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A005641 Number of 2-diregular digraphs with n nodes.
(Formerly M2757)
3
1, 3, 8, 27, 131, 711, 5055, 41607, 389759, 4065605, 46612528, 581713045, 7846380548, 113718755478, 1762208816647, 29073392136390, 508777045979418 (list; graph; refs; listen; history; text; internal format)
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 A148845

Adjacent sequences:  A005638 A005639 A005640 * A005642 A005643 A005644

KEYWORD

nonn

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified November 20 20:28 EST 2019. Contains 329347 sequences. (Running on oeis4.)