Let G be a group which acts on a contractable space X. One calls Y included in X a spine if Y is a G-equivariant deformation retract of X, the dimension of Y is the virtual cohomological dimension of G, and Y has a simplical complex structure which is compatible with the action of G. Brownstein is interested in the case where G is SL(2,0) and X is H2 x H2, where 0 is the ring of integers in a real quadratic number field and also the case where G is Sp(2g,Z) the symplectic group and X is the Siegel upper half space. He plans to compute the integral homology of G by constructing appropriate spines. The actual computation is done via the spectral sequence associated with the Borel construction.
