.

She then sends the ciphertext. If Eve gets hold of hte ciphertext, she shouldn’t be able to decode it (unless it’s a poor encryption scheme).

When Bartolo receives the ciphertext, he can decrypt it to recover the plaintext message, since he agreed on the encryption scheme beforehand.

Caesar Shift

The first known instance of cryptography came from Julius Caesar. It was not a very good method. To encrypt a message, he shifted each letter over by 2, so for instance “A” becomes “C”, and “B” becomes “D”, and so on.

Let’s see what sort of message comes out.

alpha = "abcdefghijklmnopqrstuvwxyz".upper()
punct = ",.?:;'\n "
from functools import partial

def shift(l, s=2):
    l = l.upper()
    return alpha[(alpha.index(l) + s) % 26]

def caesar_shift_encrypt(m, s=2):
    m = m.upper()
    c = "".join(map(partial(shift, s=s), m))
    return c

def caesar_shift_decrypt(c, s=-2):
    c = c.upper()
    m = "".join(map(partial(shift, s=s), c))
    return m
print "Original Message: HI"
print "Ciphertext:", caesar_shift_encrypt("hi")
Original Message: HI
Ciphertext: JK
m = """To be, or not to be, that is the question:
Whether 'tis Nobler in the mind to suffer
The Slings and Arrows of outrageous Fortune,
Or to take Arms against a Sea of troubles,
And by opposing end them."""

m = "".join([l for l in m if not l in punct])

print "Original Message:"
print m

print "Ciphertext:"
tobe_ciphertext = caesar_shift_encrypt(m)
print tobe_ciphertext
Original Message:

print "Decrypted first message:", caesar_shift_decrypt("JK")
Decrypted first message: HI
print "Decrypted second message:"
print caesar_shift_decrypt(tobe_ciphertext)
Decrypted second message:

Is this a good encryption scheme? No, not really. There are only 26 different things to try. This can be decrypted very quickly and easily, even if entirely by hand.

Substitution Cipher

A slightly different scheme is to choose a different letter for each letter. For instance, maybe “A” actually means “G” while “B” actually means “E”. We call this a substitution cipher as each letter is substituted for another.

import random
permutation = list(alpha)
# Display the new alphabet
print alpha
subs = "".join(permutation)
print subs
def subs_cipher_encrypt(m):
    m = "".join([l.upper() for l in m if not l in punct])
    return "".join([subs[alpha.find(l)] for l in m])

def subs_cipher_decrypt(c):
    c = "".join([l.upper() for l in c if not l in punct])
    return "".join([alpha[subs.find(l)] for l in c])
m1 = "this is a test"

print "Original message:", m1
c1 = subs_cipher_encrypt(m1)

print "Encrypted Message:", c1

print "Decrypted Message:", subs_cipher_decrypt(c1)
Original message: this is a test

Encrypted Message: VAYRYREVLRV

Decrypted Message: THISISATEST
print "Original message:"
print m

c2 = subs_cipher_encrypt(m)
print "Encrypted Message:"
print c2

print "Decrypted Message:"
print subs_cipher_decrypt(c2)
Original message:

Encrypted Message:

Decrypted Message:

Is this a good encryption scheme? Still no. In fact, these are routinely used as puzzles in newspapers or puzzle books, because given a reasonably long message, it’s pretty easy to figure it out using things like the frequency of letters.

For instance, in English, the letters RSTLNEAO are very common, and much more common than other letters. So one might start to guess that the most common letter in the ciphertext is one of these. More powerfully, one might try to see which pairs of letters (called bigrams) are common, and look for those in the ciphertext, and so on.

From this sort of thought process, encryption schemes that ultimately rely on a single secret alphabet (even if it’s not our typical alphabet) fall pretty quickly. So… what about polyalphabetic ciphers? For instance, what if each group of 5 letters uses a different set of alphabets?

This is a great avenue for exploration, and there are lots of such encryption schemes that we won’t discuss in this class. But a class on cryptography (or a book on cryptography) would certainly go into some of these. It might also be a reasonable avenue of exploration for a final project.

The German Enigma

One very well-known polyalphabetic encryption scheme is the German Enigma used before and during World War II. This was by far the most complicated cryptosystem in use up to that point, and the story of how it was broken is a long and tricky one. Intial successes towards breaking the Enigma came through the work of Polish mathematicians, fearful (and rightfully so) of the Germans across the border. By 1937, they had built replicas and understood many details of the Enigma system. But in 1938, the Germans shifted to a more secure and complicated cryptosystem. Just weeks before the German invasion of Poland and the beginning of World War II, Polish mathematicians sent their work and notes to mathematicians in France and Britain, who carried out this work.

