Nonzero coefficients in restrictions and tensor products of supercharacters of $U_n(q)$

Details Nonzero coefficients in restrictions and tensor products of supercharacters of $U_n(q)$ Nonzero coefficients in restrictions and tensor products of supercharacters of $U_n(q)$

2009-12-09

arxiv.org/abs/0912.1880v1

Mathematics

Combinatorics

28 pages

Scientific paper

The standard supercharacter theory of the finite unipotent upper-triangular matrices $U_n(q)$ gives rise to a beautiful combinatorics based on set partitions. As with the representation theory of the symmetric group, embeddings of $U_m(q)\subseteq U_n(q)$ for $m\leq n$ lead to branching rules. Diaconis and Isaacs established that the restriction of a supercharacter of $U_n(q)$ is a nonnegative integer linear combination of supercharacters of $U_m(q)$ (in fact, it is polynomial in $q$). In a first step towards understanding the combinatorics of coefficients in the branching rules of the supercharacters of $U_n(q)$, this paper characterizes when a given coefficient is nonzero in the restriction of a supercharacter and the tensor product of two supercharacters. These conditions are given uniformly in terms of complete matchings in bipartite graphs.

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