MixedMath - explorations in math and programinghttps://davidlowryduda.comDavid's personal blog.en-usCopyright David Lowry-Duda (2022) - All Rights Reserved.admin@davidlowryduda.comadmin@davidlowryduda.comSat, 03 Sep 2022 19:10:45 +0000Sat, 03 Sep 2022 19:10:45 +0000mixedmathapp/generate_rss.py v0.1https://cyber.harvard.edu/rss/rss.htmlhttps://davidlowryduda.com/static/images/favicon-32x32.pngMixedMathhttps://davidlowryduda.comInitial thoughts on visualizing number fieldshttps://davidlowryduda.com/pcmi-vis-nfDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/pcmi-vis-nfFri, 22 Jul 2022 03:14:15 +0000Computing coset representatives for quotients of congruence subgroupshttps://davidlowryduda.com/coset-reps-cong-subgroupsDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/coset-reps-cong-subgroupsWed, 20 Jul 2022 03:14:15 +0000Note Series on Zeros of General Dirichlet Serieshttps://davidlowryduda.com/zeros-of-dirichlet-series-mastheadDavid Lowry-Duda<p>I wrote a series of notes on some aspects of the theoretical behavior
and zeros of Dirichlet series in the <em>extended Selberg Class</em>. There are a few
different ways of extending the Selberg Class, but here I mean Dirichlet series
\begin{equation*}
L(s) = \sum_{n \geq 1} \frac{a(n)}{n^s}
\end{equation*}
with a functional equation of the shape $s \mapsto 1 - s$, satisfying</p>
<ol>
<li>A Ramanujan–Petersson bound <em>on average</em>, meaning that $\sum_{n \leq N}
\lvert a(n) \rvert^2 \ll N^{1 + \epsilon}$ for any $\epsilon > 0$.</li>
<li>$L(s)$ has <em>analytic continuation</em> to $\mathbb{C}$ to an entire function of
finite order.</li>
<li>$L(s)$ satisfies a functional equation of the typical self-dual shape.</li>
</ol>
<p>There is no assumption of an Euler product.</p>
<p>Although I write these notes for general Dirichlet series in the extended
Selberg class, I was really thinking about Dirichlet series associated to
half-integral weight modular forms.</p>
<h2>Links and Summaries of each Note</h2>
<ol>
<li>
<p><a href="https://davidlowryduda.com/zeros-of-dirichlet-series/">The first note</a> sets the stage, defines the relevant series, and
establishes fundamental results to be used later. Jensen's inequality and
Jensen's theorem are given, as are generic convexity bounds for these
Dirichlet series.</p>
<p>The first note also contains a proof of a new fact to me:<sup>1</sup>
<span class="aside"><sup>1</sup>but only new
<em>to me</em>. I based my presentation of this fact on notes from Hardy from a
century ago.</span>
if a Dirichlet series has a zero in its domain of
absolute convergence, then it has infinitely many, and these zeros are
<em>almost periodic</em>.</p>
</li>
<li>
<p><a href="https://davidlowryduda.com/zeros-of-dirichlet-series-ii/">The second note</a> is relatively short and shows that these Dirichlet
series are in fact entire of order $1$. Then it establishes weak
zero-counting results based only on this order of growth.</p>
<p>These are foundational ideas, and in essence are no different than analysis
for $\zeta(s)$ or typical $L$-functions in the Selberg class.</p>
</li>
<li>
<p><a href="https://davidlowryduda.com/zeros-of-dirichlet-series-iii/">The third note</a> describes a theorem of Potter from
1940, proving Lindelöf-on-average (in the $t$-aspect) on certain vertical
lines, depending on the degree. This suggests that for Dirichlet series
associated to half-integral weight modular forms, the Lindelöf Hypothesis
might be <em>true</em> even though the Riemann Hypothesis is false.</p>
</li>
<li>
<p><a href="https://davidlowryduda.com/zeros-of-dirichlet-series-iv/">The fourth note</a> proves that one hundred percent of zeros of
Dirichlet series associated to half-integral weight modular forms lie within
$\epsilon$ of the critical line, for any $\epsilon > 0$.</p>
<p>For these proofs, the <em>standard proofs</em> that I've seen elsewhere for Selberg
$L$-functions don't quite apply. Fundamentally, this is because zeros are
counted using some form of the argument principle, and for Selberg
$L$-functions with Euler product, the logarithmic derivative of the
$L$-functions is both easier to understand (because of the Euler product)
and gives convenient access to sum up changes in the argument. But in
