Entanglement of Modular Forms
Samuele Anni, Aix-Marseille Université

This talk addresses the compelling question of whether two residual Galois representations attached to modular forms, possibly in different residual characteristics, can correspond to the same number field or, more generally, may have common non-trivial subfields. The study of entanglement for torsion representations attached to elliptic curves over the rationals has been already addressed in several papers. This question, linked to the inverse Galois and modular forms, highlights the importance of the study of congruences. Our ongoing research, in collaboration with Luis Dieulefait (Universitat de Barcelona) and Gabor Wiese (Université du Luxembourg), aims to uncover the conditions under which entanglement arises, offering new insights into the intricate relationships between modular forms, arithmetic geometry and representation theory.

On the geometry of period lattices for elliptic curves
Lenny Fukshansky, Claremont McKenna College

Isomorphism classes of elliptic curves correspond to similarity classes of planar lattices, represented by points in the fundamental strip of the upper halfplane. The position of such a point determines certain geometric properties of the corresponding lattice and arithmetic properties of the corresponding elliptic curve. The purpose of this talk is to describe some such properties. We will focus on well-rounded, semistable, virtually rectangular and arithmetic lattices and their corresponding curves. We will also discuss the finite sequence of the so-called deep-hole lattices resulting from any point in the fundamental strip and the corresponding family of elliptic curves and their isogenies, which become especially interesting in the CM/arithmetic case. The talk is based on joint works with Pavel Guerzhoy, Stefan Kuehnlein, Florian Luca and Tanis Nielsen.

Local-global principles for abelian schemes
Wojciech Gajda, Adam Mickiewicz University

I will tell a story which originated almost 30 years ago with a paper by Corrales-Rodriganes and Schoof on an elliptic analogue of the support problem of Erdos. The paper led to investigations of many authors of certain local-global principles for detecting relations in Mordell-Weil groups of abelian varieties. In the final part of my talk I will discuss recent results on related local-global principle for homomorphisms between abelian schemes (joint work with Petersen) which are follow-ups of the paper by Corrales-Rodriganes and Schoof.

Zeta functions and arithmetic geometry
Marc Hindry, Université Paris-Diderot

We will describe several situations where one can identify interesting arithmetic quantities (regulators, heights, cardinality of arithmetic groups, etc.) inside special values of zeta functions and infer asymptotic estimates for the latter via complex analysis. The model for this pattern is the Brauer-Siegel theorem, I will show that there are many other applications of this principle.

Challenges in generating random supersingular elliptic curves over finite fields of large characteristic
Annamaria Iezzi, Université Grenoble Alpes

Constructing supersingular elliptic curves with unknown endomorphism ring over finite fields of large characteristic is an open problem with significant implications for isogeny-based cryptography. In this talk, we will provide an overview of recent attempts to tackle this problem, we will examine their limitations and we will possibly discuss new ideas.

Constructing Class Groups of Imaginary Quadratic Fields with Large n-Rank
Mike Jacobson, University of Calgary

Constructing imaginary quadratic fields whose ideal class groups have large n-rank has proved to be a challenging practical problem, due in part to fact that we believe such examples to be very rare. One of the most successful methods for producing many fields of relatively small discriminant with large 3-rank is due to Diaz y Diaz; this was part of the method used by Quer to find 3 fields with 3-rank equal to 6 in 1987, which still stands as the current record. We describe generalizations to this method for constructing fields with large n-rank for an arbitrary integer n >= 3, and practical enhancements to improve the efficiency of the search procedure. We present some preliminary numerical results, including the first known example of an imaginary quadratic field whose class group has 7-rank equal to 4.

Abelian varieties over F_2 of prescribed order and dimension
Kiran Kedlaya, University of California San Diego

For A a simple abelian variety of dimension g over a finite field F_q, when q > 2 one can give a lower bound on the order of A(F_q) which is exponential in g. By contrast, for q = 2 it has been known since the 1970s that there are infinitely many simple abelian varieties of order 1. We show that for every fixed m > 1, there exist infinitely many simple abelian varieties over F_2 of order m; for certain values of m, we also show that these occur in every sufficiently large dimension.

