AGNT seminar 2020/2021

Organizers: Anders Södergren, Christian Johansson.

Note​: For the time being, all upcoming seminars will be online. Please check the calendar

for information on how to join, or email one of the organizers. The standard time for the seminars in Spring 2021 is 15.15-16.15. However, there are exceptions --- please check the calendar above or email one of the organizers for information about the time.

Upcoming seminars

28.04.2021 - Chantal David (Concordia University)One-Level density for cubic characters over the Eisenstein field.​

We show that the one-level density for L-functions associated with the cubic residue symbols χn, with n ∈ Z[ω] square-free, satisfies the Katz-Sarnak conjecture for all test functions whose Fourier transforms are supported in (−13/11, 13/11), under GRH. This is the first result extending the support outside the trivial range (−1, 1) for a family of cubic L-functions. This implies that a positive proportion of the L-functions associated with these characters do not vanish at the central point s = 1/2. A key ingredient is a bound on an average of generalized cubic Gauss sums at prime arguments, whose proof is based on the work of Heath-Brown and Patterson. Joint work with Ahmet M. Güloglu.

19.05.2021 - Carl Wang Erickson (University of Pittsburgh)TBA

Abstract: TBA​

26.05.2021 - Ana Ros Camacho (Cardiff University)TBA

Abstract: TBA

02.06.2021 - Wushi Goldring (Stockholm University)TBA

Abstract: TBA

Past seminars

21.04.2021 - Wei Ho (University of Michigan)Splitting Brauer classes ​(Time: 16.30-17.30)

Abstract: Given a Brauer class over a field, what types of varieties split it? Or more geometrically, can we say anything about the varieties that map to a given Brauer-Severi variety? In this talk, we will discuss some open questions related to splitting Brauer classes. For example, we will review some classical algebro-geometric constructions that produce genus one curves splitting low index Brauer classes ((old) joint work with J. de Jong), and we will explain why a Brauer class of any index is split by a torsor under an abelian variety (joint work with M. Lieblich).​

14.04.2021 - David Loeffler (University of Warwick)Euler systems and the Bloch--Kato conjecture

The Bloch--Kato conjecture is a very general conjecture relating the properties of arithmetic objects over number fields to special values of L-functions, generalising a wealth of earlier​ theorems and conjectures such as the Birch--Swinnerton-Dyer conjecture for elliptic curves. I will explain a little about how this conjecture arises, and how it can be attacked using tools known as 'Euler systems'. In recent years there has been dramatic progress in the theory of Euler systems, and I'll explain recent work of Sarah Zerbes and myself from 2020, in which we use Euler systems to prove new cases of the Bloch--Kato conjecture for L-functions arising from the symplectic group GSp(4).

07.04.2021 - Kristian Holm (Chalmers/GU)A Central Limit Theorem for Symplectic Lattice Point Counting Functions

We study a sequence of normalized counting functions on the space of 2d-dimensional symplectic lattices where d is at least 3. Using a combinatorial device introduced by Björklund and Gorodnik in order to estimate cumulants (alternating sums of moments), we prove that our sequence satisfies a central limit theorem. This is work in progress.​

24.03.2021 - David Rydh (KTH)Stacky weighted blowups

Many problems, such as resolution of singularities, are solved by blowing up smooth varieties along smooth centers. This is a very useful and explicit construction but sometimes a little restrictive. If we replace (smooth) varieties with (smooth) stacks/orbifolds, we obtain several new similar operations (root stacks, stacky blowups, weighted stacky blowups) that we can add to our toolbox. I will describe these new operations and some of their applications such as easier and more efficient algorithms for resolution of singularities (Abramovich--Temkin--Wlodarczyk) and weak factorization, as well as other applications such as \'etalification and destackification. There are also close relations to toric and logarithmic geometry.

This is partly joint work with Daniel Bergh and Ming Hao Quek.

09.03.2021 - Dmitry Gourevitch (Weizmann Institute of Science)Relations between Fourier coefficients of automorphic forms, with applications to vanishing and to Eulerianity

In recent works with H. P. A. Gustafsson, A. Kleinschmidt, D. Persson, and S. Sahi, we found a way to express any automorphic form through its Fourier coefficients, using adelic integrals, period integrals and discrete summation – generalizing the Piatetski-Shapiro – Shalika decomposition for GL(n). I will explain the general idea behind our formulas, and illustrate it on examples. I will also show applications to vanishing and Eulerianity of Fourier coefficients.

