Euler trace congruences over local fields

Abstract

Let $K$ be a finite extension of \QQ_p with uniformizer $π$ and residue field \FF_q. For each $r\ge1$, we determine the largest integer $E_r$ such that \Tr(A^{q^r})\equiv\Tr(A^{q^{r-1}})\pmod{π^{E_r}} for every integral matrix $A$ with a prescribed residual similarity class. We give formulas for both nilpotent and nonnilpotent residual classes and construct integral lifts attaining the resulting exponents. We also exhibit local fields with the same ramification index and residue field for which the optimal exponents differ, even when $r$ and the residual similarity class are fixed.