A162941 Number of reduced words of length n in Coxeter group on 7 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.

1, 7, 42, 252, 1491, 8820, 52185, 308700, 1826160, 10802925, 63906150, 378045675, 2236381350, 13229622000, 78261652875, 462967596000, 2738748634125, 16201445085000, 95841881782500, 566965863568125, 3353964722666250

Offset: 0

Author

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

Keywords

• nonn

Comments

The initial terms coincide with those of A003949, 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 (5, 5, 5, -15).

Programs

Mathematica

coxG[{4,15,-5}] (* The coxG program is at A169452 *) (* or *) LinearRecurrence[ {5,5,5,-15},{1,7,42,252,1491},30] (* Harvey P. Dale, Mar 13 2018 *)

Formula

G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(15*t^4 - 5*t^3 - 5*t^2 - 5*t + 1)