03.03.2021 - Per Salberger (Chalmers/GU)On n-torsion in class groups of number fields

It is well known that the class group of a number field is of size bounded above by roughly the square root of its discriminant. But one expects by conjectures of Cohen-Lenstra that the the n-torsion part of this group should be much smaller and there have recently been several papers on this by prominent mathematicians. We present in our talk some new bounds which are sharper than those in the literature.​

17.02.2021 - Olof Sisask (Stockholm University) Breaking the logarithmic barrier in Roth's theorem

We present an improvement to Roth's theorem on arithmetic progressions, implying the first non-trivial case of a conjecture of Erdős: if a subset A of {1,2,3,...} is not too sparse, in that the sum of its reciprocals diverges, then A must contain infinitely many three-term arithmetic progressions. Although a problem in number theory and combinatorics on the surface, it turns out to have fascinating links with geometry, harmonic analysis and probability, and we shall aim to give something of a flavour of this.

10.02.2021 - Jiacheng Xia (Chalmers/GU)The unitary Kudla conjecture in the cases of norm-Euclidean imaginary quadratic fields

Modularity conjectures lie at the heart of Kudla's program, which is a historic development of the earlier work of Hirzebruch--Zagier, Gross--Zagier, Gross--Keating, and Kudla--Millson.

The modularity conjecture for orthogonal Shimura varieties over Q was fully resolved by Bruinier--Raum in 2015, thanks to earlier work of Richard Borcherds and Wei Zhang. In this talk, I will explain how to use the method of Bruinier--Raum to show the modularity conjecture for certain unitary Shimura varieties based on a result of Yifeng Liu.

If time allows, I'll also talk about recent development of certain arithmetic applications, including the work of Li--Liu on arithmetic inner product formula, which is a natural analogue of the Gross--Zagier formula in higher dimension.​

27.01.2021 - James Maynard (University of Oxford)Primes in arithmetic progressions beyond the Riemann Hypothesis

Questions about the distribution of primes in an arithmetic progression are closely linked to the Generalized Riemann Hypothesis (GRH), which unfortunately appears out of reach. A very useful unconditional substitute for the GRH is the Bombieri-Vinogradov Theorem, which shows that the GRH is true 'on average'.

I'll talk about some recent results on primes in arithmetic progressions which goes beyond the Bombieri-Vinogradov Theorem, and corresponds to proving something stronger than the Riemann Hypothesis holds 'on average'.

20.01.2021 - Christian johansson (Chalmers/GU)Langlands correspondences in families

I will survey some old and new ideas on "geometrizing" the Langlands correspondence, by viewing it as a sheaf of representations over a moduli space of Galois representations (or other "Langlands parameters"). Towards the end, I hope to briefly discuss some work in progress on p-adic endoscopy for SL(2) over totally real fields utilizing this perspective, which is joint with Judith Ludwig.​

16.12.2020 - Julia Brandes (Chalmers/GU)Diophantine systems containing both diagonal and non-diagonal equations

Abstract: We establish the analytic Hasse principle for Diophantine systems consisting of one diagonal form of degree k and one general form of degree d, where d is smaller than k. By employing a hybrid method that combines ideas from the study of general forms with techniques adapted to the diagonal case, we are able to obtain bounds that grow exponentially in d but only quadratically in k, reflecting the growth rates typically obtained for both problems separately. Time permitting, we may also discuss some of the most interesting generalisations of our approach. 
This is joint work with Scott T. Parsell.​​​​

02.12.2020 - Kevin Buzzard (Imperial College London): Formalising algebra and geometry

Abstract: Formalising mathematics is the process of typing it into a computer proof verification system, which then checks that all proofs follow from the axioms of mathematics. I will spend some time talking about why one might want to do this. I will then attempt to dispel the old-fashioned idea that it takes 100 pages to prove 1+1=2 by showing that it is now possible for mathematics undergraduates to formalise MSc level algebra and geometry in modern systems. I will be using the Lean theorem prover, although other theorem provers are available.

18.11.2020 - Tim Browning (IST Austria): Poor bounds for a rich problem

Abstract: Thanks to Neron heights and the Mordell-Weil theorem we can count rational points of bounded height on elliptic curves. Falting's theorem tells us that there are only finitely many rational points (of any height) on hyperelliptic curves of genus at least 2. Unfortunately, these estimates depend intimately on the individual curve and it is hard to apply them to study the corresponding problem for surfaces that are fibred into elliptic or hyperelliptic curves. In this talk I shall discuss an alternative approach that uses sieves, though the bounds are far from optimal! This is joint work with Dante Bonolis. ​

04.11.2020 - John Christian Ottem (University of Oslo): Enriques surface fibrations of even index

Abstract: I will explain a geometric construction of an Enriques surface fibration over P1 of even index. This answers a question of Colliot-Thèlene and Voisin, and provides new counterexamples to the integral Hodge conjecture. This is joint work with Fumiaki Suzuki. 

