%I #7 Nov 23 2016 15:41:13
%S 1,20,380,7220,136990,2599200,49316400,935712000,17753871510,
%T 336855735180,6391382632020,121267853544780,2300893742387430,
%U 43656351283440360,828320305398630840,15716259104684097960
%N Number of reduced words of length n in Coxeter group on 20 generators S_i with relations (S_i)^2 = (S_i S_j)^4 = I.
%C The initial terms coincide with those of A170739, although the two sequences are eventually different.
%C Computed with MAGMA using commands similar to those used to compute A154638.
%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (18, 18, 18, -171).
%F G.f.: (t^4 + 2*t^3 + 2*t^2 + 2*t + 1)/(171*t^4 - 18*t^3 - 18*t^2 - 18*t + 1)
%t coxG[{4,171,-18}] (* The coxG program is at A169452 *) (* _Harvey P. Dale_, Dec 14 2015 *)
%K nonn
%O 0,2
%A _John Cannon_ and _N. J. A. Sloane_, Dec 03 2009