A163988 Number of reduced words of length n in Coxeter group on 22 generators S_i with relations (S_i)^2 = (S_i S_j)^6 = I.

1, 22, 462, 9702, 203742, 4278582, 89849991, 1886844960, 39623642520, 832094358480, 17473936704840, 366951729513600, 7705966552789890, 161824882502745000, 3398313815357307000, 71364407061765925800

Offset: 0

Author

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

Keywords

• nonn

Comments

The initial terms coincide with those of A170741, 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..740
• Index entries for linear recurrences with constant coefficients, signature (20, 20, 20, 20, 20, -210).

Programs

Magma

R:=PowerSeriesRing(Integers(), 20); Coefficients(R!( (1+x)*(1-x^6)/(1-21*x+230*x^6-210*x^7) )); // G. C. Greubel, Apr 25 2019

Mathematica

CoefficientList[Series[(1+x)*(1-x^6)/(1-21*x+230*x^6-210*x^7), {x,0,20}], x] (* G. C. Greubel, Aug 24 2017 *)
coxG[{6, 210, -20, 20}] (* The coxG program is at A169452 *)

PARI

my(x='x+O('x^20)); Vec((1+x)*(1-x^6)/(1-21*x+230*x^6-210*x^7)) \\ G. C. Greubel, Aug 24 2017, modified Apr 25 2019

Sage

((1+x)*(1-x^6)/(1-21*x+230*x^6-210*x^7)).series(x, 20).coefficients(x, sparse=False) # G. C. Greubel, Apr 25 2019

Formula

G.f.: (t^6 + 2*t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(210*t^6 - 20*t^5 - 20*t^4 - 20*t^3 - 20*t^2 - 20*t + 1).

G.f.: (1+x)*(1-x^6)/(1 -21*x +230*x^6 -210*x^7). - G. C. Greubel, Apr 25 2019