A057012 Number of conjugacy classes of subgroups of index 6 in free group of rank n.

1, 624, 504243, 371515454, 268530771271, 193466859054994, 139311082645798043, 100305771690618678654, 72220370631411094037391, 51998692654400641678907114, 37439061807069469917891862243

Offset: 1

Author

N. J. A. Sloane, Sep 09 2000

Keywords

• nonn

References

• R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.13(c), pp. 76, 112.

Links

• J. H. Kwak and J. Lee, Enumeration of connected graph coverings, J. Graph Th., 23 (1996), 105-109.
• 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.
• V. A. Liskovets, Reductive enumeration under mutually orthogonal group actions, Acta Applic. Math., 52 (1998), 91-120.

Crossrefs

Cf. A057004-A057013.

Programs

PARI

a(n)=if(n<0,0,n--;720^n-120^n+24^n+18^n-16^n+12^n*2-36^n/2-9^n/2+8^n*2/3-6^n/2-4^n*3/2-3^n/2+2^n*5/6)

Extensions

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