28.10.2020 - Scott Ahlgren (University of Illinois, Urbana-Champaign): OBS! 14.30-15.30

Title:  Congruences for the partition function

Abstract:  The arithmetic properties of the ordinary partition function p(n) have been the topic of intensive study for many years. Much of the interest (and the difficulty)  in this problem arises from the fact that values of the partition function are given by coefficients of modular forms of half integral weight.  I’ll briefly discuss  the history of this problem, and focus mostly on some new joint work with Olivia Beckwith and Martin Raum which goes a long way towards explaining exactly when congruences can occur.  The main tools are techniques from the theory of modular forms, Galois representations, and analytic number theory.​

21.10.2020 - Peter Sarnak (Princeton University/Institute for Advanced Study): ​Applications of points on subvarieties of tori

Abstract: The intersection of the division group of a finitely generated subgroup of a torus with an algebraic subvariety has been understood for some time (Lang, Laurent,...). After a brief review of some of the tools in the analysis and their recent extensions (André-Oort conjectures), we give some old and new applications; in particular to the additive structure of the spectra of metric graphs and crystalline measures.

Joint work with P. Kurasov.

07.10.2020 - Jack Shotton (University of Durham)Moduli of local Galois representations and representation theory

Abstract: It is understood that there should be close connections between moduli spaces of representations of local Galois groups and modular representation theory of groups such as GL_n(Z_p).  I will survey what is expected and what is known, and talk about some of my own work in the 'l \neq p' setting.

24.09.2020 - Jan Gerken (Chalmers/GU): Single-valued maps at genus zero and one

Abstract: The single-valued map for for multiple zeta values (MZVs) due to Francis Brown and Oliver Schnetz is an intriguing algebra homomorphism for MZVs. Surprisingly, it appears in the leading contribution to scattering amplitudes in string theory, relating open- and closed-string amplitudes. Since MZVs are periods of configuration spaces of punctured genus-zero Riemann surfaces, a natural next step is the generalization of the single-valued map to genus-one surfaces. In string theory, these correspond to the subleading contributions to the scattering amplitudes. Using the structures provided by string theory, we propose a genus-one generalization of the single-valued map for MZVs which acts on the level of generating functions of genus-one periods.​

09.09.2020 - Lucile Devin (Chalmers/GU): Chebyshev’s bias and sums of two squares
Abstract: Studying the secondary terms of the Prime Number Theorem in Arithmetic Progressions, Chebyshev claimed that there are more prime numbers congruent to 3 modulo 4 than to 1 modulo 4. We will explain and qualify this claim following the framework of Rubinstein and Sarnak. Then we will see how this framework can be adapted to other questions on the distribution of prime numbers. This will be illustrated by a new Chebyshev-like claim :  there are “more” prime numbers that can be written as a sum of two squares with the even square larger than the odd square than the other way around.

11.06.2020 - Jakob Palmkvist (Chalmers/GU): Tensor hierarchy algebras

Abstract: Tensor hierarchy algebras constitute a new class of non-contragredient Lie superalgebras, whose finite-dimensional members are the simple Lie superalgebras of Cartan type in Kac’s classification. They have proven useful in describing gauge structures in physical models related to string theory. I will review their construction by generators and relations and some of the remarkable features they exhibit.

20.05.2020 - Asbjörn Nordentoft (University of Copenhagen): Reciprocity Laws, Quantum Modular Forms and Additive Twists of Modular L-functions


Abstract:  In an unpublished paper from 2007, Conrey discovered certain ‘reciprocity relations’ satisfied by twisted moments of Dirichlet L-functions, linking the arithmetics of the finite fields F_p, F_q for two different primes p,q (as is the case with quadratic reciprocity). In this talk I will discuss a generalization to twisted moments of twisted modular L-functions. This will lead to a discussion of the notion of quantum modular forms due to Zagier, and in particular we will explain that additive twists of modular L-functions define examples of quantum modular forms. 

06.05.2020 - Martin Raum (Chalmers/GU): Divisibilities of class numbers and partition counts

Abstract: Hurwitz class numbers, class numbers of imaginary quadratic fields, and partition counts are among the most classic quantities in number theory, and for each of them their factorizations, i.e. divisibilities, are celebrated open questions. In the case of class numbers the Cohen-Lehnstra Heuristics provides predictions of of statistical nature. In the case of partition counts, Ramanujan congruences opened the door to a whole new research area in~1920.

