Asymptotic results for Representation Theory

Dario De Stavola

Published 2018-05-10Version 1

Representation theory of finite groups portrays a marvelous crossroad of group theory, algebraic combinatorics, and probability. In particular the Plancherel measure is a probability that arises naturally from representation theory, and in this thesis we consider three ramifications of asymptotic questions for random Plancherel distributed representations. First we recall irreducible characters of the symmetric group, which are indexed by integer partitions. We focus on the so called 'dual approach', in which the partition indexing a character is now considered to be the variable. We extend a famous result of Kerov on the asymptotic of Plancherel distributed characters by studying partial trace and partial sum of a representation matrix. We decompose each of these objects into a main term and a remainder, and in each case we prove a central limit theorem for the main term. We apply these results to prove a law of large numbers for the partial sum. In the second part we consider projective representations of the symmetric group by converting a multirectangular approach developed by Stanley for the classical case. In particular, we present a positivity conjecture for the coefficients of the renormalized projective character, written as a polynomial in a new set of coordinates. We prove the positivity for the leading term of the polynomial. In the last part of the thesis we recall the theory of supercharacters, which is an alternative tool to representation theory when the classical approach proves to be intractable. We show that a generalization of the Plancherel measure, called the superplancherel measure, arises naturally. We focus on a particular supercharacter theory for the upper unitriangular group in which supercharacters are indexed by set partitions. We prove a limit shape result for a random set partition according to this distribution.