The second major progress towards breaking the Enigma occurred largely in Bletchley Park in Britain, a communication center with an enormous dedicated effort to breaking the Enigma. This is where the tragic tale of Alan Turing, recently popularized through the movie The Imitation Game, begins. This is also the origin tale for modern computers, as Alan Turing developed electromechanical computers to help break the Enigma.

The Enigma worked by having a series of cogs or rotors whose positions determined a substitution cipher. After each letter, the positions were changed through a mechanical process. An Enigma machine is a very impressive machine to look at [and the “computer” Alan Turing used to help break them was also very impressive].

Below, I have implemented an Enigma, by default set to 4 rotors. I don’t expect one to understand the implementation. The interesting part is how meaningless the output message looks. Note that I’ve kept the spacing and punctuation from the original message for easier comparison. Really, you wouldn’t do this.

The plaintext used for demonstration is from Wikipedia’s article on the Enigma.

from random import shuffle,randint,choice  
from copy import copy  
num_alphabet = range(26)   
def en_shift(l, n):                         # Rotate cogs and arrays
    return l[n:] + l[:n]  
class Cog:                                  # Each cog has a substitution cipher  
    def __init__(self):  
        self.shuf = copy(num_alphabet)  
        shuffle(self.shuf)                  # Create the individual substition cipher
        return                              # Really, these were not random
    def subs_in(self,i):                    # Perform a substition
        return self.shuf[i] 
    def subs_out(self,i):                   # Perform a reverse substition
        return self.shuf.index(i)
    def rotate(self):                       # Rotate the cog by 1.
        self.shuf = en_shift(self.shuf, 1)
    def setcog(self,a):                     # Set up a particular substitution
        self.shuf = a  

class Enigma:  
    def __init__(self, numcogs,readability=True):  
        self.readability = readability  
        self.numcogs = numcogs  
        self.cogs = []  
        self.oCogs = []                     # "Original Cog positions"  
        for i in range(0,self.numcogs):     # Create the cogs
        refabet = copy(num_alphabet) 
        self.reflector = copy(num_alphabet)  
        while len(refabet) > 0:             # Pair letters in the reflector
            a = choice(refabet)  
            b = choice(refabet)  
            self.reflector[a] = b  
            self.reflector[b] = a
    def print_setup(self): # Print out substituion setup.
        print "Enigma Setup:\nCogs: ",self.numcogs,"\nCog arrangement:"  
        for i in range(0,self.numcogs):  
            print self.cogs[i].shuf  
        print "Reflector arrangement:\n",self.reflector,"\n"  
    def reset(self):  
        for i in range(0,self.numcogs):  
    def encode(self,text):  
        t = 0     # Ticker counter  
        for l in text.lower():  
            num = ord(l) % 97  
            # Handle special characters for readability
            if (num>25 or num<0):  
                if (self.readability):
                    ciphertext += l   
                # Pass through cogs, reflect, then return through cogs
                t += 1  
                for i in range(self.numcogs): 
                    num = self.cogs[i].subs_in(num)  
                num = self.reflector[num]  
                for i in range(self.numcogs):  
                    num = self.cogs[self.numcogs-i-1].subs_out(num)  
                ciphertext += "" + chr(97+num)
                # Rotate cogs
                for i in range(self.numcogs):
                    if ( t % ((i*6)+1) == 0 ):
        return ciphertext.upper()  
plaintext="""When placed in an Enigma, each rotor can be set to one of 26 possible positions. 
When inserted, it can be turned by hand using the grooved finger-wheel, which protrudes from 
the internal Enigma cover when closed. So that the operator can know the rotor's position, 
each had an alphabet tyre (or letter ring) attached to the outside of the rotor disk, with 
26 characters (typically letters); one of these could be seen through the window, thus indicating 
the rotational position of the rotor. In early models, the alphabet ring was fixed to the rotor 
disk. A later improvement was the ability to adjust the alphabet ring relative to the rotor disk. 
The position of the ring was known as the Ringstellung ("ring setting"), and was a part of the 
initial setting prior to an operating session. In modern terms it was a part of the 
initialization vector."""

