I will discuss the relationship between the Kuznetsov formula and certain exponential sums that arise naturally on GL(3). This will lead us to consider the structure of GL(3) Maass forms having non-trivial dependence on the SO(3) part of the Iwasawa decomposition.

Starting from the Poisson summation formula, I discuss spectral summation formulae on GL(2) and GL(3) and present a variety of applications to automorphic forms, analytic number theory, and arithmetic.

In the late 80’s Henniart used the then recently introduced Laumon l-adic local Fourier transform to prove the numerical local Langlands correspondence for GL(n). More recently, Bloch-Esnault and independently Lopez have developed a complex version of this transform. I will explain the fascinating picture that emerges when Henniart’s strategy of proof is translated to this setting of local geometric Langlands parameters.

Shnirelman discovered in the 1970s that the eigenfunctions of the Laplacian on a compact Riemannian manifold whose flow is ergodic with respect to Liouville measure exhibit an analogue of classical ergodicity at the quantum level. This phenomenon became known as "Quantum Ergodicity" and Schnirelman's proof was completed by Zelditch and Colin de Verdiere in the 1980s. Following a brief introduction to the subject, I will show that Quantum Ergodicity can also hold in systems which are essentially integrable, provided they have some arithmetic structure. Finally, in the absence of such an arithmetic structure, a very different phenomenon occurs: scarring. This talk is based on joint work with Par Kurlberg, KTH Stockholm.

A page in Ramanujan's lost notebook contains two identities for trigonometric sums in terms of doubly infinite series of Bessel functions. One is related to the famous ``circle problem'' and the other to the equally famous ``divisor problem.'' We discuss these classical unsolved problems. Each identity can be interpreted in three distinct ways. We discuss various methods that have been devised to prove the identities under these different interpretations. Weighted divisor sums naturally arise, and new methods for estimating trigonometric sums need to be developed. Trigonometric analogues and extensions of Ramanujan's identities to Riesz and logarithmic sums are discussed. The research to be described is joint work with Sun Kim and Alexandru Zaharescu.

For a non-degenerate integral quadratic form F(x1,...,xd) in 5 (or more) variables, we prove an optimal strong approximation theorem. Fix any compact subspace Ω⊂Rd of the affine quadric F(x1,...,xd)=1. Suppose that we are given a small ball B of radius 0 < r < 1 inside Ω, and an integer m. Further assume that N is a given integer which satisfies N ≫ (r−1m)4+ε for any ε > 0. Finally assume that we are given an integral vector (λ1, . . . , λd) mod m. Then we show that there exists an integral solution x = (x1, . . . , xd) x of F(x)=N such that xi ≡λi mod m and √N ∈B, provided that all the local conditions are satisfied. We also show that 4 is the best possible exponent. Moreover, for a non-degenerate integral quadratic form F (x1 , . . . , x4 ) in 4 variables we prove the same result if N ≥ (r−1m)6+ε and N is not divisible by 2k where 2k ≫ Nε for any ε. Based on some numerical experiments on the diameter of LPS Ramanujan graphs, we conjecture that the optimal strong approximation theorem holds for any quadratic form F(X) in 4 variables with the optimal exponent 4.

This is joint work with Julee Kim and Nicolas Templier. Irreducible smooth representations of a p-adic reductive group are said to be supercuspidal if they do not appear in any induced representation from a proper parabolic subgroup. While it is still an open problem to obtain a precise character formula for them (apart from some special cases), I will explain that we can prove a reasonable upper bound and a limit formula as the formal degree tends to infinity, for a large class of supercuspidal representations. An expected application is an equidistribution result as well as a low-lying zero statistics for L-functions in a new kind of families of automorphic representations.

This talk will introduce a very natural and interesting differential complex associated with a CAT(0)-cube complex. The construction builds on ideas first introduced by Pytlik and Szwarc for the free group and extended by Julg and Valette in the case of groups acting on trees. We will extend ideas of Julg-Valette to show how this construction can be used to study K-amenability and K-homology of groups acting on CAT(0)-cube complexes. This talk is based on joint work with Erik Guentner and Nigel Higson.

