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%I #20 Oct 22 2015 14:48:25
%S 1,4,28,232,2092,19884,196096,1988424,20611116,217526524,2330681348,
%T 25296553088,277653104800,3077568629256,34410056828392,
%U 387725845018512,4399241841920428,50228061806093020,576729989899675348
%N Cogrowth function of the group Baumslag-Solitar(4,4).
%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(4,4)=<a,t | ta^4=a^4t>.
%H Murray Elder, <a href="/A229646/b229646.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