# Remove newlines for encryption
pt = "".join([l.upper() for l in plaintext if not l == "\n"])
# pt = "".join([l.upper() for l in plaintext if not l in punct])
print "Original Message:"
print pt

ciphertext = x.encode(pt)  
print "Encrypted Message"
print ciphertext

# Decryption and encryption are symmetric, so to decode we reset and re-encrypt.
decipheredtext = x.encode(ciphertext)  
print "Decrypted Message:"
print decipheredtext
Original Message:

Encrypted Message

Decrypted Message:

The advent of computers brought in a paradigm shift in approaches towards cryptography. Prior to computers, one of the ways of maintaining security was to come up with a hidden key and a hidden cryptosystem, and keep it safe merely by not letting anyone know anything about how it actually worked at all. This has the short cute name security through obscurity. As the number of type of attacks on cryptosystems are much, much higher with computers, a different model of security and safety became necessary.

It is interesting to note that it is not obvious that security through obscurity is always bad, as long as it’s really really well-hidden. This is relevant to some problems concerning current events and cryptography.

Public Key Cryptography

The new model begins with a slightly different setup. We should think of Anabel and Bartolo as sitting on opposite sides of a classroom, trying to communicate securely even though there are lots of people in the middle of the classroom who might be listening in. In particular, Anabel has something she wants to tell Bartolo.

Instead of keeping the cryptosystem secret, Bartolo tells everyone (in our metaphor, he shouts to the entire classroom) a public key K, and explains how to use it to send him a message. Anabel uses this key to encrypt her message. She then sends this message to Bartolo.

If the system is well-designed, no one will be able to understand the ciphertext even though they all know how the cryptosystem works. This is why the system is called Public Key.

Bartolo receives this message and (using something only he knows) he decrypts the message.

We will learn one such cryptosystem here: RSA, named after Rivest, Shamir, and Addleman — the first major public key cryptosystem.


Bartolo takes two primes such as $p = 12553$ and $q = 13007$. He notes their product
$$m = pq = 163276871$$
and computes $\varphi(m)$,
$$\varphi(m) = (p-1)(q-1) = 163251312.$$
Finally, he chooses some integer $k$ relatively prime to $\varphi(m)$, like say
$$k = 79921.$$
Then the public key he distributes is
$$ (m, k) = (163276871, 79921).$$

In order to send Bartolo a message using this key, Anabel must convert her message to numbers. She might use the identification A = 11, B = 12, C = 13, … and concatenate her numbers. To send the word CAB, for instance, she would send 131112. Let’s say that Anabel wants to send the message


Then she needs to convert this to numbers.

conversion_dict = dict()
alpha = "abcdefghijklmnopqrstuvwxyz".upper()
curnum = 11
for l in alpha:
    conversion_dict[l] = curnum
    curnum += 1
print "Original Message:"
print msg

def letters_to_numbers(m):
    return "".join([str(conversion_dict[l]) for l in m.upper()])

print "Numerical Message:"
msg_num = letters_to_numbers(msg)
print msg_num
Original Message:

Numerical Message:

So Anabel’s message is the number


which she wants to encrypt and send to Bartolo. To make this manageable, she cuts the message into 8-digit numbers,

$$24312312, 15283018, 15252835, 19293018, 15273115, 15242516, 30181529, 13191524, 131529.$$

To send her message, she takes one of the 8-digit blocks and raises it to the power of $k$ modulo $m$. That is, to transmit the first block, she computes

$$ 24312312^{79921} \equiv 13851252 \pmod{163276871}.$$

# Secret information
p = 12553
q = 13007
phi = (p-1)*(q-1) # varphi(pq)

# Public information
m = p*q # 163276871
k = 79921

print pow(24312312, k, m)

She sends this number
to Bartolo (maybe by shouting. Even though everyone can hear, they cannot decrypt it). How does Bartolo decrypt this message?

He computes $\varphi(m) = (p-1)(q-1)$ (which he can do because he knows $p$ and $q$ separately), and then finds a solution $u$ to
$$ uk = 1 + \varphi(m) v.$$
This can be done quickly through the Euclidean Algorithm.

def extended_euclidean(a,b):
    if b == 0:
        return (1,0,a)
    else :
        x, y, gcd = extended_euclidean(b, a % b) # Aside: Python has no tail recursion
        return y, x - y * (a // b),gcd           # But it does have meaningful stack traces
# This version comes from Exercise 6.3 in the book, but without recursion
def extended_euclidean2(a,b):
    x = 1
    g = a
    v = 0
    w = b
    while w != 0:
        q = g // w
        t = g - q*w
        s = x - q*v
        x,g = v,w
        v,w = s,t
    y = (g - a*x) / b
    return (x,y,g)
def modular_inverse(a,m) :
    x,y,gcd = extended_euclidean(a,m)
    if gcd == 1 :
        return x % m
    else :
        return None
print "k, p, q:", k, p, q
u = modular_inverse(k,(p-1)*(q-1))
print u
k, p, q: 79921 12553 13007