We give an explicit presentation for the ring of modular forms for a Fuchsian group with cofinite area, depending on the signature of the group. Our work can be seen as a generalization of the classical theorem of Petri: we give a presentation for the canonical ring of a stacky curve. This is joint work with David Zureick-Brown.

Classically the analytic properties of L-functions, in particular the functional equation, have been related to summation formulae of Poisson type. On the other hand, analytic properties of automorphic L-functions could be used to deduce functorial lifting of automorphic forms to general linear groups via the converse theorem. In his recent work, L. Lafforgue showed that a conjectural nonlinear Poisson summation formula on reductive groups is equivalent to the existence of functorial liftings to general linear groups. Here the Fourier transform is on a nonlinear space and involves nonstandard test functions. In my talk I will explain a toy model of such a summation formula for an integral transform between nonstandard spaces of test functions. The integral transform in question is the Harish-Chandra transform operating on the space of orbital integrals, and the summation formula follows from a trace formula on Lie algebras.

In this talk we use generalizations of classical geometric constructions of Kummer and Humbert to construct new higher Chow cycles on Abelian surfaces and K3 surfaces over p-adic local fields, generalising some work of Collino. The existence of these cycles is predicted by the poles of the local L-factor at p of the L-function of the Abelian surface. The techniques involve using some recent work of Bogomolov, Hassett and Tschinkel on the deformations of rational curves on K3 surfaces. As an application we use these cycles to prove an analogue of the Hodge-D-conjecture for Abelian surfaces.

Waves in free-space diffractively spread, while waves in a spatially non-homogeneous medium undergo a combination of scattering and localization.
In many applications, e.g. photonic and quantum systems, one is interested in controlled localization of wave energy. Edge states are a type of localization along a line-defect, the interface between different media.
Topologically protected edge states are a class of edge states which are robust to strong local distortions of the edge. They are therefore potential vehicles for robust energy-transfer in the presence of defects and random imperfections. These states arise, for example, in graphene and its photonic analogues.
We first review the mathematics of dispersive waves in periodic media and discuss examples of wave localization by a defect.
We then specialize to the case of honeycomb structures (such as grapheme) and discuss their novel properties. Finally we introduce and discuss a rich family of continuum partial differential equation (Schroedinger) models, admitting edge states which are topologically protected and those which are not.

We will discuss the notion of Artin root numbers attached to an n-dimensional continuous Frobenius-semisimple complex representation of the Weil-Deligne group and show their equalities with those defined by Langlands-Shahidi method through local Langlands correspondence for GL(n) and the exterior and symmetric square representation of the L-group GL(n,C) of GL(n). The proof is a robust deformation argument using local-global techniques, complemented with suitable asymptotic expansions for partial Bessel functions inspired by certain generalized Shalika germ expansions of Jacquet and Ye. This is a joint work with J. Cogdell and T.-L. Tsai.

Higher theta functions are the residues of Eisenstein series on covers of the adelic points of classical groups. On the one hand, they generalize the Jacobi theta function. On the other, their Whittaker-Fourier coefficients are not understood, even for covers of $GL_2$. In this talk I explain how, using methods of descent, one may establish a series of relations between the coefficients of theta functions on different groups. In the first instance, this allows us to prove a version of Patterson's famous conjecture relating the Fourier coefficient of the biquadratic theta function to quartic Gauss sums. This is based on joint work with David Ginzburg.

We will begin by formulating the Riemann-Roch theorem for graphs due to the speaker and Norine. We will then describe some refinements and applications. Refinements include a Riemann-Roch theorem for tropical curves, proved by Gathmann-Kerber and Mikhalkin-Zharkov, and a Riemann-Roch theorem for metrized complexes of curves, proved by Amini and the speaker. Applications (mainly due to other researchers) include new proofs of some important results in Brill-Noether theory, a generalization of the Eisenbud-Harris theory of limit linear series, and new bounds for the number of rational points on algebraic curves over number fields.

