1,3

As usual, there can be an edge both from i to j and from j to i.

R. C. Read and R. J. Wilson, An Atlas of Graphs, Oxford, 1998.

R. W. Robinson, Counting labeled acyclic digraphs, pp. 239-273 of F. Harary, editor, New Directions in the Theory of Graphs. Academic Press, NY, 1973.

R. W. Robinson, personal communication.

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1980.

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

R. W. Robinson, Table of n, a(n) for n = 1..18

Huantian Cao, AutoGF: An Automated System to Calculate Coefficients of Generating Functions.

a(3) = 18 (the symbol = denotes a pair of parallel edges): 1->2->3->1 (2 such); 1=2->3->1 (6); 1=2=3->1 (6); 1=2=3=1 (1); 1=2=3 (3).

Cf. A035512, A054946 (strongly connected labeled tournaments).

Sequence in context: A152933 A160252 A160307 this_sequence A086366 A086193 A064347

Adjacent sequences: A003027 A003028 A003029 this_sequence A003031 A003032 A003033

nonn,nice

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