A161499 Number of reduced words of length n in the Weyl group A_18.

1, 18, 170, 1121, 5795, 25005, 93651, 312646, 948328, 2651355, 6908989, 16929266, 39287173, 86859412, 183856635, 374153699, 734644291, 1396030396, 2574320659, 4617369760, 8072048882, 13779087108, 23004239550, 37616173304, 60323245445

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..24.

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: A125381 A126539 A213802 * A162346 A161878 A139618

Adjacent sequences: A161496 A161497 A161498 * A161500 A161501 A161502

Keyword

nonn

Author

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

Status

approved