Recently, Jiang made a conjecture investigating the connection between two fundamental invariants of an automorphic representation \pi appearing in the discrete spectrum of quasi-split classical groups G(A). The first invariant is the wave front of \pi, WF(\pi), which is the set of maximal unipotent orbits of G, such that \pi admits a non-trivial Fourier coefficients with respect to them. The second invariant is the Arthur parameter \psi of \pi to which one can associate a unipotent orbit \underline{p}(\psi) of the dual group of G. The conjecture says that in any Arthur packet associated to \psi, the Barbasch-Vogan duality of the orbit \underline{p}(\psi) is a sharp upper bound for the wave front of the representations of the packet. This is an important conjecture that vastly generalizes Shahidi's conjecture which claims that in every tempered packet there exists a generic representation. In this talk, I will review this conjecture and present some recent progress towards it. This is a joint work in progress with Dihua Jiang.

The prime numbers are the multiplicative building blocks of the integers, and much thought has been given towards understanding their behavior. In this talk, we will examine prime numbers from two points of view. We will first consider questions on the distribution of the primes. In particular, we will illustrate how the breakthrough work of Maynard and Tao on bounded gaps between primes settles an old problem of Erdos and Turan. Secondly, we will explore the relationship between prime numbers and zeros of the Riemann zeta-function, as a way to motivate the study of the moments of the Riemann zeta function and more general L-functions. In particular, we consider arbitrary products of L-functions attached to irreducible cuspidal automorphic representations of GL(m) over the rationals. The Langlands program suggests essentially all L-functions are of this form. Assuming some standard conjectures, I will discuss how to estimate two types of moments: the continuous moment of an arbitrary product of primitive automorphic L-functions and the discrete moment (taken over fundamental discriminants) of an arbitrary product of primitive automorphic L-functions twisted by quadratic Dirichlet characters.

We discuss special value formulas for a p-adic L-function L_p(f, \chi), where f is a fixed newform and \chi varies over the space of Hecke characters of a fixed imaginary quadratic field, as well as some recent applications. These formulas, first found by Bertolini, Darmon, and Prasanna, relate L_p(f, \mathbb{1}), a value which is outside the range of interpolation defining L_p, to arithmetic invariants of cycles on varieties fibered over modular curves.

Inter-universal Teichmuller theory, as developed by Mochizuki in the past decade, is an analogue for number fields of the classical Teichmuller theory, and also of the p-adic Teichmuller theory of Mochizuki. In this theory, the ring structure of a number field is subject to non-ring theoretic deformation. Absolute anabelian geometry, a refinement of anabelian geometry, plays a crucial role in inter-universal Teichmuller theory. In this talk, we will try to give an introduction to these ideas.

Hyperbolic 3-manifolds have been studied intensely by topologists since the mid-1970's. When the fundamental group arises from a certain number theoretic construction (in this case, the manifold is called "arithmetic"), the manifold acquires extra features that lead to important connections with number theory. Accordingly, arithmetic hyperbolic 3-manifolds have been studied by number theorists (perhaps not as intensely as the topologists) with different motivations. Very recently, number theorists have started to study the torsion in the homology of arithmetic hyperbolic 3-manifolds. The aim of the first half of this introductory talk, where we will touch upon notions like "arithmeticity", "Hecke operators", will be to illustrate the importance of torsion from the perspective of number theory. In the second half, I will present new joint work with N.Bergeron and A.Venkatesh which relates the topological complexity of homology cycles to the asymptotic growth of torsion in the homology. I will especially focus on the interesting use of the celebrated "Cheeger-Mueller Theorem" from global analysis.

We will formulate a conjectural identity relating the Fourier--Jacobi periods on unitary groups and the central value of certain Rankin--Selberg $L$-functions. This refines the famous Gan--Gross--Prasad conjecture. We will give some examples supporting this conjecture.

