A007108 Number of connected labeled 2-regular digraphs with n nodes. (Formerly M4667)

0, 0, 0, 1, 9, 216, 7560, 357120, 22025430, 1720751760, 166198926600, 19453788144000, 2714247736061400, 445133524289731200, 84788348720139464400, 18565341317290193025600, 4631048864538293219022000, 1305644758257711732981120000, 413133912085072442199311376000

Offset

0,5

Comments

From R. J. Mathar, Apr 29 2019: (Start)

The numbers of labeled 2-regular digraphs with n >= 1 nodes and 1 <= k <= n components are

0;

0, 0;

1, 0, 0;

9, 0, 0, 0;

216, 0, 0, 0, 0;

7560, 10, 0, 0, 0, 0;

357120, 315, 0, 0, 0, 0, 0;

22025430, 14931, 0, 0, 0, 0, 0, 0;

1720751760, 879984, 280, 0, 0, 0, 0, 0, 0;

...

with row sums A007107. (End)

References

R. W. Robinson, personal communication.

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1982.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

Alois P. Heinz, Table of n, a(n) for n = 0..253 (first 49 terms from R. W. Robinson)

Crossrefs

Cf. A007107 (EXP-transform).

Sequence in context: A109587 A067426 A250548 * A007107 A217042 A064633

Adjacent sequences: A007105 A007106 A007107 * A007109 A007110 A007111

Keyword

nonn,nice

Author

N. J. A. Sloane

Status

approved