Invariant Hopf 2-Cocycles for Affine Algebraic Groups

We generalize the theory of the second invariant cohomology group H-inv(2)(G) for finite groups G, developed in [3, 4, 14], to the case of affine algebraic groups G, using the methods of [9, 10, 12]. In particular, we show that for connected affine algebraic groups G over an algebraically closed field of characteristic 0, the map Theta from [14] is bijective (unlike for some finite groups, as shown in [14]). This allows us to compute H-inv(2)(G) in this case, and in particular show that this group is commutative (while for finite groups it can be noncommutative, as shown in [14]).