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%I #5 Feb 24 2017 05:22:29
%S 0,0,0,1,9,55,286,1365,6188,27132,116281,490337,2043275,8439210,
%T 34621041,141290436,574274008,2326683921,9402807817,37923176863,
%U 152705590518,614111175965,2467123420524,9903167265124,39725253489545
%N Number of words of length 2n+1 generated by the two letters s and t that reduce to the identity 1 by using the relations sssssss=1, tt=1 and stst=1. The generators s and t along with the three relations generate the 14-element dihedral group D7.
%D H.S.M. Coxeter and W.O.J. Moser, Generators and Relations for Discrete Groups, Fourth Edition, (p.134).
%F Conjecture: a(n) = 9*a(n-1) - 26*a(n-2) + 25*a(n-3) - 4*a(n-4).
%F Empirical g.f.: x^4 / ((1 - 4*x)*(1 - 5*x + 6*x^2 - x^3)). - _Colin Barker_, Feb 24 2017
%Y Cf. A072266.
%K nonn
%O 1,5
%A Jamaine Paddyfoot and _John W. Layman_, Jul 24 2002
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