0,2

Each node may or may not be related to itself.

Also the number of rooted graphs on n+1 nodes.

The 1-1-correspondence is as follows. Given a rooted graph on n+1 nodes, replace each edge joining the root node to another node by a self-loop at that node and erase the root node. The result is an undirected graph on n nodes which is the graph of the symmetric relation.

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. Davies, The numbers of structures of finite relations, Proc. Amer. Math. Soc., 4 (1953), 486-494.

F. Harary, The number of linear, directed, rooted and connected graphs, Trans. Amer. Math. Soc., 78 (1955), 445-463.

F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, pp. 101, 241.

Harary, Frank; Palmer, Edgar M.; Robinson, Robert W.; Schwenk, Allen J.; Enumeration of graphs with signed points and lines. J. Graph Theory 1 (1977), no. 4, 295-308.

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.

W. Oberschelp, Kombinatorische Anzahlbestimmungen in Relationen, Math. Ann., 174 (1967), 53-78.

R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976.

David Applegate, Table of n, a(n) for n = 0..80 [Shortened file because terms grow rapidly: see Applegate link below for additional terms]

David Applegate, Table of n, a(n) for n = 0..100

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, Rooted Graph

Let G_{n+1,k} be the number of rooted graphs on n+1 nodes with k edges and let G_{n+1}(x) = Sum_{k=0..n(n+1)/2} G_{n+1,k} x^k. Thus a(n) = G_{n+1}(1). Let S_n(x_1, ..., x_n) denote the cycle index for Sym_n. (cf. the link in A000142).

Compute x_1*S_n and regard it as the cycle index of a set of permutations on n+1 points and find the corresponding cycle index for the action on the n(n+1)/2 edges joining those points (the corresponding "pair group"). Finally, by replacing each x_i by 1+x^i gives G_{n+1}(x). [Harary]

Example, n=2. S_2 = (1/2)*(x_1^2+x_2), x_1*S_2 = (1/2)*(x_1^3+x_1*x_2). The pair group is (1/2)*(x_1^2+x_1*x_2) and so G_3(x) = (1/2)*((1+x)^3+(1+x)*(1+x^2)) = 1+2*x+2*x^2+x^3; set x=1 to get a(2) = 6.

Cf. A000595, A001172, A001174, A006905, A000250.

Sequence in context: A004104 A079468 A124382 this_sequence A027321 A027315 A005965

Adjacent sequences: A000663 A000664 A000665 this_sequence A000667 A000668 A000669

nonn,nice

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

Description corrected by Christian Bower (bowerc(AT)usa.net). More terms from Vladeta Jovovic (vladeta(AT)eunet.rs), Apr 18 2000

Entry revised by N. J. A. Sloane (njas(AT)research.att.com), Mar 06 2007