In particular, Bartolo computes that his $u = 145604785$. To recover the message, he takes the number $13851252$ sent to him by Anabel and raises it to the $u$ power. He computes
$$ 13851252^{u} \equiv 24312312 \pmod{pq},$$
which we can see must be true as
$$ 13851252^{u} \equiv (24312312^{k})^u \equiv 24312312^{1 + \varphi(pq)v} \equiv 24312312 \pmod{pq}.$$
In this last step, we have used Euler’s Theorem to see that
$$ 24312312^{\varphi(pq)v} \equiv 1 \pmod{pq}.$$

# Checking this power explicitly.
print pow(13851252, 145604785, m)

Now Bartolo needs to perform this process for each 8-digit chunk that Anabel sent over. Note that the work is very easy, as he computes the integer $u$ only once. Each other time, he simply computes $c^u \pmod m$ for each ciphertext $c$, and this is very fast with repeated-squaring.

We do this below, in an automated fashion, step by step.

First, we split the message into 8-digit chunks.

# Break into chunks of 8 digits in length.
def chunk8(message_number):
    cp = str(message_number)
    ret_list = []
    while len(cp) > 7:
        cp = cp[8:]
    if cp:
    return ret_list

msg_list = chunk8(msg_num)
print msg_list
['24312312', '15283018', '15252835', '19293018', '15273115', 
'15242516', '30181529', '13191524', '131529']

This is a numeric representation of the message Anabel wants to send Bartolo. So she encrypts each chunk. This is done below

# Compute ciphertexts separately on each 8-digit chunk.
def encrypt_chunks(chunked_list):
    ret_list = []
    for chunk in chunked_list:
        #print chunk
        #print int(chunk)
        ret_list.append(pow(int(chunk), k, m))
    return ret_list

cipher_list = encrypt_chunks(msg_list)
print cipher_list
[13851252, 14944940, 158577269, 117640431, 139757098, 25099917, 
88562046, 6640362, 10543199]

This is the encrypted message. Having computed this, Anabel sends this message to Bartolo.

To decrypt the message, Bartolo uses his knowledge of $u$, which comes from his ability to compute $\varphi(pq)$, and decrypts each part of the message. This looks like below.

# Decipher the ciphertexts all in the same way
def decrypt_chunks(chunked_list):
    ret_list = []
    for chunk in chunked_list:
        ret_list.append(pow(int(chunk), u, m))
    return ret_list

decipher_list = decrypt_chunks(cipher_list)
print decipher_list
[24312312, 15283018, 15252835, 19293018, 15273115, 15242516, 
30181529, 13191524, 131529]

Finally, Bartolo concatenates these numbers together and translates them back into letters. Will he get the right message back?


# Collect deciphered texts into a single list, and translate back into letters.
def chunks_to_letters(chunked_list):
    s = "".join([str(chunk) for chunk in chunked_list])
    ret_str = ""
    while s:
        ret_str += alpha[int(s[:2])-11].upper()
        s = s[2:]
    return ret_str

print chunks_to_letters(decipher_list)

Yes! Bartolo successfully decrypts the message and sees that Anabel thinks that Number Theory is the Queen of the Sciences. This is a quote from Gauss, the famous mathematician who has been popping up again and again in this course.

Why is this Secure?

Let’s pause to think about why this is secure.

What if someone catches the message in transit? Suppose Eve is eavesdropping and hears Anabel’s first chunk, $13851252$. How would she decrypt it?

Eve knows that she wants to solve
$$ x^k \equiv 13851252 \pmod {pq}$$
or rather
$$ x^{79921} \equiv 13851252 \pmod {163276871}.$$
How can she do that? We can do that because we know to raise this to a particular power depending on $\varphi(163276871)$. But Eve doesn’t know what $\varphi(163276871)$ is since she can’t factor $163276871$. In fact, knowing $\varphi(163276871)$ is exactly as hard as factoring $163276871$.

But if Eve could somehow find $79921$st roots modulo $163276871$, or if Eve could factor $163276871$, then she would be able to decrypt the message. These are both really hard problems! And it’s these problems that give the method its security.

