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%I #10 Oct 22 2015 14:48:58
%S 1,4,28,232,2092,19864,195352,1970896,20275660,211823800,2240795888,
%T 23951292204,258255572584,2805537209648,30675548482880,
%U 337307986673572,3727607821613388,41377406950962504,461130952671387592,5157535231753964268
%N Cogrowth function of the group Baumslag-Solitar(9,9).
%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(9,9)=<a,t | ta^9=a^9t>.
%H Murray Elder, <a href="/A229651/b229651.txt">Table of n, a(n) for n = 0..49</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 28 2013