Existence of an invariant form under a linear map

Abstract

Let $\F$ be a field of characteristic different from $2$ and $\V$ be a vector space over $\F$. Let $J: \alpha \to \alpha^J$ be a fixed involutory automorphism on $\F$. In this paper we answer the following question: given an invertible linear map $T: \V \to \V$, when does the vector space $\V$ admit a $T$-invariant non-degenerate $J$-hermitian, resp. $J$-skew-hermitian, form?