Quadratic principal indecomposable modules and strongly real elements of finite Groups

Abstract

Let $P$ be a principal indecomposable module of a finite group $G$ in characteristic $2$ and let $φ$ be the Brauer character of the corresponding simple $G$-module. We show that $P$ affords a non-degenerate $G$-invariant quadratic form if and only if there are involutions $s,t\in G$ such that $st$ has odd order and $φ(st)/2$ is not an algebraic integer. We then show that the number of isomorphism classes of quadratic principal indecomposable $G$-modules is equal to the number of strongly real conjugacy classes of odd order elements of $G$.

14 pages