Second cohomology groups for algebraic groups and their Frobenius kernels

Abstract

Let $G$ be a simple simply connected algebraic group scheme defined over an algebraically closed field of characteristic $p>0$. Let $T$ be a maximal split torus in $G$, $B\supset T$ be a Borel subgroup of $G$ and $U$ its unipotent radical. Let $F:G\to G$ be the Frobenius morphism. For $r\ge 1$ define the Frobenius kernel, $G_r$, to be the kernel of $F$ iterated with itself $r$ times. Define $U_r$ (respectively $B_r$) to be the kernel of the Frobenius map restricted to $U$ (respectively $B$). Let $X(T)$ be the integral weight lattice and $X(T{)}_{+}$ be the dominant integral weights. The computations of particular importance are $\h^2(U_1,k)$, $\h^2(B_r,\la)$ for $\la \in X(T)$, $\h^2(G_r,H^0(\la))$ for $\la \in X(T)_+$, and $\h^2(B,\la)$ for $\la \in X(T)$. The above cohomology groups for the case when the field has characteristic 2 one computed in this paper. These computations complete the picture started by Bendel, Nakano, and Pillen for $p\ge 3$ \cite{BNP2}.

49 pages, 4 appendices, 6 tables