A162807 Number of reduced words of length n in Coxeter group on 21 generators S_i with relations (S_i)^2 = (S_i S_j)^3 = I.

1, 21, 420, 8190, 159600, 3108210, 60532290, 1178845500, 22957618110, 447091673490, 8706955895400, 169564956368010, 3302218915041690, 64309571936658300, 1252406684472377910, 24390187277913766890

Offset

0,2

Comments

The initial terms coincide with those of A170740, although the two sequences are eventually different.

Computed with MAGMA using commands similar to those used to compute A154638.

Links

Table of n, a(n) for n=0..15.

Index entries for linear recurrences with constant coefficients, signature (19, 19, -190).

Formula

G.f.: (t^3 + 2*t^2 + 2*t + 1)/(190*t^3 - 19*t^2 - 19*t + 1).

a(n) = 19*a(n-1)+19*a(n-2)-190*a(n-3). - Wesley Ivan Hurt, May 02 2021

Crossrefs

Sequence in context: A020534 A106656 A083043 * A097833 A163145 A163503

Adjacent sequences: A162804 A162805 A162806 * A162808 A162809 A162810

Keyword

nonn

Author

John Cannon and N. J. A. Sloane, Dec 03 2009

Status

approved