Relative $(\varphi, Γ)$-modules and $p$-adic differential equations

Abstract: Let $X$ be an affinoid rigid analytic space over a $p$-adic field, equipped with a suitable étale map to a unit polydisk. We provide a formalism of \emph{imperfect relative period rings} over $X$, which lie inside the corresponding perfect relative period rings constructed by Kedlaya--Liu. Then we establish a relative version of the Fontaine--Cherbonnier--Colmez equivalence between $p$-adic local systems on $X$ and étale $(\varphi, Γ)$-modules over such imperfect relative period rings, generalizing previous works of Andreatta--Brinon and others. Using this equivalence, we construct $p$-adic differential equations attached to de Rham local systems, carrying both geometric and arithmetic differential operators. This generalizes the work of Berger to the relative geometric setting. Along the way, we study a relative Fontaine--Sen theory on the decompletion of $Γ$-modules over the relative $\mathbf{B}_{\mathrm{dR}}<sup>+$-period rings.