We survey recent progress on divisibilities of class numbers and partition counts on arithmetic progressions. These result rely on a two new methods exploiting the finer structure of Fourier coefficients of real-analytic and meromorphic modular forms.

The project on class numbers is partially based on joint work with Olivia Beckwith and Olav Richter. The project on partition counts is partially based on joint work with Olivia Beckwith and Scott Ahlgren.​

04.03.2020 - Jasmin Matz (University of Copenhagen): Distribution of Hecke eigenvalues

Abstract: There are many difficult conjectures about automorphic representations, many of which seem to be out of reach at the moment. It has therefore become increasingly popular to study instead families of automorphic representations and their statistical properties, which allows for additional analytic techniques to be used.

In my talk I want to discuss the distribution of Hecke eigenvalues or, in other words, Satake parameters in the family of spherical unramified automorphic representations of split classical groups.  We obtain an effective distribution of the Satake parameters, when we order the family according to the size of analytic conductor. This has applications to various questions in number theory, for example, low-lying zeros in families of automorphic L-functions, but also yields an effective Weyl law for the underlying locally symmetric space. This is joint work with T. Finis.

26.02.2020 - Kirsti Biggs (Chalmers/GU): Efficient congruencing in ellipsephic sets
 Abstract: An ellipsephic set is a subset of the natural numbers whose elements have digital restrictions in some fixed prime base---for example, the set of positive integers whose digits in the given base are squares. Such sets have a fractal structure and can be viewed as p-adic Cantor sets analogous to those studied over the real numbers. The results of this talk can similarly be viewed from either a number theoretic or a harmonic analytic perspective: we bound the number of ellipsephic solutions to a system of diagonal equations, or, alternatively, we obtain discrete restriction estimates for the moment curve over ellipsephic sets. In this talk, I will outline the key ideas from the proof, which uses Wooley's efficient congruencing method, give motivating examples and highlight the importance of the additive structure of our ellipsephic sets.


12.02.2020 - Kevin Hughes (University of Bristol): Discrete restriction to the curve (x,x^3)

Abstract: In this talk I will motivate the problem of discrete restriction to the curve (x,x^3). This is one of the simplest cases outside the recently introduced and powerful machinery of decoupling and efficient congruencing. While the expected 10th decoupling inequality fails, we show that the nigh-optimal discrete restriction estimate holds. This is work with Trevor Wooley.

19.02.2020 - Nils Matthes (University of Oxford): Motivic periods
Abstract: A period is a complex number which can be written as the integral of an algebraic differential form over a semialgebraic set. This is a classical notion whose roots can be traced back at least to Euler and which conjecturally contains all special values of L-functions of algebraic varieties. Beginning in the 1960s it was realized that the study of periods may be viewed as part of Grothendieck's vision of motives which very recently lead to the notion of "motivic period". Although progress has been made, many fundamental questions about (motivic) periods remain.


05.02.2020 - Pankaj Vishe (Durham University): Rational points on complete intersections over global fields

Abstract: The quantitative arithmetic of the set of rational points on a smooth complete intersection of two quadrics over the function field F_q(t) is obtained, under the assumption that q is odd and n9. The main ingredient here is the development of a Kloosterman refinement over global fields.

28.01.2020 - Jonas Bergström (Stockholm University): Traces of Hecke operators on spaces of Siegel modular forms modulo prime powers

Abstract: I will report on ongoing work, where I apply the Lefschetz fixed point theorem to local systems on the moduli space of abelian varieties of dimension at most 3, and use simple equalities in modular arithmetic, to study traces of Hecke operators on spaces of Siegel modular forms (of degree at most 3) modulo prime powers.

15.01.2020 - Dennis Eriksson (Chalmers/GU): Genus one mirror symmetry

Abstract: Mirror symmetry, in a crude formulation, is usually presented as a correspondence between curve counting on a Calabi--Yau variety X, and some invariants extracted from a mirror family of Calabi--Yau varieties. After the physicists Bershadsky--Cecotti--Ooguri--Vafa (henceforth BCOV), this is organised according to the genus of the curves in X we wish to enumerate, and gives rise to an infinite recurrence of differential equations. In this talk, I will give a general introduction to these problems, and present a rigorous mathematical formulation of the BCOV conjecture at genus one, in terms of a lifting of the Grothendieck--Riemann--Roch. I will explain a proof of the conjecture for Calabi--Yau hypersurfaces in projective space, based on the Riemann--Roch theorem in Arakelov geometry. Our results generalise from dimension 3 to arbitrary dimensions previous work of Fang--Lu--Yoshikawa.
This is joint work with G. Freixas and C. Mourougane.

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