Bounds on Negligible Thresholds in Group Cohomology

Abstract

Let $F$ be a field, $K/F$ a field extension with ${\Gamma }_K:=\text{Gal}(K_{\text{sep}}/K)$, $G$ a finite group, $j:{\Gamma }_K\to G$ a continuous group homomorphism, and $M$ an abelian group. There is an induced map of profinite cohomology $j^{\ast }:H^n(G;M)\to H^n(K;M)=:H^n({\Gamma }_K;M)$. An element $u\in H^n(G;M)$ is negligible over $F$ if $j^{\ast }(u)=0$ for all such $j$ and field extensions $K/F$. The subset of negligible elements forms a subgroup $H^n(G;M{)}_{\text{neg},F}\subseteq H^n(G;M)$. The negligible threshold ${\text{nthres}}_F^G(M)$ is the smallest $N\in {\mathbb{N}}_{\ge 0}\cup \{\infty \}$ such that $H^n(G;M{)}_{\text{neg},F}=H^n(G;M)$ for all $n\ge N$. In this paper, we first establish some general bounds for negligible thresholds of arbitrary triples $(G,M,F)$ consisting of a finite group $G$, trivial $G$-module $M$, and field $F$. We then compute the negligible thresholds of an arbitrary group when $F$ is a field of characteristic $p$ and $M=\mathbb{Z}/p^k\mathbb{Z}$. Continuing in the characteristic $p$ setting, we compute these thresholds for $G$ a $p$-group and $M=\mathbb{Z}$, and we compute the thresholds for $G=\mathbb{Z}/\ell \mathbb{Z},M=\mathbb{Z}/s\mathbb{Z}$ for integers $s,\ell \in {\mathbb{N}}_{>1}$ satisfying certain number theoretic conditions. For fields containing sufficiently many roots of unity, we show that the negligible threshold of odd exponent finitely-generated trivial $G$-modules is finite. For $G$-modules of the form $\mathbb{Z}/p\mathbb{Z}$, $p$ an odd prime, we compute an explicit upper bound for the negligible threshold in terms of $p$ and $|G|$. We conclude by providing explicit computations of the negligible thresholds of elementary abelian $p$-groups with mod-$p$ coefficients, negligible thresholds of cyclic groups, and bounds on mod-$p$ negligible thresholds of general finite abelian $p$-groups.