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A163878
Number of reduced words of length n in Coxeter group on 5 generators S_i with relations (S_i)^2 = (S_i S_j)^6 = I.
1
1, 5, 20, 80, 320, 1280, 5110, 20400, 81450, 325200, 1298400, 5184000, 20697690, 82637820, 329940630, 1317324420, 5259563280, 20999387520, 83842374870, 334749945240, 1336526142210, 5336228292840, 21305481048360, 85064487085440
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.
FORMULA
G.f.: (t^6 + 2*t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(6*t^6 - 3*t^5 - 3*t^4 - 3*t^3 - 3*t^2 - 3*t + 1).
a(n) = -6*a(n-6) + 3*Sum_{k=1..5} a(n-k). - Wesley Ivan Hurt, May 11 2021
MAPLE
seq(coeff(series((1+t)*(1-t^6)/(1-4*t+9*t^6-6*t^7), t, n+1), t, n), n = 0 .. 30); # G. C. Greubel, Aug 10 2019
MATHEMATICA
CoefficientList[Series[(1+t)*(1-t^6)/(1-4*t+9*t^6-6*t^7), {t, 0, 30}], t] (* G. C. Greubel, Aug 07 2017 *)
coxG[{6, 6, -3}] (* The coxG program is at A169452 *) (* G. C. Greubel, Aug 10 2019 *)
PROG
(PARI) my(t='t+O('t^30)); Vec((1+t)*(1-t^6)/(1-4*t+9*t^6-6*t^7)) \\ G. C. Greubel, Aug 07 2017
(Magma) R<t>:=PowerSeriesRing(Integers(), 30); Coefficients(R!( (1+t)*(1-t^6)/(1-4*t+9*t^6-6*t^7) )); // G. C. Greubel, Aug 10 2019
(Sage)
def A163878_list(prec):
P.<t> = PowerSeriesRing(ZZ, prec)
return P((1+t)*(1-t^6)/(1-4*t+9*t^6-6*t^7)).list()
A163878_list(30) # G. C. Greubel, Aug 10 2019
(GAP) a:=[5, 20, 80, 320, 1280, 5110];; for n in [7..30] do a[n]:=3*(a[n-1] +a[n-2]+a[n-3]+a[n-4]+a[n-5]) -6*a[n-6]; od; Concatenation([1], a); # G. C. Greubel, Aug 10 2019
CROSSREFS
Sequence in context: A214939 A162925 A163316 * A164354 A164706 A165185
KEYWORD
nonn
AUTHOR
John Cannon and N. J. A. Sloane, Dec 03 2009
STATUS
approved