A161878 Number of reduced words of length n in the Weyl group B_18.

1, 18, 170, 1122, 5813, 25176, 94791, 318630, 974643, 2752112, 7253764, 18003544, 42378246, 95162260, 204856291, 424515042, 849825768, 1648470894, 3106669575, 5701318544, 10209535182, 17871860844, 30631158960, 51476623220, 84931612739

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

Table of n, a(n) for n=0..24.

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: A213802 A161499 A162346 * A139618 A162638 A010970

Adjacent sequences: A161875 A161876 A161877 * A161879 A161880 A161881

Keyword

nonn

Author

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

Status

approved