0,2
P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 160, Sigma(Gamma_2, S_2; z).
Table of n, a(n) for n=0..19.
G.f.: (1+x)^2*(x^2+1)/(x^4-6*x^3-6*x^2-6*x+1).
Sequence in context: A010556 A010575 A162949 * A234274 A163347 A163924
Adjacent sequences: A063809 A063810 A063811 * A063813 A063814 A063815
nonn
N. J. A. Sloane, Aug 21 2001
approved