A161518 Number of reduced words of length n in the Weyl group A_21.

1, 21, 230, 1749, 10373, 51107, 217602, 822504, 2814141, 8843868, 25820674, 70673825, 182690494, 448730093, 1052649257, 2368601685, 5131231668, 10736569602, 21758936146, 42815701060, 81978913225, 153027659730

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 I.)

Links

Table of n, a(n) for n=0..21.

Formula

G.f. for A_m is the polynomial Prod_{k=1..m}(1-x^(k+1))/(1-x). Only finitely many terms are nonzero. This is a row of the triangle in A008302.

Crossrefs

Sequence in context: A027508 A221037 A221500 * A162360 A161899 A220721

Adjacent sequences: A161515 A161516 A161517 * A161519 A161520 A161521

Keyword

nonn

Author

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

Status

approved