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

1,2

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).

Links

Table of n, a(n) for n=1..21.

Formula

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

Crossrefs

Cf. A027841, A027839, A027840.

Sequence in context: A231339 A222178 A117110 * A194996 A330089 A000835

Adjacent sequences: A027835 A027836 A027837 * A027839 A027840 A027841

Keyword

nonn

Author

N. J. A. Sloane

Extensions

More terms from Sean A. Irvine, Dec 06 2019

Status

approved