More generally, one might use primes $p$ and $q$ that are each about $200$ digits long, and a fairly large $k$. Then the resulting $m$ would be about $400$ digits, which is far larger than we know how to factor effectively. The reason for choosing a somewhat large $k$ is for security reasons that are beyond the scope of this segment. The relevant idea here is that since this is a publicly known encryption scheme, many people have strenghtened it over the years by making it more secure against every clever attack known. This is another, sometimes overlooked benefit of public-key cryptography: since the methodology isn’t secret, everyone can contribute to its security — indeed, it is in the interest of anyone desiring such security to contribute. This is sort of the exact opposite of Security through Obscurity.

The nature of code being open, public, private, or secret is also very topical in current events. Recently, Volkswagen cheated in its car emissions-software and had it report false outputs, leading to a large scandal. Their software is proprietary and secret, and the deliberate bug went unnoticed for years. Some argue that this means that more software, and especially software that either serves the public or that is under strong jurisdiction, should be publicly viewable for analysis.

Another relevant current case is that the code for most voting machines in the United States is proprietary and secret. Hopefully they aren’t cheating!

On the other side, many say that it is necessary for companies to be able to have secret software for at least some time period to recuperate the expenses that go into developing the software. This is similar to how drug companies have patents on new drugs for a number of years. This way, a new successful drug pays for its development since the company can charge above the otherwise-market-rate.

Further, many say that opening some code would open it up to attacks from malicious users who otherwise wouldn’t be able to see the code. Of course, this sounds a lot like trying for security through obscurity.

This is a very relevant and big topic, and the shape it takes over the next few years may very well have long-lasting impacts.

Condensed Version

Now that we’ve gone through it all once, we have a condensed RSA system set up with our $p, q,$ and $k$ from above. To show that this can be done quickly with a computer, let’s do another right now.

Let us encrypt, transmit, and decrypt the message

“I have never done anything useful. No discovery of mine has made, or is likely to make, directly or indirectly, for good or ill, the least difference to the amenity of the world”.

First, we prepare the message for conversion to numbers.

message = """I have never done anything useful. No discovery of mine has made, 
or is likely to make, directly or indirectly, for good or ill, the least 
difference to the amenity of the world"""

message = "".join([l.upper() for l in message if not l in "\n .,"])
print "Our message:\n"+message
Our message:

Now we convert the message to numbers, and transform those numbers into 8-digit chunks.

numerical_message = letters_to_numbers(message)
print "Our message, converted to numbers:"
print numerical_message

plaintext_chunks = chunk8(numerical_message)
print "Separated into 8-digit chunks:"
print plaintext_chunks
Our message, converted to numbers:

Separated into 8-digit chunks:
['19181132', '15241532', '15281425', '24151124', '35301819', 
'24173129', '15163122', '24251419', '29132532', '15283525', 
'16231924', '15181129', '23111415', '25281929', '22192115', 
'22353025', '23112115', '14192815', '13302235', '25281924', 
'14192815', '13302235', '16252817', '25251425', '28192222', 
'30181522', '15112930', '14191616', '15281524', '13153025', 
'30181511', '23152419', '30352516', '30181533', '25282214']

We encrypt each chunk by computing $P^k \bmod {m}$ for each plaintext chunk $P$.

ciphertext_chunks = encrypt_chunks(plaintext_chunks)
print ciphertext_chunks
[99080958, 142898385, 80369161, 11935375, 108220081, 82708130,
 158605094, 96274325, 154177847, 121856444, 91409978, 47916550,
 155466420, 92033719, 95710042, 86490776, 15468891, 139085799,
 68027514, 53153945, 139085799, 68027514, 9216376, 155619290,
 83776861, 132272900, 57738842, 119368739, 88984801, 83144549,
 136916742, 13608445, 92485089, 89508242, 25375188]

This is the message that Anabel can sent Bartolo.

To decrypt it, he computes $c^u \bmod m$ for each ciphertext chunk $c$.

deciphered_chunks = decrypt_chunks(ciphertext_chunks)
print "Deciphered chunks:"
print deciphered_chunks
Deciphered chunks:
[19181132, 15241532, 15281425, 24151124, 35301819, 24173129,
 15163122, 24251419, 29132532, 15283525, 16231924, 15181129,
 23111415, 25281929, 22192115, 22353025, 23112115, 14192815,
 13302235, 25281924, 14192815, 13302235, 16252817, 25251425,
 28192222, 30181522, 15112930, 14191616, 15281524, 13153025,
 30181511, 23152419, 30352516, 30181533, 25282214]

Finally, he translates the chunks back into letters.

decoded_message = chunks_to_letters(deciphered_chunks)
print "Decoded Message:"
print decoded_message
Decoded Message:

Even with large numbers, RSA is pretty fast. But one of the key things that one can do with RSA is securely transmit secret keys for other types of faster-encryption that don’t work in a public-key sense. There is a lot of material in this subject, and it’s very important.

