A trace formula for the distribution of rational $G$-orbits in ramified covers, adapted to representation stability

TL;DR

This paper develops a trace formula tailored for analyzing the distribution of rational G-orbits in ramified covers, especially in the context of representation stability, with detailed examples including polynomial covers.

Contribution

It introduces a specialized trace formula for studying Frobenius distributions in stable cohomological sequences of varieties, extending classical methods to new stable contexts.

Findings

Derived a trace formula suited for representation stability scenarios.

Analyzed the distribution of Frobenius actions in stable cohomology.

Worked out explicit examples for polynomial covers and cycle decompositions.

Abstract

A standard observation in algebraic geometry and number theory is that a ramified cover of an algebraic variety $\widetilde{X}\rightarrow X$ over a finite field $F_q$ furnishes the rational points $x\in X(F_q)$ with additional arithmetic structure: the Frobenius action on the fiber over $x$. For example, in the case of the Vieta cover of polynomials over $F_q$ this structure describes a polynomial's irreducible decomposition type. Furthermore, the distribution of these Frobenius actions is encoded in the cohomology of $\widetilde{X}$ via the Grothendieck-Lefschetz trace formula. This note presents a version of the trace formula that is suited for studying the distribution in the context of representation stability: for certain sequences of varieties $(\widetilde{X}_n)$ the cohomology, and therefore the distribution of the Frobenius actions, stabilizes in a precise sense. We conclude by fully working out the example of the Vieta cover of the variety of polynomials. The calculation includes the distribution of cycle decompositions on cosets of Young subgroups of the symmetric group, which might be of independent interest.