Galois cohomology groups given by étale cohomology - MathOverflow most recent 30 from http://mathoverflow.net 2013-06-20T06:05:37Z http://mathoverflow.net/feeds/question/60310 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/60310/galois-cohomology-groups-given-by-etale-cohomology Galois cohomology groups given by étale cohomology unknown (google) 2011-04-01T17:10:03Z 2011-04-02T01:48:51Z <p>What are cases when Galois cohomology groups are given by étale cohomology?</p> <p>Example: $S = Spec(K)$ the spectrum of a field, $F \in Sh(K)$, then $H^p(K, F) = H^p(G_K, F_{\bar{K}})$.</p> <p>What if $G = \pi_1(X)$ and $F \in Sh(X)$? Under what conditions do we have $H^p(X, F) = H^p(G, [F])$, where $[F]$ denotes a suitable $\pi_1(X)$-module associated with $F$? (Example for this: $X = Spec(O_K)\setminus S$)</p> http://mathoverflow.net/questions/60310/galois-cohomology-groups-given-by-etale-cohomology/60335#60335 Answer by mephisto for Galois cohomology groups given by étale cohomology mephisto 2011-04-02T01:48:51Z 2011-04-02T01:48:51Z <p>All Galois cohomology groups are given by etale cohomology, but you seem to be asking the opposite. For an etale sheaf $F$ on $X$, there is a Hochschild-Serre spectral sequence $H^{p}(\pi_{1}(X),H^{q}(\tilde{X},F))\implies H^{p+q}(X,F)$ where $\tilde{X}$ is the "universal covering scheme" of $X$. Don't expect this to collapse to give isomorphisms $H^{p}(\pi _{1}(X),F)\approx H^{p}(X,F)$ except when $X$ is spec of a field or a henselian local ring. There are a few fragmentary results. For example, $H^{1}(\pi _{1}(X),A)\approx H^{1}(X,A)$ when $X$ is an open subscheme of the spec of the ring of integers in a number field and $A$ is an abelian scheme on $X$. </p>