OFFSET
0,2
COMMENTS
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(2,2)=<a,t | ta^2=a^2t>.
LINKS
Murray Elder, Table of n, a(n) for n = 0..50
M. Elder, A. Rechnitzer, E. J. Janse van Rensburg, T. Wong, The cogrowth series for BS(N,N) is D-finite
Wikipedia, Baumslag-Solitar group
EXAMPLE
For n=1 there are 4 words of length 2 equal to the identity: aa^{-1}, a^{-1}a, tt^{-1}, t^{-1}t.
CROSSREFS
KEYWORD
nonn,walk
AUTHOR
Murray Elder, Sep 27 2013
STATUS
approved