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A167896
Number of reduced words of length n in Coxeter group on 5 generators S_i with relations (S_i)^2 = (S_i S_j)^16 = I.
7
1, 5, 20, 80, 320, 1280, 5120, 20480, 81920, 327680, 1310720, 5242880, 20971520, 83886080, 335544320, 1342177280, 5368709110, 21474836400, 85899345450, 343597381200, 1374389522400, 5497558080000, 21990232281600, 87960928972800
OFFSET
0,2
COMMENTS
The initial terms coincide with those of A003947, 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 (3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,-6).
FORMULA
G.f.: (t^16 + 2*t^15 + 2*t^14 + 2*t^13 + 2*t^12 + 2*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)/ ( 6*t^16 - 3*t^15 - 3*t^14 - 3*t^13 - 3*t^12 - 3*t^11 - 3*t^10 - 3*t^9 - 3*t^8 - 3*t^7 - 3*t^6 - 3*t^5 - 3*t^4 - 3*t^3 - 3*t^2 - 3*t + 1).
From G. C. Greubel, Dec 06 2024: (Start)
a(n) = 3*Sum_{j=1..15} a(n-j) - 6*a(n-16).
G.f.: (1+x)*(1-x^16)/(1 - 4*x + 9*x^16 - 6*x^17). (End)
MATHEMATICA
CoefficientList[Series[(1+t)*(1-t^16)/(1-4*t+9*t^16-6*t^17), {t, 0, 50}], t] (* G. C. Greubel, Jul 01 2016; Dec 06 2024 *)
coxG[{16, 6, -3, 40}] (* The coxG program is at A169452 *) (* G. C. Greubel, Dec 06 2024 *)
PROG
(Magma)
R<x>:=PowerSeriesRing(Integers(), 40); Coefficients(R!( (1+x)*(1-x^16)/(1-4*x+9*x^16-6*x^17) )); // G. C. Greubel, Dec 06 2024
(SageMath)
def A167896_list(prec):
P.<x> = PowerSeriesRing(ZZ, prec)
return P( (1+x)*(1-x^16)/(1-4*x+9*x^16-6*x^17) ).list()
print(A167896_list(40)) # G. C. Greubel, Dec 06 2024
(PARI) Vec((x^16+2*x^15+2*x^14+2*x^13+2*x^12+2*x^11+2*x^10+2*x^9+2*x^8+2*x^7+2*x^6+2*x^5+2*x^4+2*x^3+2*x^2+2*x+1)/(6*x^16-3*x^15-3*x^14-3*x^13-3*x^12-3*x^11-3*x^10-3*x^9-3*x^8-3*x^7-3*x^6-3*x^5-3*x^4-3*x^3-3*x^2-3*x+1)+O(x^99)) \\ Charles R Greathouse IV, May 15 2026
CROSSREFS
KEYWORD
nonn,easy,changed
AUTHOR
John Cannon and N. J. A. Sloane, Dec 03 2009
STATUS
approved