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.

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).

Adjacent sequences: A003027 A003028 A003029 this_sequence A003031 A003032 A003033

Sequence in context: A052135 A033518 A064564 this_sequence A086366 A086193 A064347

nonn,nice

njas