Modular entanglements of elliptic curves
David Kohel, Aix-Marseille Université

Let N₁ and N₂ be two levels in {3,4,5}. We describe explicit 1-dimensional families parametrizing pairs of elliptic curves with projective A₄-entanglements between the Galois representations on the respective N₁-torsion and N₂-torsion subgroups. Following the work of Silverberg, Rubin and Silverberg, and Fisher, we derive explicit models for the associated modular surfaces parametrizing all such pairs of elliptic curves over a suitable cyclotomic extension of ℚ. This is joint work with Samuele Anni and Zoé Yvon.

Explicit Formula for the Coefficients of the L-Polynomial of the Zeta Function of a Function Field
Mahdi Koutchoukali, Aix-Marseille Université

We establish an explicit formula for the coefficients of the L-polynomial associated with the zeta function of a function field defined over a finite field. This formula is obtained using the parapermanent of a matrix and involves integer compositions. We then adopt a combinatorial approach to study specific properties of these coefficients in the case of defect 2 curves defined over F_2. The talk will conclude with a research direction currently under exploration: the asymptotic existence of non-special divisors of degree g-1 on certain towers of function fields, using this explicit form as a starting point.

Point counting without points
Antoine Leudière, University of Calgary

Drinfeld modules—the function field counterpart to elliptic curves—do not have points, in the traditional sense. But we're going to count them anyway! The first methods achieving this were inspired by classical elliptic curve arithmetics; we will instead explore an algorithm that is based on so-called Anderson motives (joint work with X. Caruso).

Automorphisms of modular curves
Pietro Mercuri, Sapienza Università di Roma

First we recall the definition of modular curve and of modular automorphism of a modular curve. After this, we try to and (partially) successfully answer the question: Is every automorphism of a modular curve a modular automorphism?

Computing supersingular traces
Travis Morrison, Virginia Tech

Let p > 3 be a prime, let E be a supersingular elliptic curve defined over F_{p²} with j(E) ≠ 0, 1728, and let α be an endomorphism of E represented as a sequence of L isogenies of degree at most d. We prove that the trace of α may be computed in time O(n⁴(log n)² + dLn³) bit operations using a generalization of the SEA algorithm for computing the trace of the Frobenius endomorphism of an ordinary curve. When L = O(log p) and d = O(1), this complexity is quasi-quadratic in the size of the input and matches the heuristic complexity of the SEA algorithm. Our theorem is unconditional, unlike the SEA algorithm, since all isogenies of a supersingular elliptic curve are defined over an extension whose degree is bounded by a constant, independent of p. We also give practical speedups, including a fast algorithm to compute the trace of α modulo p, that provide a substantial speedup to the computation of the trace of α in practice.

Structure of Supersingular Elliptic Curve Isogeny Graphs
Renate Scheidler, University of Calgary

Supersingular elliptic curve isogeny graphs have isomorphism classes of supersingular elliptic curves over a finite field as their vertices and isogenies of some fixed degree between them as their edges. Due to their apparent "random" nature, supersingular isogeny graphs – which are optimal expander graphs – have been used as a setting for certain cryptographic schemes that are resistant to attacks by quantum computers. However, recent research, including the work presented here, reveals hidden structures in these graphs that may have implications for the security of supersingular isogeny based cryptosystems. This is joint work with Sarah Arpin (Virginia Tech) and our co-supervised undergraduate student Taha Hedayat (University of Calgary).

The kernel of the Gysin homomorphism for positive characteristic - From algebraic varieties to smooth schemes
Claudia Schoemann, HAWK University Göttingen

Let S be a smooth projective connected surface over an algebraically closed field k embedded into a projective space P^d and let C be a smooth projective curve embedded into S. Let CH0(S)_{deg=0} and CH0(C)_{deg=0} be the Chow groups of zero-cycles of degree 0 on S and C, respectively. The kernel of the Gysin homomorphism from CH0(C)_{deg=0} to CH0(S)_{deg=0} is a countable union of translates of an abelian subvariety A inside the Jacobian J of the curve C. Further there is a countable open subset U₀ contained in the subset U of (P^d)^* parametrizing the smooth curves such that A=B or A=0 for all curves parametrized by U₀, where B is the abelian subvariety of J corresponding to the vanishing cohomology H¹(C, k')_{van} of C. Hence the kernel of the Gysin homomorphism either is a countable union of shifts of B or, if A=0, it is countable. Passing to finite type smooth schemes as a generalisation of algebraic varieties we extend this result to the subset U of (P^d)^*, i.e. to all smooth curves C. Using the language of algebraic stacks this is done by constructing an increasing filtration of Zariski countable open substacks U_i of U for i in a countable set I, that respects the required isomorphism between geometric generic and special point and by applying a convergence argument. This is joint work with Rina Paucar (IMCA/UNI, Lima in Peru).

