# mixedmath

Explorations in math and programming
David Lowry-Duda

I'm in San Diego, and it's charming here. (It's certainly much nicer outside than the feet of snow in Boston. I've apparently brought some British rain with me, though).

Today I give a talk on counting lattice points on one-sheeted hyperboloids. These are the shapes described by $$X_1^2 + \cdots + X_{d-1}^2 = X_d^2 + h,$$ where $h > 0$ is a positive integer. The question is: how many lattice points $x$ are on such a hyperboloid with $| x |^2 \leq R$; or equivalently, how many lattice points are on such a hyperboloid and contained within a ball of radius $\sqrt R$ centered at the origin?

I describe my general approach of transforming this into a question about the behavior of modular forms, and then using spectral techniques from the theory of modular forms to understand this behavior. This becomes a question of understanding the shifted convolution Dirichlet series $$\sum_{n \geq 0} \frac{r_{d-1}(n+h)r_1(n)}{(2n + h)^s}.$$ Ultimately this comes from the modular form $\theta^{d-1}(z) \overline{\theta(z)}$, where $$\theta(z) = \sum_{m \in \mathbb{Z}} e^{2 \pi i m^2 z}.$$

Here are the slides for this talk. Note that this talk is based on chapter 5 of my thesis.

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