Title: Contraction par Frobenius et modules de Steinberg,
Abstract: For a reductive group G defined over an algebraically closed field of
positive characteristic, we show that the Frobenius contraction functor of
G-modules is right adjoint to the Frobenius twist of the modules tensored with
the Steinberg module twice. It follows that the Frobenius contraction functor
preserves injectivity, good filtrations, but not semisiplicity.