Divisors on Bott-Samelson varieties
-- Dave Anderson, February 27, 2015
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I will explain how to compute the cones of effective divisors
on a Bott-Samelson variety.  These varieties are certain towers
of  P<sup>1</sup>-bundles associated to sequences of simple roots for a
reductive algebraic group.  When the sequence is "reduced," the
Bott-Samelson variety resolves singularities of a Schubert
variety, and in this case the cone is very easy. For general
sequences, the cone is somewhat complicated, but tractable.
The main new tool is a theorem which describes the effective
cones of special equivariant P<sup>1</sup>-bundles.