OFFSET
0,2
REFERENCES
P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 23.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(b).
LINKS
Harvey P. Dale, Table of n, a(n) for n = 0..1000
M. Hall, Subgroups of finite index in free groups, Canad. J. Math., 1 (1949), 187-190.
V. A. Liskovets and A. Mednykh, Enumeration of subgroups in the fundamental groups of orientable circle bundles over surfaces, Commun. in Algebra, 28, No. 4 (2000), 1717-1738.
Index entries for linear recurrences with constant coefficients, signature (9,-20,12).
FORMULA
a(n) = 3*6^n-3*2^n+1.
G.f.: (1+4*x)/((1-x)*(1-2*x)*(1-6*x)). [Colin Barker, May 08 2012]
MATHEMATICA
LinearRecurrence[{9, -20, 12}, {1, 13, 97}, 20] (* Harvey P. Dale, Sep 24 2017 *)
CROSSREFS
KEYWORD
easy,nice,nonn
AUTHOR
EXTENSIONS
More terms from Karen Richardson (s1149414(AT)cedarville.edu)
STATUS
approved