A005641 Number of 2-diregular digraphs with n nodes. (Formerly M2757)

1, 3, 8, 27, 131, 711, 5055, 41607, 389759, 4065605, 46612528, 581713045, 7846380548, 113718755478, 1762208816647, 29073392136390, 508777045979418

Offset

2,2

Comments

These graphs may have loops, whereas A219889 does not admit them. Both sequences do not admit multiedges. - R. J. Mathar, Mar 27 2019

References

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

Links

Table of n, a(n) for n=2..18.

Richard J. Mathar, 2-regular Digraphs of the Lovelock Lagrangian, arXiv:1903.12477 [math.GM], 2019.

M. V. S. Ramanath and T. R. Walsh, Enumeration and generation of a class of regular digraphs, J. Graph Theory 11 (1987), no. 4, 471-479. (Annotated scanned copy)

Crossrefs

Cf. A005642 (inverse Euler transform, connected graphs).

Sequence in context: A294197 A030495 A074271 * A260961 A230822 A332727

Adjacent sequences: A005638 A005639 A005640 * A005642 A005643 A005644

Keyword

nonn

Author

N. J. A. Sloane

Status

approved