Algorithmic universal étale $(φ,Γ)$-modules

Abstract

Let $F/\mathbb{Q}_p$ be a finite extension and let $\breve{G}$ be a connected reductive group with connected center satisfying $p>h_{\breve{G}}$ (the Coxeter number). We write down explicit polynomial equations for the reduced Emerton-Gee stacks (the Borel and the twisted Borel versions that jointly cover the entire $\mathcal{X}_{\breve{G},\mathrm{red}}^{\mathrm{EG}}$), by introducing the mod $p$ Weil-Deligne stacks capturing derived structures of $(φ, Γ)$-modules in reduced families. This allows us to algorithmically compute the set of irreducible components of $\mathcal{X}_{\breve{G},\mathrm{red}}^{\mathrm{EG}}$, thereby establishing its equidimensionality, and the existence of crystalline lifts by showing the total number of irreducible components equals the number of (mod $p$) crystalline (or potentially semistable) components. The last step of the arguments is to interpolate the rigid analytic Weil-Deligne stacks and the mod $p$ Weil-Deligne stacks to deduce the number of potentially semistable components. An initial implementation of the algorithms based on Gröbner basis establishes the existence of crystalline lifts in the $\breve{G}=\mathrm{F}_4$ case for all $F/\mathbb{Q}_p$, and in the remaining $\breve{G}=\mathrm{E}_6, \mathrm{E}_7$, and $\mathrm{E}_8$ cases for all but finitely many $F/\mathbb{Q}_p$.