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A161880
Number of reduced words of length n in the Weyl group B_20.
0
1, 20, 209, 1520, 8644, 40944, 168035, 613756, 2034120, 6206596, 17632836, 47062620, 118870650, 285840940, 657667521, 1454009144, 3100176535, 6394814820, 12796122680, 24898749084, 47210910670, 87394933100
OFFSET
0,2
COMMENTS
Computed with MAGMA using commands similar to those used to compute A161409.
REFERENCES
J. E. Humphreys, Reflection Groups and Coxeter Groups, Cambridge, 1990. See under Poincaré polynomial.
N. Bourbaki, Groupes et alg. de Lie, Chap. 4, 5, 6. (The group is defined in Planche II.)
FORMULA
G.f. for B_m is the polynomial Prod_{k=1..m}(1-x^(2k))/(1-x). Only finitely many terms are nonzero. This is a row of the triangle in A128084.
CROSSREFS
Sequence in context: A231059 A161513 A162359 * A074668 A139620 A094311
KEYWORD
nonn
AUTHOR
John Cannon and N. J. A. Sloane, Nov 30 2009
STATUS
approved