Motivic decompositions of projective homogeneous varieties and change of coefficients

Abstract

We prove that under some assumptions on an algebraic group $G$, indecomposable direct summands of the motive of a projective $G$-homogeneous variety with coefficients in ${\mathbb{F}}_p$ remain indecomposable if the ring of coefficients is any field of characteristic $p$. In particular for any projective $G$-homogeneous variety $X$, the decomposition of the motive of $X$ in a direct sum of indecomposable motives with coefficients in any finite field of characteristic $p$ corresponds to the decomposition of the motive of $X$ with coefficients in ${\mathbb{F}}_p$. We also construct a counterexample to this result in the case where $G$ is arbitrary.