A163124 Number of reduced words of length n in Coxeter group on 20 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.

1, 20, 380, 7220, 136990, 2599200, 49316400, 935712000, 17753871510, 336855735180, 6391382632020, 121267853544780, 2300893742387430, 43656351283440360, 828320305398630840, 15716259104684097960

Offset: 0

Author

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

Keywords

• nonn

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

• Index entries for linear recurrences with constant coefficients, signature (18, 18, 18, -171).

Programs

Mathematica

coxG[{4,171,-18}] (* The coxG program is at A169452 *) (* Harvey P. Dale, Dec 14 2015 *)

Formula

G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(171*t^4 - 18*t^3 - 18*t^2 - 18*t + 1)