Let $R$ be a complete DVR with algebraically closed residue field $k$ and fractional field $K$ , $PGL(2)$ the automorphic group of projective line over $\overline K$.


My question is:


When $H^{1}(Gal(\overline K/K), PGL(2))=0$ ? Is this group trivial if $Char(K) \neq 2$ ?



a question of Galois cohomology
The claimed triviality holds (the nonabelian cohomology set is not a group though), and I don't think you need $Char(K) \neq 2$. To argue this, I will use the long exact nonabelian cohomology sequence of the central extension $1 \rightarrow \mathbf{G}\_m \rightarrow GL\_2 \rightarrow PGL\_2 \rightarrow 1$, a segment of which reads $H^1(K, GL\_2) \rightarrow H^1(K, PGL\_2) \rightarrow H^2(K, \mathbf{G}\_m)$. Firstly, $H^1(K, GL\_2)$ is the one-point set because it classifies rank 2 vector bundles over $Spec(K)$, of which there is only the trivial one. Secondly, $K$ is a $C\_1$ field by a theorem of Lang (see Serre "Galois cohomology", p. 80, II.3.3 c)), hence is of $dim \le 1$, so its Brauer group $H^2(K, \mathbf{G}\_m)$ vanishes (loc. cit. for more details).


The same argument shows that $H^1(K, PGL\_n)$ is the one-point set for any $n$ and any $C\_1$ field $K$.


