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A057006 Number of conjugacy classes of subgroups of index n in free group of rank 3. 2
1, 7, 41, 604, 13753, 504243, 24824785, 1598346352, 129958211233, 13030565312011, 1579721338432537, 227804599861102676, 38541084552054952009, 7560534755192908672087, 1702288146755359962223409 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

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.

P. Vrana, On the algebra of local unitary invariants of pure and mixed quantum states, J. Phys A: Math. Theor. 44 (2011) 225304 doi:10.1088/1751-8113/44/22/225304, Table 2.

LINKS

Table of n, a(n) for n=1..15.

J. B. Geloun, S. Ramgoolam, Counting Tensor Model Observables and Branched Covers of the 2-Sphere, arXiv preprint arXiv:1307.6490, 2013

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.

CROSSREFS

Cf. A057004-A057013. Inverse Euler transform of A152612.

Sequence in context: A146991 A053676 A002701 * A144747 A176090 A183064

Adjacent sequences:  A057003 A057004 A057005 * A057007 A057008 A057009

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Sep 09 2000

EXTENSIONS

More terms from Francisco Salinas (franciscodesalinas(AT)hotmail.com), Dec 25 2001

STATUS

approved

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Last modified July 23 22:18 EDT 2019. Contains 325269 sequences. (Running on oeis4.)