Non-Abelian p-Curvature and a Non-Abelian Katz's Formula

Abstract: Let $k$ be a field of characteristic $p,$ and $f : X \to S$ a smooth proper morphism of smooth $k$-schemes. Katz's formula gives a relationship between the Kodaira--Spencer map of $f,$ and an invariant called the $p$-curvature of the Gauss--Manin connection associated to $f.$ Recently, Lam--Litt proved a variant of Katz's formula in non-abelian Hodge theory, and suggested that it should be possible to give a more conceptual proof of their formula using the stacky approach to $p$-adic Hodge theory. In this article, we realize their suggestion, explaining how the rather concrete phenomena observed by Katz and Lam--Litt can be explained in a conceptual way using sheared de Rham stacks, as developed by Bhatt--Kanaev--Vologodsky--Zhang and Drinfeld (though we prove a slightly different statement than Lam--Litt do). We do not assume the reader has any background in the theory of de Rham stacks.