1,2

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

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

R. L. Davis, The number of structures of finite relations, Proc. Amer. Math. Soc. 4 (1953), 486-495.

F. Harary and E. M. Palmer, Enumeration of mixed graphs, Proc. Amer. Math. Soc., 17 (1966), 682-687.

F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 133, c_p.

M. D. McIlroy, Calculation of numbers of structures of relations on finite sets, Massachusetts Institute of Technology, Research Laboratory of Electronics, Quarterly Progress Reports, No. 17, Sept. 15, 1955, pp. 14-22.

P. J. Cameron, Sequences realized by oligomorphic permutation groups, J. Integ. Seqs. Vol. 3 (2000), #00.1.5.

G. Pfeiffer, Counting Transitive Relations, Journal of Integer Sequences, Vol. 7 (2004), Article 04.3.2.

Eric Weisstein's World of Mathematics, Oriented Graph

There's an explicit formula - see for example Harary and Palmer (book), Eq. (5.4.14).

Cf. A000595, A001173.

Cf. A047656 (labeled case), A054941 (connected labeled case), A086345 (connected unlabeled case).

Adjacent sequences: A001171 A001172 A001173 this_sequence A001175 A001176 A001177

Sequence in context: A091669 A108042 A152559 this_sequence A067975 A065298 A091877

nonn,nice,easy

N. J. A. Sloane (njas(AT)research.att.com).

More terms from Vladeta Jovovic (vladeta(AT)eunet.rs)

Revised description from Vladeta Jovovic (vladeta(AT)eunet.rs), Jan 20 2005