A163451 Number of reduced words of length n in Coxeter group on 17 generators S_i with relations (S_i)^2 = (S_i S_j)^5 = I.

1, 17, 272, 4352, 69632, 1113976, 17821440, 285108360, 4561178880, 72969984000, 1167377713080, 18675771192000, 298775988016200, 4779834262113600, 76468044587443200, 1223339873805905400, 19571056837109136000

Offset: 0

Author

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

Keywords

• nonn

Comments

The initial terms coincide with those of A170736, although the two sequences are eventually different.

Computed with MAGMA using commands similar to those used to compute A154638.

Links

• G. C. Greubel, Table of n, a(n) for n = 0..825
• Index entries for linear recurrences with constant coefficients, signature (15, 15, 15, 15, -120).

Programs

Magma

R:=PowerSeriesRing(Integers(), 20); Coefficients(R!( (1+x)*(1-x^5)/(1-16*x+135*x^5-120*x^6) )); // G. C. Greubel, May 13 2019

Mathematica

CoefficientList[Series[(1+x)*(1-x^5)/(1-16*x+135*x^5-120*x^6), {x, 0, 20}], x] (* G. C. Greubel, Dec 24 2016 *)
coxG[{5, 120, -15}] (* The coxG program is at A169452 *) (* G. C. Greubel, May 13 2019 *)

PARI

my(x='x+O('x^20)); Vec((1+x)*(1-x^5)/(1-16*x+135*x^5-120*x^6)) \\ G. C. Greubel, Dec 24 2016

Sage

((1+x)*(1-x^5)/(1-16*x+135*x^5-120*x^6)).series(x, 20).coefficients(x, sparse=False) # G. C. Greubel, May 13 2019

Formula

G.f.: (t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(120*t^5 - 15*t^4 - 15*t^3 - 15*t^2 - 15*t + 1).

a(n) = 15*a(n-1)+15*a(n-2)+15*a(n-3)+15*a(n-4)-120*a(n-5). - Wesley Ivan Hurt, May 10 2021