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P. de la Harpe, Topics in Geometric Group Theory, Univ. Chicago Press, 2000, p. 23.
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.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(b).
Table of n, a(n) for n=1..9.
J. H. Kwak and J. Lee, Enumeration of graph coverings and surface branched coverings, Lecture Note Series 1 (2001), Com^2MaC-KOSEF, Korea. See chapter 3.
Main diagonal of A049290.
Sequence in context: A201843 A278202 A246537 * A167582 A068420 A276188
Adjacent sequences: A057011 A057012 A057013 * A057015 A057016 A057017
N. J. A. Sloane, Sep 09 2000
More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001