Published online by Cambridge University Press: 20 November 2018
A diagonal ind-group is a direct limit of classical affine algebraic groups of growing rank under a class of inclusions that contains the inclusion
$$SL(n)\,\to \,SL(2n),\,\,M\mapsto \,\left( \begin{matrix}
M & 0 \\
0 & M \\
\end{matrix} \right)$$
as a typical special case. If $G$ is a diagonal ind-group and
$B\,\subset \,G$ is a Borel ind-subgroup, we consider the ind-variety
$G/B$ and compute the cohomology
${{H}^{\ell }}(G/B,\,{{\mathcal{O}}_{-\lambda }})$ of any
$G$-equivariant line bundle
${{\mathcal{O}}_{-\lambda }}$ on
$G/B$. It has been known that, for a generic
$\lambda $, all cohomology groups of
${{\mathcal{O}}_{-\lambda }}$ vanish, and that a non-generic equivariant line bundle
${{\mathcal{O}}_{-\lambda }}$ has at most one nonzero cohomology group. The new result of this paper is a precise description of when
${{H}^{j}}(G/B,\,{{\mathcal{O}}_{-\lambda }})$ is nonzero and the proof of the fact that, whenever nonzero,
${{H}^{j}}(G/B,\,{{\mathcal{O}}_{-\lambda }})$ is a
$G$-module dual to a highest weight module. The main difficulty is in defining an appropriate analog
${{W}_{B}}$ of the Weyl group, so that the action of
${{W}_{B}}$ on weights of
$G$ is compatible with the analog of the Demazure “action” of the Weyl group on the cohomology of line bundles. The highest weight corresponding to
${{H}^{j}}(G/B,\,{{\mathcal{O}}_{-\lambda }})$ is then computed by a procedure similar to that in the classical Bott–Borel–Weil theorem.