Galois cohomology and component group of a real reductive group

Abstract

Let G be a connected reductive group over the field of real numbers R. Using results of our previous joint paper, we compute combinatorially the first Galois cohomology set H^1(R,G) in terms of reductive Kac labelings. Moreover, we compute the group of connected components \pi_0 G(R) of the real Lie group G(R) and the maps in exact sequences containing \pi_0 G(R) and H^1(R,G).