A164911 Number of reduced words of length n in Coxeter group on 20 generators S_i with relations (S_i)^2 = (S_i S_j)^8 = I.

1, 20, 380, 7220, 137180, 2606420, 49521980, 940917620, 17877434590, 339671253600, 6453753750000, 122621319950400, 2329805054365200, 44266295563783200, 841059606797924400, 15980132359795392000

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, 18, 18, 18, 18, 18, -171).

Formula

G.f. (t^8 + 2*t^7 + 2*t^6 + 2*t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(171*t^8 -

18*t^7 - 18*t^6 - 18*t^5 - 18*t^4 - 18*t^3 - 18*t^2 - 18*t + 1)

Crossrefs

Sequence in context: A163454 A163969 A164633 * A165348 A165882 A063815

Adjacent sequences: A164908 A164909 A164910 * A164912 A164913 A164914

Keyword

nonn

Author

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

Status

approved