## M. B. Porter Lectures, 2006-2007.

**Rice University**

Department of Mathematics

# Alexei Borodin

Caltech

### March 12 - March 21, 2007

## Representations of symmetric groups and determinantal point processes

The infinite symmetric group S(\infty) is a big group in the sense
that the space of its irreducible characters is infinite-dimensional. The
description of this space is a classical result known as Thoma's theorem
(1964). It is (nontrivially) equivalent to another well-known theorem --
classification of totally positive sequences due to Edrei and Schoenberg
(1953). The main goal of the first two lectures will be to explain these
theorems as well as ideas behind one of the proofs.

The regular representation of S(\infty) is irreducible
as opposed to the case of finite groups when the decomposition of the
regular representation yields all irreducible ones. In the third lecture
we will provide a geometric construction of a deformation of the regular
representation of S(\infty) which makes the problem of harmonic analysis
(=decomposition on irreducibles) meaningful.

In the last lecture we will solve the problem of harmonic analysis for
the generalized regular representations of S(\infty) using tools from
probability theory (determinantal point processes). Connections with
random matrices will be emphasized.

Monday, March 12

4:00 - 5:00 PM Keck Lecture Hall 100

Lecture 1. **Discrete probabilistic models of random matrix type**
In recent years there has been a lot of progress in understanding the
objects like
increasing subsequences of random permutations, random directed polymers,
various models
of vicious walkers, random tilings etc. that miraculously exhibit similar
behavior as they become
``large''. The goal of the talk is to give a survey of such objects with
emphasis on unexpected connections between various domains of mathematics
and mathematical physics.

Wednesday, March 14

4:00 - 5:00 PM Herman Brown 227

Lecture 2. **Thoma's theorem I**

Thursday, March 15

4:00 - 5:00 PM Herman Brown 227

Lecture 3. **Thoma's theorem II**

Monday, March 19

4:00 - 5:00 PM Herman Brown 227

Lecture 4. **Generalized regular representations of the infinite symmetric group **

Wednesday, March 21

4:00 - 5:00 PM Herman Brown 227

Lecture 5. **Harmonic analysis on S(\infty) and determinantal point
processes**

There will be a tea preceding each lecture in the Mathematics Commons Room, Herman Brown 438.

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