A161859 Number of reduced words of length n in the Weyl group B_13.

1, 13, 90, 442, 1728, 5720, 16653, 43745, 105586, 237354, 502113, 1007773, 1931631, 3554747, 6308719, 10837683, 18078554, 29362618, 46541560, 72140848, 109543070, 163203326, 238898101, 344008185, 487835165, 681949801, 940569228, 1280958420, 1723849738, 2293872698, 3019984381

Offset: 0

Author

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

Keywords

• nonn
• fini
• full

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..169

Crossrefs

Row n=13 of A128084.

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.