$R$-trivial Galois cohomology classes and cohomological $p$-dimension

Source abstract

Given a prime $p$, a field $F$ of characteristic different from $p$ and a finite $p$-primary Galois module $V$, a Galois cohomology class in $H^n(F,V)$ is called $R$-trivial if it can be connected to $0$ by a class defined over an open subscheme of the projective line. All classes of $H^n(F,V)$ are $R$-trivial if $F$ has cohomological $p$-dimension at most $n$, by the Bloch-Kato conjecture. We show that this bound is sharp: working over fields of iterated Laurent series, we produce, for every $n\geq 1$, a field $F$ of cohomological $p$-dimension $n+1$ and a Galois module $V$ such that $H^n(F,V)$ contains classes which are not $R$-trivial. The module $V$ is a quotient of the permutation module on the affine hyperplanes of an elementary abelian $p$-group, arising as a quotient of the absolute Galois group of $F$. We also describe $R$-trivial classes as sums of corestrictions of cup products involving a Kummer class.