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%I #14 Oct 22 2015 14:48:53
%S 1,4,28,232,2092,19864,195352,1970924,20277036,211864264,2241723728,
%T 23969620844,258583473640,2811005437348,30762114003572,
%U 338624821158892,3747021722921964,41656518905688504,465062224305678280,5211973807553021868
%N Cogrowth function of the group Baumslag-Solitar(6,6).
%C a(n) is the number of words of length 2n in the letters a,a^{-1},t,t^{-1} that equal the identity of the group BS(6,6)=<a,t | ta^6=a^6t>.
%H Murray Elder, <a href="/A229648/b229648.txt">Table of n, a(n) for n = 0..50</a>
%H M. Elder, A. Rechnitzer, E. J. Janse van Rensburg, T. Wong, <a href="http://arxiv.org/abs/1309.4184">The cogrowth series for BS(N,N) is D-finite</a>
%H <a href="/index/Gre#groups">Index entries for sequences related to groups</a>
%e For n=1 there are 4 words of length 2 equal to the identity: aa^{-1}, a^{-1}a, tt^{-1}, t^{-1}t.
%Y The cogrowth sequences for BS(N,N) for N = 1..10 are A002894, A229644, A229645, A229646, A229647, A229648, A229649, A229650, A229651, A229652.
%K nonn,walk
%O 0,2
%A _Murray Elder_, Sep 27 2013