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Search: id:A057010
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| A057010 |
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Number of conjugacy classes of subgroups of index 4 in free group of rank n. |
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+0
1
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| 1, 26, 604, 14120, 334576, 7987616, 191318464, 4588288640, 110090411776, 2641931680256, 63404394241024, 1521689370306560, 36520413978750976, 876488875160477696, 21035724442850934784, 504857317670233210880
(list; graph; listen)
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OFFSET
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1,2
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REFERENCES
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J. H. Kwak and J. Lee, J. Graph Th., 23 (1996), 105-109.
V. A. Liskovets, Reductive enumeration under mutually orthogonal group actions, Acta Applic. Math., 52 (1998), 91-120.
R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(c), pp. 76, 112.
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LINKS
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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.
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FORMULA
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G.f.: x(1-12x)/((1-6x)(1-8x)(1-24x)). a(n)=24^(n-1)+8^(n-1)-6^(n-1).
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PROGRAM
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(PARI) a(n)=if(n<0, 0, 24^(n-1)+8^(n-1)-6^(n-1))
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CROSSREFS
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Cf. A057004-A057013.
Sequence in context: A018208 A001723 A058461 this_sequence A014913 A106793 A097309
Adjacent sequences: A057007 A057008 A057009 this_sequence A057011 A057012 A057013
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KEYWORD
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nonn
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AUTHOR
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njas, Sep 09 2000
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EXTENSIONS
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More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001
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