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

0,2

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, -171).

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)

Mathematica

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

Crossrefs

Sequence in context: A084329 A097832 A342886 * A163454 A163969 A164633

Adjacent sequences: A163121 A163122 A163123 * A163125 A163126 A163127

Keyword

nonn

Author

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

Status

approved