practice the actual methods I use were mostly lightly modified from the
vast, extensive literature on various techniques applied to study $\zeta(s)$
(before better techniques arose using the Euler product in more
sophisticated ways).</p>
<p>I note that I do not know how to prove a lower bound for the percentage of
zeros lying <em>directly on the critical line</em> for series without an Euler
product. It is possible to apply a generalized form of the classical
argument of Hardy to show that there are infinitely many zeros on the line,
but I haven't managed to modify any of the various results for lower bounds.</p>
</li>
</ol>
<p>I will also note that with Thomas Hulse, Mehmet Kiral, and Li-Mei Lim, I've
computed many examples of zeros of half-integral weight forms and I
conjecture that 100 percent of their zeros lie <em>directly on the critical line</em>.
(But there are many, many zeros not on the critical line). I described some of
those computations <a href="https://davidlowryduda.com/slides-from-a-talk-on-half-integral-weight-dirichlet-series/">in this
talk</a>.</p>https://davidlowryduda.com/zeros-of-dirichlet-series-mastheadSat, 09 Jul 2022 03:14:15 +0000Zeros of Dirichlet Series IVhttps://davidlowryduda.com/zeros-of-dirichlet-series-ivDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/zeros-of-dirichlet-series-ivFri, 08 Jul 2022 03:14:15 +0000Zeros of Dirichlet Series IIIhttps://davidlowryduda.com/zeros-of-dirichlet-series-iiiDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/zeros-of-dirichlet-series-iiiThu, 07 Jul 2022 03:14:15 +0000Visualizations for Quanta's 'What is the Langlands Program?'https://davidlowryduda.com/quanta-langlands-vizDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/quanta-langlands-vizWed, 01 Jun 2022 03:14:15 +0000Now pagehttps://davidlowryduda.com/nowDavid Lowry-Duda<h1>What I'm doing Now</h1>
<p>Last updated <strong>3 September 2022</strong>.</p>
<p>This is a <a href="https://nownownow.com/about">now page</a>. It is a written version of
what I might say if we met in person and you asked me what I'm up to. The date
at the top is important: it helps the reader determine if this is a <em>now</em> page
or a <em>then</em> page.</p>
<h2>Research Travel</h2>
<ul>
<li>I will be near Berkeley during the week of September 12th.</li>
<li>I will be at <a href="https://archimede.mat.ulaval.ca/QUEBEC-MAINE/">Maine-Quebec</a>
on October 15-16.</li>
</ul>
<p>Other than that, I'm typically in either Boston or Providence.</p>
<h2>Research</h2>
<p>I'm currently actively working on several projects.</p>
<ul>
<li>I'm working to develop methods to explicitly and rigorously compute Maass
forms. I'm currently implementing a rigorous form of Hejhal's algorithm for
weight $0$ Maass forms in <a href="https://arblib.org/">arb</a>.</li>
<li>At the same time, I'm currently writing down interrelated things about Maass
forms, including implementation notes for my computational work, the theory
behind rigorous evaluation, and work towards an explicit and computable trace
formula for forms of nontrivial character on non-squarefree level.</li>
<li>With my frequent collaborators Chan Ieong Kuan and Alex Walker, I'm
investigating discrete reduction problems for the divisor function. We will
release a preprint for this soon.</li>
<li>I supervised a project with some students at PROMYS. We're writing down
several of their observations.</li>
</ul>
<h2>Teaching</h2>
<p>I'm not currently teaching.</p>https://davidlowryduda.com/nowTue, 10 May 2022 03:14:15 +0000Slides from a talk on Improved Bounds for Number Fieldshttps://davidlowryduda.com/talk-improved-bounds-number-fieldsDavid Lowry-Duda<p>At a meeting of the Algebraic Geometry, Number Theory, and Computation Simons
Collaboration, I gave a short talk surveying the results and ideas of
<a href="https://arxiv.org/abs/2204.01651"><em>Improved bounds on number fields of small
degree</em></a>, joint work with Anderson, Gafni,
Hughes, Lemke Oliver, Thorne, Wang, and Zhang.</p>
<p>In many ways this is a follow-up to an <a href="/slides-from-a-talk-on-quantitative-hilbert-irreducibility/">earlier talk I gave to the Simons
Collaboration</a>
about work towards a quantitative form of Hilbert's irreducibility theorem.