The study of sending and receiving secret messages is called Cryptography. There are lots of interesting related topics, some of which we’ll touch on in class.

The study of analyzing and breaking cryptosystems is called Cryptanalysis, and is something I find quite fun. But it’s also quite intense.

I should mention that in practice, RSA is performed a little bit differently to make it more secure.

Aside: this is the first time I’ve converted a Jupyter Notebook, and it turns out it’s very easy. However, there are a few formatting annoyances that I didn’t consider, but which are too minor to spend time fixing. But now I know!
Posted in Brown University, Expository, Math 420, Math.NT, Mathematics, Programming, Python, Teaching | Tagged , , , , | 3 Comments

Paper: The Second Moments of Sums of Fourier Coefficients of Cusp Forms

This is joint work with Thomas Hulse, Chan Ieong Kuan, and Alex Walker.

We have just uploaded a paper to the arXiv on the second moment of sums of Fourier coefficients of cusp forms. This is the first in a trio of papers that we will be uploading and submitting in the near future.

Suppose $latex {f(z)}$ and $latex {g(z)}$ are weight $latex {k}$ holomorphic cusp forms on $latex {\text{GL}_2}$ with Fourier expansions

$$\begin{align} f(z) &= \sum_{n \geq 1} a(n) e(nz) \\
g(z) &= \sum_{n \geq 1} b(n) e(nz). \end{align}$$

Denote the sum of the first $latex {n}$ coefficients of a cusp form $latex {f}$ by $$ S_f(n) := \sum_{m \leq n} a(m). \tag{1}$$

We consider upper bounds for the second moment of $latex {S_f(n)}$.

The famous Ramanujan-Petersson conjecture gives us that $latex {a(n)\ll n^{\frac{k-1}{2} + \epsilon}}$. So one might assume $latex {S_f(X) \ll X^{\frac{k-1}{2} + 1 + \epsilon}}$. However, we expect the better bound $$ S_f(X) \ll X^{\frac{k-1}{2} + \frac{1}{4} + \epsilon}, \tag{2}$$

which we refer to as the “Classical Conjecture,” echoing Hafner and Ivić [HI].

Chandrasekharan and Narasimhan [CN] proved that the Classical Conjecture is true on average by showing that $$ \sum_{n \leq X} \lvert S_f(n) \rvert^2 = CX^{k- 1 + \frac{3}{2}} + B(X), \tag{3}$$

where $latex {B(x)}$ is an error term, $$ B(X) = \begin{cases} O(X^{k}\log^2(X)) \ \Omega\left(X^{k – \frac{1}{4}}\frac{(\log \log \log X)^3}{\log X}\right), \end{cases} \tag{4}$$

and $latex {C}$ is the constant, $$ C = \frac{1}{(4k + 2)\pi^2} \sum_{n \geq 1}\frac{\lvert a(n) \rvert^2}{n^{k + \frac{1}{2}}}. \tag{5}$$

A application of the Cauchy-Schwarz inequality to~(3) leads to the on-average statement that $$ \frac{1}{X} \sum_{n \leq X} |S_f(n)| \ll X^{\frac{k-1}{2} + \frac{1}{4}}. \tag{6}$$

From this, [HI] were able to show in some cases that $$ S_f(X) \ll X^{\frac{k-1}{2} + \frac{1}{3}}. \tag{7}$$

Better lower bounds are known for $latex {B(X)}$. In the same work [HI] improved the lower bound of [CN] for full-integral weight forms of level one and showed that $$ B(X) = \Omega\left(X^{k – \frac{1}{4}}\exp\left(D \tfrac{(\log \log x )^{1/4}}{(\log \log \log x)^{3/4}}\right)\right), \tag{8}$$

for a particular constant $latex {D}$.

The question of better understanding $latex {B(X)}$ is analogous to understanding the error term in the circle problem or divisor problem. In our paper, we introduce the Dirichlet series $$D(s, S_f \times S_g) := \sum_{n \geq 1} \frac{S_f(n) \overline{S_g(n)}}{n^{s + k – 1}}$$

D(s, S_f \times \overline{S_g}) &:= \sum_{n \geq 1} \frac{S_f(n)S_g(n)}{n^{s + k – 1}} and provide their meromorphic continuations. From our review of the literature, these Dirichlet series and their meromorphic continuations are new and provide new approaches to the classical problems related to $latex {S_f(n)}$.

