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%I A163391 #18 Sep 08 2022 08:45:46
%S A163391 1,9,72,576,4608,36828,294336,2352420,18801216,150264576,1200956652,
%T A163391 9598382640,76712967828,613111567824,4900159716480,39163451657148,
%U A163391 313005296651040,2501626174048260,19993698450611424,159795249138713664
%N A163391 Number of reduced words of length n in Coxeter group on 9 generators S_i with relations (S_i)^2 = (S_i S_j)^5 = I.
%C A163391 The initial terms coincide with those of A003951, although the two sequences are eventually different.
%C A163391 Computed with MAGMA using commands similar to those used to compute A154638.
%H A163391 G. C. Greubel, <a href="/A163391/b163391.txt">Table of n, a(n) for n = 0..1000</a>
%H A163391 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (7, 7, 7, 7, -28).
%F A163391 G.f.: (t^5 + 2*t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(28*t^5 - 7*t^4 - 7*t^3 - 7*t^2 - 7*t + 1).
%F A163391 a(n) = 7*a(n-1)+7*a(n-2)+7*a(n-3)+7*a(n-4)-28*a(n-5). - _Wesley Ivan Hurt_, May 10 2021
%t A163391 CoefficientList[Series[(1+x)*(1-x^5)/(1-8*x+35*x^5-28*x^6), {x, 0, 30}], x] (* or *) LinearRecurrence[{7,7,7,7,-28}, {1,9,72,576,4608,36828}, 30] (* _G. C. Greubel_, Dec 21 2016 *)
%t A163391 coxG[{5, 28, -7}] (* The coxG program is at A169452 *) (* _G. C. Greubel_, May 12 2019 *)
%o A163391 (PARI) my(x='x+O('x^30)); Vec((1+x)*(1-x^5)/(1-8*x+35*x^5-28*x^6)) \\ _G. C. Greubel_, Dec 21 2016
%o A163391 (Magma) R<x>:=PowerSeriesRing(Integers(), 30); Coefficients(R!( (1+x)*(1-x^5)/(1-8*x+35*x^5-28*x^6) )); // _G. C. Greubel_, May 12 2019
%o A163391 (Sage) ((1+x)*(1-x^5)/(1-8*x+35*x^5-28*x^6)).series(x, 30).coefficients(x, sparse=False) # _G. C. Greubel_, May 12 2019
%K A163391 nonn
%O A163391 0,2
%A A163391 _John Cannon_ and _N. J. A. Sloane_, Dec 03 2009

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