Results in the theory of linear recurrences
László Szalay, University of Sopron

The theory of linear recurrences often plays an important role in solving combinatorial and number theoretical problems. On the other hand, there exist challenging questions associated to the theory itself. In the talk, we focus on linear vector recurrences of certain types. Consider the vector sequence v_n = A_1 v_{n-1} + ... + A_s v_{n-s} where n ≥ s, v_i is a column vector with k components and A_i is a k×k complex matrix. For example, if s=1 and k=2, then the system x_n = a x_{n-1} + b y_{n-1}, y_n = c x_{n-1} + d y_{n-1} is equivalent to a particular case of the vector sequence. The main question is how to separate the mixed component sequences. The general results on the vector sequence makes it possible to handle the problem of so-called k-periodic linear recurrences.

Speeding up the computation of group order of genus 2 curves over finite fields
Nicolas Thériault, Universidad de Santiago de Chile

In 2012, Gaudry and Schost obtained the first known randomly selected cryptographically interesting hyperelliptic curve of genus 2 over a field of 127 bits. Producing this curve required a very significant computation, in the order of one million CPU-hours, in which computing the order of a specific curve (after a comparatively fast selection process) would take on average close to 1000 CPU-hours. Furthermore, the curves considered had full 2-torsion groups, i.e. the curve had group order 16*prime. We present new developments in the computation of the group order of specific curves which allow us to significantly reduce the cost of several of the sub-algorithms used by Gaudry and Schost, specifically: improved division-by-L algorithms, specialized factorization algorithms and the use of special towers of field extensions, which allow us to compute the group order of each hyperelliptic curve in less than 100 CPU-hour. Furthermore, our algorithm applies to more general curves and we are able to obtain examples of curves with prime order (giving slightly stronger curves) over fields of 127 bits.

Lagrange’s theorem for certain finite flat group schemes
Emiliano Torti, University of French Polynesia

Around 1960s, Grothendieck asked if every finite flat group scheme over any base is killed by its order, i.e. the morphism multiplication-by-#G factors via the identity section. In this talk, we discuss the state of the art of the question as well as giving a positive answer in a new case when the special fiber of G belongs to a certain family of non-commutative finite flat group scheme. This extends partially the current best known result due to Schoof.

The first level of ℤₚ-extensions and compatibility of heuristics
Lawrence C. Washington, University of Maryland

Let K be an imaginary quadratic field in which the odd prime p does not split. When the p-part of the class group of K is cyclic, we describe the possible structures for the p-part of the class group of the first level of the cyclotomic ℤₚ-extension of K. This allows us to show the compatibility of the heuristics of Cohen--Lenstra--Martinet for class groups with the heuristics of Ellenberg--Jain--Venkatesh for how often the cyclotomic Iwasawa invariant lambda equals 1. This is joint work with Debanjana Kundu.

Rigorous methods in computational algebraic number theory
Benjamin Wesolowski, École normale supérieure de Lyon

Many number theoretic algorithms resort to heuristic assumptions for their analysis. This issue concerns some of the most fundamental problems of the field, such as the computation of class groups in subexponential time. In this talk, we will present new methods for the design of algorithms which, instead of ad hoc heuristics, only rely on the Extended Riemann Hypothesis.

When cluster algebras imply finiteness or infinitude of positive integral points
Robin Zhang, Massachusetts Institute of Technology

General finiteness results for positive integral solutions to Diophantine equations are rare. The theory of cluster algebras is a remarkable source of finiteness & infinitude for positive integral points in Diophantine geometry. On the finiteness of positive integral points, I will highlight 7-dimensional and 8-dimensional affine varieties with infinitely many integral points that arise in the proof of the Fontaine–Plamondon conjecture for Dynkin friezes. On the infinitude of positive integral points, I will highlight surfaces and threefolds that arise in the Mordell–Schinzel program.