A161899 Number of reduced words of length n in the Weyl group B_21.

1, 21, 230, 1750, 10394, 51338, 219373, 833129, 2867249, 9073845, 26706681, 73769301, 192639951, 478480891, 1136148412, 2590157556, 5690334091, 12085148911, 24881271591, 49780020675, 96990931345, 184385864445, 342595978935, 623097898035, 1110823234484, 1943507590140

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.)

Links

Andrew Howroyd, Table of n, a(n) for n = 0..441

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

Row n=21 of A128084.

Sequence in context: A221500 A161518 A162360 * A220721 A383714 A320588

Adjacent sequences: A161896 A161897 A161898 * A161900 A161901 A161902

Keyword

nonn,fini,full

Author

John Cannon and N. J. A. Sloane, Nov 30 2009

Status

approved