A162806 Number of reduced words of length n in Coxeter group on 20 generators S_i with relations (S_i)^2 = (S_i S_j)^3 = I.

1, 20, 380, 7030, 129960, 2400840, 44352270, 819332820, 15135787980, 279607936230, 5165281123560, 95419783331640, 1762718203098270, 32563220683609620, 601550117011031580, 11112615265773737430

Offset

0,2

Comments

The initial terms coincide with those of A170739, 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 (18, 18, -171).

Formula

G.f.: (t^3 + 2*t^2 + 2*t + 1)/(171*t^3 - 18*t^2 - 18*t + 1)

Mathematica

coxG[{3, 171, -18}] (* The coxG program is at A169452 *) (* Harvey P. Dale, Jul 10 2020 *)

Crossrefs

Sequence in context: A266736 A019580 A177109 * A324949 A084329 A097832

Adjacent sequences: A162803 A162804 A162805 * A162807 A162808 A162809

Keyword

nonn

Author

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

Status

approved