Published online by Cambridge University Press: 25 September 2019
Let $k$ be a perfect field of characteristic
$p>0$ and let
$\operatorname{W}$ be the ring of Witt vectors of
$k$. In this article, we give a new proof of the Frobenius descent for convergent isocrystals on a variety over
$k$ relative to
$\operatorname{W}$. This proof allows us to deduce an analogue of the de Rham complexes comparison theorem of Berthelot [
$\mathscr{D}$-modules arithmétiques. II. Descente par Frobenius, Mém. Soc. Math. Fr. (N.S.) 81 (2000)] without assuming a lifting of the Frobenius morphism. As an application, we prove a version of Berthelot’s conjecture on the preservation of convergent isocrystals under the higher direct image by a smooth proper morphism of
$k$-varieties.