A166415 - OEIS

A166415

Number of reduced words of length n in Coxeter group on 21 generators S_i with relations (S_i)^2 = (S_i S_j)^11 = I.

1, 21, 420, 8400, 168000, 3360000, 67200000, 1344000000, 26880000000, 537600000000, 10752000000000, 215039999999790, 4300799999991600, 86015999999748210, 1720319999993288400, 34406399999832252000

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OFFSET

0,2

COMMENTS

The initial terms coincide with those of A170740, 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..500

Index entries for linear recurrences with constant coefficients, signature (19,19,19,19,19,19,19,19,19,19,-190).

FORMULA

G.f.: (t^11 + 2*t^10 + 2*t^9 + 2*t^8 + 2*t^7 + 2*t^6 + 2*t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(190*t^11 - 19*t^10 - 19*t^9 - 19*t^8 - 19*t^7 - 19*t^6 - 19*t^5 - 19*t^4 - 19*t^3 - 19*t^2 - 19*t + 1).

From G. C. Greubel, Jul 23 2024: (Start)

a(n) = 19*Sum_{j=1..10} a(n-j) - 190*a(n-11).

G.f.: (1+x)*(1-x^11)/(1 - 20*x + 209*x^11 - 190*x^12). (End)

MATHEMATICA

CoefficientList[Series[(1+t)*(1-t^11)/(1-20*t+209*t^11-190*t^12), {t, 0, 50}], t] (* G. C. Greubel, May 13 2016; Jul 23 2024 *)

coxG[{11, 190, -19, 30}] (* The coxG program is at A169452 *) (* G. C. Greubel, Jul 23 2024 *)

PROG

(Magma)

R<x>:=PowerSeriesRing(Integers(), 30);

Coefficients(R!( (1+x)*(1-x^11)/(1-20*x+209*x^11-190*x^12) )); // G. C. Greubel, Jul 23 2024

(SageMath)

def A166415_list(prec):