Seminars and Colloquia by Series

Sqrt and Levers

Series
Athens-Atlanta Number Theory Seminar
Time
Tuesday, October 24, 2023 - 17:15 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Vesselin Dimitrov – Georgia Tech –

If the formal square root of an abelian surface over Q looks like an elliptic curve, it has to be an elliptic curve."


We discuss what such a proposition might mean, and prove the most straightforward version where the precise condition is simply that the L-function of the abelian surface possesses an entire holomorphic square root. The approach follows the Diophantine principle that algebraic numbers or zeros of L-functions repel each other, and is in some sense similar in spirit to the Gelfond--Linnik--Baker solution of the class number one problem.

We discuss furthermore this latter connection: the problems that it raises under a hypothetical presence of Siegel zeros, and a proven analog over finite fields. The basic remark that underlies and motivates these researches is the well-known principle (which is a consequence of the Deuring--Heilbronn phenomenon, to be taken with suitable automorphic forms $f$ and $g$): an exceptional character $\chi$ would cause the formal $\sqrt{L(s,f)L(s,f \otimes \chi)}L(s,g)L(s, g \otimes \chi)$ to have a holomorphic branch on an abnormally big part of the complex plane, all the while enjoying a Dirichlet series formal expansion with almost-integer coefficients. This leads to the kind of situation oftentimes amenable to arithmetic algebraization methods. The most basic (qualitative) form of our main tool is what we are calling the "integral converse theorem for GL(2)," and it is a refinement of a recent Unbounded Denominators theorem that we proved jointly with Frank Calegari and Yunqing Tang. 

 

Moments of Dirichlet L-functions

Series
Athens-Atlanta Number Theory Seminar
Time
Tuesday, October 24, 2023 - 16:00 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Caroline Turnage-Butterbaugh – Carleton College –

In recent decades there has been much interest and measured progress in the study of moments of the Riemann zeta-function and, more generally, of various L-functions. Despite a great deal of effort spanning over a century, asymptotic formulas for moments of L-functions remain stubbornly out of reach in all but a few cases. In this talk, we consider the problem for the family of all Dirichlet L-functions of even primitive characters of bounded conductor. I will outline how to harness the asymptotic large sieve to prove an asymptotic formula for the general 2kth moment of an approximation to this family. The result, which assumes the generalized Lindelöf hypothesis for large values of k, agrees with the prediction of Conrey, Farmer, Keating, Rubenstein, and Snaith. Moreover, it provides the first rigorous evidence beyond the so-called “diagonal terms” in their conjectured asymptotic formula for this family of L-functions

Athens-Atlanta Number Theory Seminar

Series
Athens-Atlanta Number Theory Seminar
Time
Tuesday, April 23, 2019 - 16:00 for 2.5 hours
Location
Skiles 311
Speaker
Ananth Shankar, Jordan Ellenberg – MIT, University of Wisconsin, Madison

First talk at 4:00 by by Ananth Shankar (MIT http://math.mit.edu/~ananths/)

Exceptional splitting of abelian surfaces over global function fields.

Let A denote a non-constant ordinary abelian surface over a global function field (of characteristic p > 2) with good reduction everywhere. Suppose that $A$ does not have real multiplication by any real quadratic field with discriminant a multiple of $p$. Then we prove that there are infinitely many places modulo which $A$ is isogenous to the product of two elliptic curves. If time permits, I will also talk about applications of our results to the p-adic monodromy of such abelian surfaces. This is joint work with Davesh Maulik and Yunqing Tang.

Second talk at 5:15 Jordan Ellenberg (University of Wisconsin http://www.math.wisc.edu/~ellenber/)

What is the tropical Ceresa class and what should it be?

This is a highly preliminary talk about joint work with Daniel Corey and Wanlin Li.  The Ceresa cycle is an algebraic cycle canonically attached to a curve C, which appears in an intriguing variety of contexts; its height can sometimes be interpreted as a special value, the vanishing of its cycle class is related to the Galois action on the nilpotent fundamental group, it vanishes on hyperelliptic curves, etc.  In practice it is not easy to compute, and we do not in fact know an explicit non-hyperelliptic curve whose Ceresa class vanishes.  We will discuss a definition of the Ceresa class for a tropical curve, explain how to compute it in certain simple cases, and describe progress towards understanding whether it is possible for the Ceresa class of a non-hyperelliptic tropical curve to vanish.  (The answer is:  "sort of”.)  The tropical Ceresa class sits at the interface of algebraic geometry, graph theory (because a tropical curve is more or less a metric graph), and topology: for we can also frame the tropical Ceresa class as an entity governed by the mapping class group, and in particular by the question of when a product of commuting Dehn twists can commute with a hyperelliptic involution in the quotient of the mapping class group by the Johnson kernel.

Periods, motivic Gamma functions, and Hodge structures

Series
Athens-Atlanta Number Theory Seminar
Time
Monday, October 30, 2017 - 17:15 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Spencer Bloch – University of Chicago
Golyshev and Zagier found an interesting new source of periods associated to (eventually inhomogeneous) solutions generated by the Frobenius method for Picard Fuchs equations in the neighborhood of singular points with maximum unipotent monodromy. I will explain how this works, and how one can associate "motivic Gamma functions" and generalized Beilinson style variations of mixed Hodge structure to these solutions. This is joint work with M. Vlasenko.

Gonality and the strong uniform boundedness conjecture for periodic points

Series
Athens-Atlanta Number Theory Seminar
Time
Monday, October 30, 2017 - 16:05 for 1 hour (actually 50 minutes)
Location
Skiles 005
Speaker
Bjorn Poonen – Massachusetts Institute of Technology
The function field case of the strong uniform boundedness conjecturefor torsion points on elliptic curves reduces to showing thatclassical modular curves have gonality tending to infinity.We prove an analogue for periodic points of polynomials under iterationby studying the geometry of analogous curves called dynatomic curves.This is joint work with John R. Doyle.

Intersection numbers and higher derivatives of L-functions for function fields

Series
Athens-Atlanta Number Theory Seminar
Time
Thursday, April 14, 2016 - 17:15 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Zhiwei Yun – Stanford University
In joint work with Wei Zhang, we prove a higher derivative analogue of the Waldspurger formula and the Gross-Zagier formula in the function field setting under the assumption that the relevant objects are everywhere unramified. Our formula relates the self-intersection number of certain cycles on the moduli of Shtukas for GL(2) to higher derivatives of automorphic L-functions for GL(2).

Nonabelian Cohen-Lenstra Heuristics and Function Field Theorems

Series
Athens-Atlanta Number Theory Seminar
Time
Thursday, April 14, 2016 - 16:05 for 1 hour (actually 50 minutes)
Location
Skiles 006
Speaker
Melanie Matchett-Wood – University of Wisconsin
The Cohen-Lenstra Heuristics conjecturally give the distribution of class groups of imaginary quadratic fields. Since, by class field theory, the class group is the Galois group of the maximal unramified abelian extension, we can consider the Galois group of the maximal unramified extension as a non-abelian generalization of the class group. We will explain non-abelian analogs of the Cohen-Lenstra heuristics due to Boston, Bush, and Hajir and joint work with Boston proving cases of the non-abelian conjectures in the function field analog.