Our primary result is the meromorphic continuation of $latex {D(s, S_f \times S_g)}$. As a first application, we prove a smoothed generalization to~(3).

Theorem 1 Suppose either that $latex {f = g}$ is a Hecke eigenform or that $latex {f}$ and $latex {g}$ have real coefficients. \begin{equation*} \frac{1}{X} \sum_{n \geq 1}\frac{S_f(n)\overline{S_g(n)}}{n^{k – 1}}e^{-n/X} = CX^{\frac{1}{2}} + O_{f,g,\epsilon}(X^{-\frac{1}{2} + \theta + \epsilon}) \end{equation*} where \begin{equation*} C = \frac{\Gamma(\tfrac{3}{2}) }{4\pi^2} \frac{L(\frac{3}{2}, f\times g)}{\zeta(3)}= \frac{\Gamma(\tfrac{3}{2})}{4\pi ^2} \sum_{n \geq 1} \frac{a(n)\overline{b(n)}}{n^{k + \frac{1}{2}}}, \end{equation*} and $latex {\theta}$ denotes progress towards Selberg’s Eigenvalue Conjecture. Similarly, \begin{equation*} \frac{1}{X} \sum_{n \geq 1}\frac{S_f(n)S_g(n)}{n^{k – 1}}e^{-n/X} = C’X^{\frac{1}{2}} + O_{f,g,\epsilon}(X^{-\frac{1}{2} + \theta + \epsilon}), \end{equation*} where \begin{equation*} C’ = \frac{\Gamma(\tfrac{3}{2})}{4\pi^2} \frac{L(\frac{3}{2}, f\times \overline{g})}{\zeta(3)} = \frac{\Gamma(\tfrac{3}{2})}{4\pi ^2} \sum_{n \geq 1} \frac{a(n)b(n)}{n^{k + \frac{1}{2}}}.\end{equation*}

We have a complete meromorphic continuation, and it would not be hard to give additional terms in the asymptotic. But the next terms come from zeroes of the zeta function and are complicated to nail down exactly.

Choosing $latex {f = g}$, we recover a proof of the Classical Conjecture on Average. More interestingly, we show that the secondary growth terms do not arise from a pole, nor are there prescribed polar reasons for growth. The secondary growth in the classical result comes from choosing a sharp cutoff instead of the nicely behaving and natural smooth cutoffs.

We prove analogous results for sums of normalized Fourier coefficients $$ S_f^\alpha(n) := \sum_{m \leq n} \frac{a(m)}{m^\alpha} \tag{9}$$

for $latex {0 \leq \alpha < k}$.

In the path to proving these results, we explicitly demonstrate remarkable cancellation between Rankin-Selberg convolution $latex {L}$-functions $latex {L(s, f\times g)}$ and shifted convolution sums $$ Z(s, 0; f,g) := \sum_{n, h} \frac{a(n)\overline{b(n-h)}}{n^{s + k – 1}}. \tag{10}$$

Comparing our results and methodologies with the main results of [CN] guarantees similar cancellation for general level and general weight, including half-integral weight forms.

We provide additional applications of the meromorphic continuation of $latex {D(s, S_f \times S_g)}$ in forthcoming works, which will be uploaded to the arXiv and described briefly here soon.

For exact references, see the paper.

Posted in Math.NT, Mathematics, Uncategorized | Tagged , | 1 Comment

Estimating the number of squarefree integers up to $X$

I recently wrote an answer to a question on MSE about estimating the number of squarefree integers up to $X$. Although the result is known and not too hard, I very much like the proof and my approach. So I write it down here.

First, let’s see if we can understand why this “should” be true from an analytic perspective.

We know that
$$ \sum_{n \geq 1} \frac{\mu(n)^2}{n^s} = \frac{\zeta(s)}{\zeta(2s)},$$
and a general way of extracting information from Dirichlet series is to perform a cutoff integral transform (or a type of Mellin transform). In this case, we get that
$$ \sum_{n \leq X} \mu(n)^2 = \frac{1}{2\pi i} \int_{(2)} \frac{\zeta(s)}{\zeta(2s)} X^s \frac{ds}{s},$$
where the contour is the vertical line $\text{Re }s = 2$. By Cauchy’s theorem, we shift the line of integration left and poles contribute terms or large order. The pole of $\zeta(s)$ at $s = 1$ has residue
$$ \frac{X}{\zeta(2)},$$
so we expect this to be the leading order. Naively, since we know that there are no zeroes of $\zeta(2s)$ on the line $\text{Re } s = \frac{1}{2}$, we might expect to push our line to exactly there, leading to an error of $O(\sqrt X)$. But in fact, we know more. We know the zero-free region, which allows us to extend the line of integration ever so slightly inwards, leading to a $o(\sqrt X)$ result (or more specifically, something along the lines of $O(\sqrt X e^{-c (\log X)^\alpha})$ where $\alpha$ and $c$ come from the strength of our known zero-free region.

