152.
In (Deodhar,
Geom. Dedicata, 36(1) (1990), 95–119), Deodhar proposes a combinatorial framework for determining the Kazhdan-Lusztig polynomials
P
x
,
w
in the case where
W is any Coxeter group. We explicitly describe the combinatorics in the case where
(the symmetric group on
n letters) and the permutation
w is 321-hexagon-avoiding. Our formula can be expressed in terms of a simple statistic on all subexpressions of any fixed reduced expression for
w. As a consequence of our results on Kazhdan-Lusztig polynomials, we show that the Poincaré polynomial of the intersection cohomology of the Schubert variety corresponding to
w is (1+
q)
l(w)
if and only if
w is 321-hexagon-avoiding. We also give a sufficient condition for the Schubert variety
X
w
to have a small resolution. We conclude with a simple method for completely determining the singular locus of
X
w
when
w is 321-hexagon-avoiding. The results extend easily to those Weyl groups whose Coxeter graphs have no branch points (
B
C
n
,
F
4,
G
2).
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