A027839 Number of subgroups of index n in fundamental group of a certain fiber space.

1, 15, 124, 2431, 68766, 3025596, 173773496, 12786773247, 1169623901221, 130305653188890, 17376934722757908, 2733655198336260124, 501034099176714376118, 105847486572700895182728, 25534322201330399433420024, 6976464857439636995547805183

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, A027838, A027840.

Formula

a(n) = Sum_{m|n} A027837(m) * (n/m)^(2*m+1). - Sean A. Irvine, Dec 06 2019

Extensions

More terms from Sean A. Irvine, Dec 06 2019