The behaviour at infinity of the Bruhat decomposition

Abstract.

For a connected reductive group G and a Borel subgroup B, we study the closures of double classes BgB in a $(G \times G)$-equivariant "regular" compactification of G. We show that these closures $\overline {BgB}$ intersect properly all $(G \times G)$-orbits, with multiplicity one, and we describe the intersections. Moreover, we show that almost all $\overline {BgB}$ are singular in codimension two exactly. We deduce this from more general results on B-orbits in a spherical homogeneous space G/H; they lead to formulas for homology classes of H-orbit closures in G/B, in terms of Schubert cycles.