arXiv subjects

Will Sawin

Publications and source records attributed to Will Sawin.

64 records · Page 4Linked to original sources

arXiv2019

A representation theory approach to integral moments of L-functions over function fields

We propose a new heuristic approach to integral moments of L-functions over function fields, which we demonstrate in the case of Dirichlet characters ramified at one place (the function field analogue of the moments of the Riemann zeta function, where we think of the character n^{it} as ramified at the infinite place). We represent the moment as a sum of traces of Frobenius on cohomology groups associated to irreducible representations. Conditional on a hypothesis on the vanishing of some of these cohomology groups, we calculate the moments of the L-function and they match the predictions of the Conrey-Farmer-Keating-Rubinstein-Snaith recipe. In this case, the decomposition into irreducible representations seems to separate the main term and error term, which are mixed together in the long sums obtained from the approximate functional equation, even when it is dyadically decomposed. This makes our heuristic statement relatively simple, once the geometric background is set up. We hope that this will clarify the situation in more difficult cases like the L-functions of quadratic Dirichlet characters to squarefree modulus. There is also some hope for a geometric proof of this cohomological hypothesis, which would resolve the moment problem for these L-functions in the large degree limit over function fields.

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arXiv2019

On the Chowla and twin primes conjectures over $\mathbb F_q[T]$

Using geometric methods, we improve on the function field version of the Burgess bound, and show that, when restricted to certain special subspaces, the Möbius function over $\mathbb F_q[T]$ can be mimicked by Dirichlet characters. Combining these, we obtain a level of distribution close to $1$ for the Möbius function in arithmetic progressions, and resolve Chowla's $k$-point correlation conjecture with large uniformity in the shifts. Using a function field variant of a result by Fouvry-Michel on exponential sums involving the Möbius function, we obtain a level of distribution beyond $1/2$ for irreducible polynomials, and establish the twin prime conjecture in a quantitative form. All these results hold for finite fields satisfying a simple condition.

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arXiv2019

Lectures on Applied $\ell$-adic Cohomology

We describe how a systematic use the deep methods from $\ell$-adic cohomology pioneered by Grothendieck and Deligne and further developed by Katz, Laumon allow to make progress on various classical questions from analytic number theory. This text is an extended version of a series of lectures given by the third and fourth authors during the 2016 Arizona Winter School.

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arXiv2019

$\overline{M}_{1,n}$ is usually not uniruled in characteristic $p$

Using etale cohomology, we define a birational invariant for varieties in characteristic $p$ that serves as an obstruction to uniruledness - a variant on an obstruction to unirationality due to Ekedahl. We apply this to $\overline{M}_{1,n}$ and show that $\overline{M}_{1,n}$ is not uniruled in characteristic $p$ as long as $n \geq p \geq 11$. To do this, we use Deligne's description of the etale cohomology of $\overline{M}_{1,n}$ and apply the theory of congruences between modular forms.

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arXiv2018

Monodromy of elliptic curve convolution, seven-point sheaves of $G_2$-type and motives of Beauville type

We study the Tannakian properties of the category of perverse sheaves on elliptic curves endowed with the convolution product. We establish that for certain sheaves with unipotent local monodromy over seven points the corresponding Tannaka group is isomorphic to $G_2$. This monodromy approach generalizes a result of Katz on the existence of $G_2$-motives in the middle cohomology of deformations of Beauville surfaces.

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arXiv2018

The equidistribution of L-functions of twists by Witt vector Dirichlet characters over function fields

Katz showed that the L-functions of all Dirichlet characters of F_q(t), with conductor a fixed power of a degree one prime, are equidistributed in the limit as q goes to infinity. We generalize this statement to the L-functions of twists of an arbitrary Galois representation by Dirichlet characters, including independence of the L-functions of twists of different representations by the same Dirichlet character. A similar generalization, without the independence statement, for characters with squarefree conductor, was proven by Hall, Keating, and Roditty-Gershon.

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arXiv2018

Dynamical models for Liouville and obstructions to further progress on sign patterns

We define a class of dynamical systems by modifying a construction due to Tao, which includes certain Furstenburg limits arising from the Liouville function. Most recent progress on the Chowla conjectures and sign patterns of the Mobius and Liouville functions uses methods that apply to any dynamical system in this class. Hence dynamical systems in this class with anomalous local behavior present obstructions to further progress on these problems by the same techniques. We construct straightforward examples of dynamical systems in this class based on polynomial phases and calculate the resulting obstruction. This requires explicit bounds for the number of sign patterns arising in a certain way from polynomials, which is elementary but not completely trivial.

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arXiv2018

The Growth Rate of Tri-Colored Sum-Free Sets

Let $G$ be an abelian group. A tri-colored sum-free set in $G^n$ is a collection of triples $({\bf a}_i, {\bf b}_i, {\bf c}_i)$ in $G^n$ such that ${\bf a}_i+{\bf b}_j+{\bf c}_k=0$ if and only if $i=j=k$. Fix a prime $q$ and let $C_q$ be the cyclic group of order $q$. Let $θ= \min_{ρ>0} (1+ρ+\cdots + ρ^{q-1}) ρ^{-(q-1)/3}$. Blasiak, Church, Cohn, Grochow, Naslund, Sawin, and Umans (building on previous work of Croot, Lev and Pach, and of Ellenberg and Gijswijt) showed that a tri-colored sum-free set in $C_q^n$ has size at most $3 θ^n$. Between this paper and a paper of Pebody, we will show that, for any $δ> 0$, and $n$ sufficiently large, there are tri-colored sum-free sets in $C_q^n$ of size $(θ-δ)^n$. Our construction also works when $q$ is not prime.

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arXiv2017

Improved estimates for polynomial Roth type theorems in finite fields

We prove that, under certain conditions on the function pair $φ_1$ and $φ_2$, bilinear average $p^{-1}\sum_{y\in \mathbb{F}_p}f_1(x+φ_1(y)) f_2(x+φ_2(y))$ along curve $(φ_1, φ_2)$ satisfies certain decay estimate. As a consequence, Roth type theorems hold in the setting of finite fields. In particular, if $φ_1,φ_2\in \mathbb{F}_p[X]$ with $φ_1(0)=φ_2(0)=0$ are linearly independent polynomials, then for any $A\subset \mathbb{F}_p, |A|=δp$ with $δ>c p^{-\frac{1}{12}}$, there are $\gtrsim δ^3p^2$ triplets $x,x+φ_1(y), x+φ_2(y)\in A$. This extends a recent result of Bourgain and Chang who initiated this type of problems, and strengthens the bound in a result of Peluse, who generalized Bourgain and Chang's work. The proof uses discrete Fourier analysis and algebraic geometry.

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arXiv2017

Bounds for Matchings in Nonabelian Groups

We give upper bounds for triples of subsets of a finite group such that the triples of elements that multiply to 1 form a perfect matching. Our bounds are the first to give exponential savings in powers of an arbitrary finite group. Previously, Blasiak-Church-Cohn-Grochow-Naslund-Sawin-Umans gave similar bounds in abelian groups of bounded exponent, and Petrov gave exponential bounds in certain p-groups.

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