The goal of this lecture course is to prove the universal optimality of the E8 and Leech lattices.

This theorem is the main result of a recent preprint « Universal optimality of the E8 and Leech lattices and interpolation formulas » written in collaboration with Henry Cohn, Abhinav Kumar, Stephen D. Miller and Danylo Radchenko. We prove that E8 and Leech lattices minimize energy of every potential function that is a completely monotonic function of squared distance (for example, inverse power laws of Gaussians).

This theorem implies recently proven optimality of E8 and Leech lattices as sphere packings and broadly generalizes it to long-range interactions. The key ingredient of the proof is sharp linear programming bounds. To construct the optimal auxiliary functions attaining these bounds, we prove a new interpolation theorem.

At the last lecture, we will discuss open questions and conjectures which arose from our work.

In the talk I will present a mathematically challenging and difficult problem of the (non)existence of compact Clifford-Klein forms of homogeneous spaces G/H. These are quotients of such spaces by  discrete subgroups of G acting freely, properly and co-compactly. I will formulate the challenging Toshiyuki Kobayashi conjecture and present several partial results supporting it. The results basically are negative in the sense that I will prove the non-existence of compact Clifford-Klein forms for large families of homogeneous spaces, and the non-existence of standard compact Clifford-Klein forms for all homogeneous spaces of exceptional simple real Lie groups. The methods are purely Lie-theoretical. The approach is quite computational: after expressing the problem as some conditions on Lie subalgebras, we develop algorithms checking known obstructions to the existence of compact Clifford-Klein forms. Algorithms are implemented in the computer algebra system GAP and use classifying algorithms of semisimple Lie subalgebras developed by Willem De Graaf. We use the works of Yosuke Morita and Nicolas Tholozan. The talk is based on my joint work with Maciej Bochenski and Piotr Jastrzebski.

The goal of this lecture course is to prove the universal optimality of the E8 and Leech lattices.

This theorem is the main result of a recent preprint « Universal optimality of the E8 and Leech lattices and interpolation formulas » written in collaboration with Henry Cohn, Abhinav Kumar, Stephen D. Miller and Danylo Radchenko. We prove that E8 and Leech lattices minimize energy of every potential function that is a completely monotonic function of squared distance (for example, inverse power laws of Gaussians).

This theorem implies recently proven optimality of E8 and Leech lattices as sphere packings and broadly generalizes it to long-range interactions. The key ingredient of the proof is sharp linear programming bounds. To construct the optimal auxiliary functions attaining these bounds, we prove a new interpolation theorem.

At the last lecture, we will discuss open questions and conjectures which arose from our work.

It is well known that any smooth projective toric surface has a full exceptional collection. I will talk about a generalization of this fact for singular surfaces. First, if the class group of Weil divisors of the surface is torsion free (for instance, this holds for all weighted projective planes), I will construct a semiorthogonal decomposition of the derived category with components equivalent to derived categories of modules over certain local finite dimensional algebras. When the class group has torsion, a similar semiorthogonal decomposition will be constructed for an appropriately twisted derived category. Many of these results extend to non-necessarily toric rational surfaces. This is a joint work with Joseph Karmazyn and Evgeny Shinder.

We shall review how supergravity theories can emerge from an exceptional field theory based on the Kac-Moody group E11 (i.e. E8+++) with gauge symmetry a set of `generalised diffeomorphisms’ acting on the fundamental module while preserving E11. The construction relies on a super-algebra T  that extends E11 and provides a differential complex for the exceptional fields. A twisted self-duality equation underlying the dynamics can be shown to be invariant under generalised diffeomorphisms provided a certain algebraic identity holds for structure coefficients of the super-algebra T. The fermions of the theory belong to an unfaithful representation of the double cover of a maximal Lorentzian subgroup K(E11). We conjecture that certain tensor products of unfaithful representations are homomorphic to the quotient of specific indecomposable modules of E11. Using these conjectures, we can write a linearised Rarita-Schwinger equation and show that the E11 twisted self-duality equations are supercovariant. The conjectures are checked through computations in level decompositions with respect to maximal parabolic subgroups.

Many molecules in living cells are present in such low numbers that individual probabilistic chemical events can have a great randomizing effect on the whole system. I will describe how this radically changes the dynamics of several core cellular processes, from cell fate decisions to the oscillators and DNA repair. The presentation will convey some analytical results for classes of stochatic processes, with an emphasis on unsolved problems, but also experimental results, aimed at a board audience, to illustrate what is known about these processes.

Infinite-dimensional conformal symmetry in two dimensions renders conformal field theories integrable with an infinite hierarchy of quantum KdV charges being in involution. These charges govern the structure of Virasoro descendant states and provide correct formulation for the Eigenstate Thermalization in 2d theories. After covering recent results on Eigenstate Thermalization, I will talk about an ongoing progress of calculating the spectrum of quantum KdV charges and generalized partition function of two dimensional theories in the limit of large central charge. The talk is based on https://arxiv.org/abs/1903.03559 as well as https://arxiv.org/abs/1812.05108 and https://arxiv.org/abs/1810.11025

A brief survey will be given of the use of KP and 2D-Toda tau functions  of special “hypergeometric type” as generating functions for weighted Hurwitz numbers (i.e. weighted enumerations of N-sheeted branched coverings of the Riemann sphere,  or equivalently,  weighted paths in the Cayley graph of the symmetric group S_N generated  by transpositions). The weights depend on parametric families of auxiliary parameters, and consist of evaluations of basis elements of the algebra of symmetric functions of the latter.   An alternative generating function is provided by certain correlation  functions W_{n,g}(x_1,. …, x_n) depending on a pair of integers that play a role analogous  to the multidifferentials in the Topological Recursion approach to intersection theory on moduli spaced of marked Riemann surfaces. As in that case, an associated  invariant classical and quantum “spectral curve” is derived and a set of recursion relations that determine the general term quadratically in terms of finite sums over preceding ones.

 

Examples include: 1) the “simple” (double or single) Hurwitz numbers studied originally by Okounkov and Pandharipande, 2) The case of "Belyi curves”, having just three branch points, one of them weighted, and the related “dessins d’enfants”; 3) The “weakly monotonic” paths in the Cayley graph, for which the generating tau function is the Itzykson-Zuber-Harish-Chandra integral and (if time permits) 4) The case of simple "quantum Hurwitz numbers", in which the weighting is shown to coincide with that of a quantum Bose-Einstein gas.  (Partly based on joint work with M. Guay-Paquet, A. Orlov, B. Eynard, A. Alexandrov and G. Chapuy)

One can formulate the quantum theory taking as the starting point the cone of states. The probabilities can be derived from the first principles in this approach. The formulation in terms of states is useful, in particular, in statistical physics. It leads to the notion of inclusive scattering matrix related to inclusive cross-sections. This notion can be applied to the scattering of quasiparticles, where the usual notion of scattering matrix does not make sense.

 

 

Let G be a finitely generated group endowed with some probability measure μ and $(rho_{lambda})$ be a non-compact algebraic family of representations of G into SL(2,C). This gives rise to a random product of matrices depending on the parameter λ, so the upper Lyapunov exponent defines a function on the parameter space. Using techniques from non-Archimedean analysis and algebraic geometry, we study the asymptotics of the Lyapunov exponent when λ goes to infinity. This is joint work with Charles Favre.