Past exam of the mathematics course of the University of Cambridge / 2012 / iii / Paper 21 / 4 / 1 / Solution

We use the standard characteristic-$p$ absolute Galois-group theorem, ${\mathrm{cd}}_p(K)\le 1$, and the cohomological identification $\mathrm{Br}(K)=H^2(K,K_s^{\times })$. Both are clearly stated inputs permitted for this question.

In characteristic $p$, the map $a\mapsto a^p$ on $K_s^{\times }$ is injective, although it need not be surjective: $p$th roots are purely inseparable. Thus it gives an exact sequence of discrete Galois modulesThe quotient $Q$ is killed by $p$. Since ${\mathrm{cd}}_p(K)\le 1$, $H^2(K,Q)=0$. The long exact cohomology sequence containsTherefore multiplication by $p$ on $\mathrm{Br}(K)$ is surjective. This is Brauer p-divisibility in characteristic p. The quotient-module sequence is the relevant one in characteristic $p$; the separable Kummer sequence with a nontrivial ${\mu }_p$ is not available there.