This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A168781 #18 Oct 02 2024 16:24:09
%S A168781 1,8,56,392,2744,19208,134456,941192,6588344,46118408,322828856,
%T A168781 2259801992,15818613944,110730297608,775112083256,5425784582792,
%U A168781 37980492079544,265863444556808,1861044111897656,13027308783283564
%N A168781 Number of reduced words of length n in Coxeter group on 8 generators S_i with relations (S_i)^2 = (S_i S_j)^19 = I.
%C A168781 The initial terms coincide with those of A003950, although the two sequences are eventually different.
%C A168781 First disagreement at index 19: a(19) = 13027308783283564, A003950(19) = 13027308783283592. - _Klaus Brockhaus_, Mar 25 2011
%C A168781 Computed with MAGMA using commands similar to those used to compute A154638.
%H A168781 G. C. Greubel, <a href="/A168781/b168781.txt">Table of n, a(n) for n = 0..500</a>
%H A168781 <a href="/index/Rec#order_19">Index entries for linear recurrences with constant coefficients</a>, signature (6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, -21).
%F A168781 G.f.: (t^19 + 2*t^18 + 2*t^17 + 2*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)/(21*t^19 - 6*t^18 - 6*t^17 - 6*t^16 - 6*t^15 - 6*t^14 - 6*t^13 - 6*t^12 - 6*t^11 - 6*t^10 - 6*t^9 - 6*t^8 - 6*t^7 - 6*t^6 - 6*t^5 - 6*t^4 - 6*t^3 - 6*t^2 - 6*t + 1).
%t A168781 CoefficientList[Series[(t^19 + 2*t^18 + 2*t^17 + 2*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)/(21*t^19 - 6*t^18 - 6*t^17 - 6*t^16 - 6*t^15 - 6*t^14 - 6*t^13 - 6*t^12 - 6*t^11 - 6*t^10 - 6*t^9 - 6*t^8 - 6*t^7 - 6*t^6 - 6*t^5 - 6*t^4 - 6*t^3 - 6*t^2 - 6*t + 1), {t, 0, 50}], t] (* _G. C. Greubel_, Aug 12 2016 *)
%t A168781 coxG[{19,21,-6}] (* The coxG program is at A169452 *) (* _Harvey P. Dale_, Oct 02 2024 *)
%Y A168781 Cf. A003950 (G.f.: (1+x)/(1-7*x)).
%K A168781 nonn,easy
%O A168781 0,2
%A A168781 _John Cannon_ and _N. J. A. Sloane_, Dec 03 2009