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A007108
Number of connected labeled 2-regular digraphs with n nodes.
(Formerly M4667)
2
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
KEYWORD
nonn,nice
STATUS
approved