Both of these results grew out of an AIM workshop.</p>
<p>The slides for this talk <a href="/wp-content/uploads/2022/05/SimonsMay2022_Schmidt.pdf">are available here</a>.</p>https://davidlowryduda.com/talk-improved-bounds-number-fieldsMon, 09 May 2022 03:14:15 +0000Simplified proofs and reasoning in "Improved Bounds on Number Fields of Small Degree"https://davidlowryduda.com/simplified-improved-boundsDavid Lowry-DudaThis post is larger than 10000 bytes, which is above the limit for this RSS feed. Perhaps it is long or has embedded images or code. Please view it directly at the url.https://davidlowryduda.com/simplified-improved-boundsSat, 30 Apr 2022 03:14:15 +0000Paper: Counting number fields of small degreehttps://davidlowryduda.com/paper-counting-number-fieldsDavid Lowry-Duda<h1>Counting number fields of small degree</h1>
<p>Recently, my collaborators Theresa C. Anderson, Ayla Gafni, Kevin Hughes,
Robert J. Lemke Oliver, Frank Thorne, Jiuya Wang, Ruixiang Zhang, and I
uploaded a <a href="https://arxiv.org/abs/2204.01651">preprint to the arxiv</a> called
"Improved bounds on number fields of small degree". This collaboration is a
continuation<sup>1</sup>
<span class="aside"><sup>1</sup>though with a few different cast members</span>
of our
<a href="https://arxiv.org/abs/2107.02914">previous work on quantitative Hilbert
irreducibility</a>, which will appear in IMRN.</p>
<p>In this paper, we improve the upper bound due to Schmidt for estimates on the
number of number fields of degree $6 \leq n \leq 94$. Actually, we improve on
Schmidt for all $n \geq 6$, but for $n \geq 95$ Lemke Oliver and Thorne have
different, better bounds.</p>
<p>Schmidt proved the following.</p>
<div class="theorem" data-text="Schmidt 95">
<p>For $n \geq 6$, there are $\ll X^{(n+2)/4}$ number fields of degree $n$ and
having discriminant bounded by $X$.</p>
</div>
<p>We prove a polynomial improvement that decays with the degree.</p>
<div class="theorem" data-text="AGHLDLOTWZ">
<p>For $n \geq 6$, there are
\begin{equation*}
\ll_\epsilon X^{\frac{n + 2}{4} - \frac{1}{4n - 4} + \epsilon}
\end{equation*}
number fields of degree $n$ and having discriminant bounded by $X$.</p>
</div>
<p>Towards the end of this project, we learned that Bhargava, Shankar, and Wang
were also producing improvements over Schmidt in this range. On the same day
that we posted our paper to the arxiv, they posted <a href="https://arxiv.org/abs/2204.01331">their
paper</a>, in which they prove the following.</p>
<div class="theorem" data-text="BSW">
<p>For $n \geq 6$, there are
\begin{equation*}
\ll_\epsilon X^{\frac{n + 2}{4} - \frac{1}{2n - 2} + \frac{1}{2^{2g}(2n-2)} + \epsilon}
\end{equation*}
number fields of degree $n$ and having discriminant bounded by $X$, where $g
= \lfloor \frac{n-1}{2} \rfloor$.</p>
</div>
<p>In both our work and in BSW, the broad strategy is based on Schmidt's approach.
