cp's OEIS Frontend

A168911 Number of reduced words of length n in Coxeter group on 42 generators S_i with relations (S_i)^2 = (S_i S_j)^21 = I.

%I A168911 #8 Nov 25 2016 09:43:28
%S A168911 1,42,1722,70602,2894682,118681962,4865960442,199504378122,
%T A168911 8179679503002,335366859623082,13750041244546362,563751691026400842,
%U A168911 23113819332082434522,947666592615379815402,38854330297230572431482
%N A168911 Number of reduced words of length n in Coxeter group on 42 generators S_i with relations (S_i)^2 = (S_i S_j)^21 = I.
%C A168911 The initial terms coincide with those of A170761, although the two sequences are eventually different.
%C A168911 First disagreement at index 21: a(21) = 7567046884169679039127253534720781, A170761(21) = 7567046884169679039127253534721642. - Klaus Brockhaus, Apr 08 2011
%C A168911 Computed with MAGMA using commands similar to those used to compute A154638.
%H A168911 <a href="/index/Rec#order_21">Index entries for linear recurrences with constant coefficients</a>, signature (40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, 40, -820).
%F A168911 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)/(820*t^21 - 40*t^20 - 40*t^19 - 40*t^18 - 40*t^17 - 40*t^16 - 40*t^15 - 40*t^14 - 40*t^13 - 40*t^12 - 40*t^11 - 40*t^10 - 40*t^9 - 40*t^8 - 40*t^7 - 40*t^6 - 40*t^5 - 40*t^4 - 40*t^3 - 40*t^2 - 40*t + 1).
%Y A168911 Cf. A170761 (G.f.: (1+x)/(1-41*x)).
%K A168911 nonn
%O A168911 0,2
%A A168911 _John Cannon_ and _N. J. A. Sloane_, Dec 03 2009