Effective divisors on Bott-Samelson varieties

Abstract

We compute the cone of effective divisors on a Bott-Samelson variety corresponding to an arbitrary sequence of simple roots. The main tool is a general result concerning effective cones of certain $B$-equivariant ${\mathbb{P}}^1$ bundles. As an application, we compute the cone of effective codimension-two cycles on Bott-Samelson varieties corresponding to reduced words. We also obtain an auxiliary result giving criteria for a Bott-Samelson variety to contain a dense $B$-orbit, and we construct desingularizations of intersections of Schubert varieties. An appendix exhibits an explicit divisor showing that any Bott-Samelson variety is log Fano.

22 pages; v2: added application to codimension-2 cycles (Theorem 1.2), computations of nef and effective cones of 2-cycles on fourfolds (Section 5), and a remark on automorphism groups (Remark 6.3); v3: final version to appear in Transformation Groups