If $K$ is the rational function field $K=\mathbb{Q}(t)$, then a polynomial $f$ in $K[x]$ can be regarded as a one-parameter family of polynomials over $\mathbb{Q}$. If $f$ is irreducible, then a basic form of Hilbert's irreducibility theorem states that there are infinitely many $t$ in $\mathbb{Q}$ for which the specialized polynomial $f_t$ is irreducible over $\mathbb{Q}$. In this talk we will discuss analogous theorems for an abelian variety $A/K$ regarded as a one-parameter family of abelian varieties over $K$. For example, we will exhibit $A$ which are simple over $K$ and for which there are only finitely many $t$ in $\mathbb{Q}$ such that the abelian variety $A_t$ is not simple over $\mathbb{Q}$.

So far, the circle method has been a very useful tool to prove many cases of Manin's conjecture on the number of rational points of bounded anticanonical height on Fano varieties. Work of B. Birch back in 1962 establishes this for smooth complete intersections in projective space as soon as the number of variables is large enough depending on the degree and number of equations. In this talk we are interested in subvarieties of biprojective space. There is not much known so far, unless the underlying polynomials are of bidegree (1,1) or (1,2). In this talk we present recent work which combines the circle method with the generalised hyperbola method developed by V. Blomer and J. Bruedern. This allows us to verify Manin's conjecture for certain smooth hypersurfaces in biprojective space of general bidegree.

We will discuss the conjecture of Kolyvagin on the indivisibility of Heegner points and its role in constructing rational points on elliptic curves over rational numbers, particularly in the proof of a recent result of this type: "the Selmer rank being one implies that the Mordell--Weil rank being one".

Rankin-Selberg convolution L-functions are important functions in number theory. Their properties play a central role in many of deepest works on the Ramanujan-Petersson Conjecture. In a recent paper, Hoffstein and Hulse defined generalizations of these L-functions, the so-called "shifted-convolution" L-functions. They obtained the meromorphic continuation of the functions in many cases. Here we consider symmetrizations of these L-functions, and we exactly evaluate their special values at diagonal weights for all shifts. This is joint work with Michael Mertens.

I will begin with an introduction to Galois deformation theory and its role in modularity lifting. This will motivate the study of local deformation rings and more specifically flat deformation rings. I will then discuss Kisin's work on flat deformations and explain how to generalize to Galois representations valued in an arbitrary reductive group. Kisin's techniques led to the successful determination of the connected components of the flat deformation ring in the 2-dimensional case. If time permits, I will touch on difficulties of going beyond GL_2.

(joint w/ B.Bakker) For an elliptic curve E over a field k, the p-torsion E[p] gives a 2-dimensional representation of the Galois group G_k over F_p. For k=Q and p>13, the Frey-Mazur conjecture famously states that one can recover the isogeny class of E from the representaiton E[p]. We state and prove a direct analogue of this question over function fields of complex algebraic curves. Specifically, for any complex algebraic curve C, let k(C) be its field of rational functions. Then there exists a constant A(C), such that for all primes p>A(C), isogeny classes of elliptic curves E over k(C) can be recovered from E[p]. Moreover, we show that A(C) can be made to depend only on the gonality of C, which can be thought of as the analogous notion of degree for number fields. The study of this question will lead us into the realm of moduli spaces and hyperbolic geometry. The use of the latter means that, unfortunately, these methods don't apply in finite characteristic.

Let M be a compact Riemannian manifold, and f an L^2 normalised Laplace eigenfunction on M. A popular question in semiclassical analysis is how well one can bound the other L^p norms of f, or its restriction to a submanifold. I will give an introduction to this problem, and describe how one can make progress on it using the additional assumptions that M is arithmetic and f is a Hecke-Maass form.

Fourier coefficients of automorphic forms are invariants which encode the analytic and arithmetic properties of automorphic forms. In this talk, we introduce the general notion of Fourier coefficients for automorphic representations of reductive groups and use them to construct explicit endoscopy correspondences, which construct via integral transforms with automorphic kernel functions members in global Arthur packets for classical groups. For instance, we will discuss with some details the recent work joint with Lei Zhang.