OFFSET
0,2
REFERENCES
P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 160, Sigma(Gamma_2, S_2; z).
LINKS
Colin Barker, Table of n, a(n) for n = 0..1000
Index entries for linear recurrences with constant coefficients, signature (6,6,6,-1).
FORMULA
G.f.: (1 + x)^2*(1 + x^2) / (1 - 6*x - 6*x^2 - 6*x^3 + x^4).
a(n) = 6*a(n-1) + 6*a(n-2) + 6*a(n-3) - a(n-4) for n>4. - Colin Barker, Jun 13 2019
PROG
(PARI) Vec((1 + x)^2*(1 + x^2) / (1 - 6*x - 6*x^2 - 6*x^3 + x^4) + O(x^20)) \\ Colin Barker, Jun 13 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Aug 21 2001
STATUS
approved