cp's OEIS Frontend

A168876 Number of reduced words of length n in Coxeter group on 7 generators S_i with relations (S_i)^2 = (S_i S_j)^21 = I.

%I A168876 #8 Nov 25 2016 09:32:47
%S A168876 1,7,42,252,1512,9072,54432,326592,1959552,11757312,70543872,
%T A168876 423263232,2539579392,15237476352,91424858112,548549148672,
%U A168876 3291294892032,19747769352192,118486616113152,710919696678912,4265518180073472
%N A168876 Number of reduced words of length n in Coxeter group on 7 generators S_i with relations (S_i)^2 = (S_i S_j)^21 = I.
%C A168876 The initial terms coincide with those of A003949, although the two sequences are eventually different.
%C A168876 First disagreement at index 21: a(21) = 25593109080440811, A003949(21) = 25593109080440832. - Klaus Brockhaus, Apr 04 2011
%C A168876 Computed with MAGMA using commands similar to those used to compute A154638.
%H A168876 <a href="/index/Rec#order_21">Index entries for linear recurrences with constant coefficients</a>, signature (5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, -15).
%F A168876 G.f.: (t^21 + 2*t^20 + 2*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)/(15*t^21 - 5*t^20 - 5*t^19 - 5*t^18 - 5*t^17 - 5*t^16 - 5*t^15 - 5*t^14 - 5*t^13 - 5*t^12 - 5*t^11 - 5*t^10 - 5*t^9 - 5*t^8 - 5*t^7 - 5*t^6 - 5*t^5 - 5*t^4 - 5*t^3 - 5*t^2 - 5*t + 1).
%Y A168876 Cf. A003949 (G.f.: (1+x)/(1-6*x)).
%K A168876 nonn
%O A168876 0,2
%A A168876 _John Cannon_ and _N. J. A. Sloane_, Dec 03 2009