The kernel generating condition and absolute Galois groups

Abstract

For a list $\cal{L}$ of finite groups and for a profinite group $G$, we consider the intersection $T(G)$ of all open normal subgroups $N$ of $G$ with $G/N$ in $\cal{L}$. We give a cohomological characterization of the epimorphisms $π\colon S\to G$ of profinite groups (satisfying some additional requirements) such that $π[T(S)]=T(G)$. For $p$ prime, this is used to describe cohomologically the profinite groups $G$ whose $n$th term $G_{(n,p)}$ (resp., $G^{(n,p)}$) in the $p$-Zassenhaus filtration (resp., lower $p$-central filtration) is an intersection of this form. When $G=G_F$ is the absolute Galois group of a field $F$ containing a root of unity of order $p$, we recover as special cases results by Minac, Spira and the author, describing $G_{(3,p)}$ and $G^{(3,p)}$ as $T(G)$ for appropriate lists $\cal{L}$.