Branching Rules for Specht Modules

Abstract

Let n be a positive integer and let Sigma_n be the symmetric group of degree n. Let S^lambda be the Specht module for Sigma_n corresponding to a partition lambda of n, defined over a field F of odd characteristic. We find the indecomposable components of the restriction of S^lambda to Sigma_{n-1}, and of the induction of S^lambda to Sigma_{n+1}. Namely, if b and B are block idempotents of FSigma_{n-1} and FSigma_{n+1} respectively, then the modules S^lambda b and S^lambda B are 0 or indecomposable. We give examples to show that the assumption that F has odd characteristic cannot be dropped.