For a monic polynomial
\begin{equation*}
f(x) = x^n + c_1 x^{n-1} + \cdots + c_n,
\end{equation*}
we define the height $H(f)$ to be
\begin{equation*}
H(f) := \max( \lvert c_i \rvert^{1/i} ).
\end{equation*}
Then Schmidt showed that to count number fields of discriminant up to $X$, it
suffices to count polynomials of height roughly up to $X^{1/(2n - 2)}$.</p>
<p>The challenge is that <em>most</em> of these polynomials cut out number fields of
discriminant <em>much larger</em> than $X$. The challenge is then to count relevant
polynomials and to identify irrelevant polynomials.</p>
<p>Remarkably, the broad strategy in out work and in BSW for identifying
irrelevant polynomials is similar. For a prototypical polynomial $f$ of degree
$n$ and of height $X^{1/(2n-2)}$, we should expect the discriminant
of $f$ to be approximately $X^{n/2}$. We should also expect the field cut
out by $f$ to have discriminant roughly this size. Recalling that we are
counting number fields of discriminant only up to $X$, this means that a
<strong>relevant</strong> polynomial of this height must be exceptional in one of two ways:</p>
<ol>
<li>either the discriminant of $f$ is unusually small, or</li>
<li>the discriminant of the number field cut out by $f$ is much smaller than the
discriminant of $f$.</li>
</ol>
<p>In both out work and in BSW, those $f$ with unusually small discriminant
are bounded straightforwardly and lossily.</p>
<p>The heart of the argument is in the latter case. Here, the ratio of the two
discriminants is the square of the index
$[\mathcal{O}_K : \mathbb{Z}[\alpha]]$, where $\alpha$ is a root of $f$. Thus
we bound the number of polynomials whose discriminants have large square
divisors.</p>
<p>In establishing bounds for polynomials with particularly squarefull
discriminants that our ideas and those in BSW significantly diverge.</p>
<p>In our work, we study the problem locally. That is, we study the behavior of
$\psi_{p^{2k}}$, the characteristic function for monic polynomials of degree
$n$ over $\mathbb{Z}/p^{2k}\mathbb{Z}$ having discriminant congruent to $0
\bmod p^{2k}$. As in our work on quantitative Hilbert irreducibility, we
translate this problem into a sieve problem with local weights coming from
Fourier transforms $\widehat{\psi_{p^{2k}}}$ after passing through Poisson
summation, and we study the Fourier transforms using a variety of somewhat
ad-hoc techniques.</p>
<p>In BSW, they reason differently. They use recent explicit quantitative Hilbert
irreducibility work from Castillo and Dietmann to replace the fundamental
underlying sieve. To do this, they translate the task of counting relevant
polynomials into the task of counting <strong>distinguished</strong> points in spaces of
$n \times n$ symmetric matrices — and then show that Castillo and
Dietmann's work bounds these points.</p>
<p>Even though the number field count in BSW is stronger than our number field
count, we think that our methods and ideas will have other applications.
Further, we've noticed remarkable interactions between local Fourier analysis
and discriminants of polynomials.</p>
<h2>See also</h2>
<ul>
<li>See also my note on <a href="/simplified-improved-bounds/">a description and simplified proofs of many of the ideas in this paper</a>.</li>
</ul>https://davidlowryduda.com/paper-counting-number-fieldsThu, 28 Apr 2022 03:14:15 +0000