In this heuristic analysis, I have omitted bounding the top, bottom, and left boundaries of the rectangles of integration. But proceeding in a similar way as in the proof of the analytic prime number theorem, you could proceed here. So we expect the answer to look like
$$ \frac{X}{\zeta(2)} + O(\sqrt X e^{-c (\log X)^\alpha})$$
using no more than the zero-free region that goes into the prime number theorem.

We will now prove this result, but in an entirely elementary way (except that I will refer to a result from the prime number theorem). This is below the fold.


Posted in Expository, Math.NT, Mathematics | Tagged , , , , | Leave a comment

Another proof of Wilson’s Theorem

While teaching a largely student-discovery style elementary number theory course to high schoolers at the Summer@Brown program, we were looking for instructive but interesting problems to challenge our students. By we, I mean Alex Walker, my academic little brother, and me. After a bit of experimentation with generators and orders, we stumbled across a proof of Wilson’s Theorem, different than the standard proof.

Wilson’s theorem is a classic result of elementary number theory, and is used in some elementary texts to prove Fermat’s Little Theorem, or to introduce primality testing algorithms that give no hint of the factorization.

Theorem 1 (Wilson’s Theorem) For a prime number $latex {p}$, we have $$ (p-1)! \equiv -1 \pmod p. \tag{1}$$

The theorem is clear for $latex {p = 2}$, so we only consider proofs for “odd primes $latex {p}$.”

The standard proof of Wilson’s Theorem included in almost every elementary number theory text starts with the factorial $latex {(p-1)!}$, the product of all the units mod $latex {p}$. Then as the only elements which are their own inverses are $latex {\pm 1}$ (as $latex {x^2 \equiv 1 \pmod p \iff p \mid (x^2 – 1) \iff p\mid x+1}$ or $latex {p \mid x-1}$), every element in the factorial multiples with its inverse to give $latex {1}$, except for $latex {-1}$. Thus $latex {(p-1)! \equiv -1 \pmod p.} \diamondsuit$

Now we present a different proof.

Take a primitive root $latex {g}$ of the unit group $latex {(\mathbb{Z}/p\mathbb{Z})^\times}$, so that each number $latex {1, \ldots, p-1}$ appears exactly once in $latex {g, g^2, \ldots, g^{p-1}}$. Recalling that $latex {1 + 2 + \ldots + n = \frac{n(n+1)}{2}}$ (a great example of classical pattern recognition in an elementary number theory class), we see that multiplying these together gives $latex {(p-1)!}$ on the one hand, and $latex {g^{(p-1)p/2}}$ on the other.

As $latex {g^{(p-1)/2}}$ is a solution to $latex {x^2 \equiv 1 \pmod p}$, and it is not $latex {1}$ since $latex {g}$ is a generator and thus has order $latex {p-1}$. So $latex {g^{(p-1)/2} \equiv -1 \pmod p}$, and raising $latex {-1}$ to an odd power yields $latex {-1}$, completing the proof $\diamondsuit$.

After posting this, we have since seen that this proof is suggested in a problem in Ireland and Rosen’s extremely good number theory book. But it was pleasant to see it come up naturally, and it’s nice to suggest to our students that you can stumble across proofs.

It may be interesting to question why $latex {x^2 \equiv 1 \pmod p \iff x \equiv \pm 1 \pmod p}$ appears in a fundamental way in both proofs.

This post appears on the author’s personal website and on the Math.Stackexchange Community Blog It is also available in pdf note form. It was typeset in \TeX, hosted on WordPress sites, converted using the utility, and displayed with MathJax.

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Functional Equations for L-Functions arising from Modular Forms

In this note, I remind myself of the functional equations for the $latex {L}$-functions $latex {\displaystyle \sum_{n\geq 0} \frac{a(n)}{n^s}}$ and $latex {\displaystyle \sum_{n\geq 0} \frac{a(n)}{n^s}e(\frac{n\overline{r}}{c})}$, where $latex {\overline{r}}$ is the multiplicative inverse of $latex {r \bmod c}$.


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