A007108 Number of connected labeled 2-regular digraphs with n nodes.

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

Offset: 0

Author

N. J. A. Sloane

Keywords

• nonn
• nice

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).