A027838 Number of subgroups of index n in fundamental group of a certain fiber space.

1, 7, 22, 111, 486, 3772, 29142, 275871, 2830459, 32028882, 392744078, 5201524044, 73943424582, 1123603726896, 18176728661832, 311951284854975, 5661698774848910, 108355867352001811, 2181096921557783966, 46066653293892718506, 1018705098450761473704

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn

References

• 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(c), Eq. (5.125).

Crossrefs

Cf. A027841, A027839, A027840.

Formula

a(n) = Sum_{m|n} A003319(m+1) * (n/m)^(m+1). - Sean A. Irvine, Dec 06 2019

Extensions

More terms from Sean A. Irvine, Dec 06 2019