On complete reducibility of tensor products of simple modules over simple algebraic groups

Abstract: Let $G$ be a simply connected simple algebraic group over an algebraically closed field $k$ of characteristic $p>0$. The category of rational $G$-modules is not semisimple. We consider the question of when the tensor product of two simple $G$-modules $L(\lambda)$ and $L(\mu)$ is completely reducible. Using some technical results about weakly maximal vectors (i.e. maximal vectors for the action of the Frobenius kernel $G_1$ of $G$) in tensor products, we obtain a reduction to the case where the highest weights $\lambda$ and $\mu$ are $p$-restricted. In this case, we also prove that $L(\lambda)\otimes L(\mu)$ is completely reducible as a $G$-module if and only if $L(\lambda)\otimes L(\mu)$ is completely reducible as a $G_1$-module.