Rigid inner forms of real and p-adic groups

Abstract: We define a new cohomology set for an affine algebraic group G and a multiplicative finite central subgroup Z, both defined over a local field of characteristic zero, which is an enlargement of the usual first Galois cohomology set of G. We show how this set can be used to normalize the Langlands-Shelstad endoscopic transfer factors and to give a conjectural description of the internal structure and endoscopic transfer of L-packets for arbitrary connected reductive groups that extends the well-known conjectural description for quasi-split groups. In the real case, we show that this description is correct using Shelstad's work.