0,3

CRC Handbook of Combinatorial Designs, 1996, p. 651.

J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 522.

F. Harary, Graph Theory. Addison-Wesley, Reading, MA, 1969, p. 225.

F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 124, Table 5.1.2 and p. 241, Table A4.

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.

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

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

Keith Briggs, Table of n, a(n) for n = 0..64

R. Absil and H Mélot, Digenes: genetic algorithms to discover conjectures about directed and undirected graphs, arXiv preprint arXiv:1304.7993, 2013

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

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

A. Iványi, Leader election in synchronous networks, Acta Univ. Sapientiae, Mathematica, 5, 2 (2013) 54-82.

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, Sep. 15, 1955, pp. 14-22. [Annotated scanned copy]

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

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

J. Qian, Enumeration of unlabeled directed hypergraphs, Electronic Journal of Combinatorics, 20(1) (2013), #P46. - From N. J. A. Sloane, Mar 01 2013

J. M. Tangen and N. J. A. Sloane, Correspondence, 1976-1976

L. Travis, [math/9811127] Graphical Enumeration: A Species-Theoretic Approach

Eric Weisstein's World of Mathematics, Directed Graph

Index entries for "core" sequences

with(combinat):with(numtheory):

for n from 1 to 20 do p:=partition(n):

s:=0:for k from 1 to nops(p) do

q:=convert(p[k], multiset):

for i from 1 to n do a(i):=0:od:for i from 1 to nops(q) do a(q[i][1]):=q[i][2]:od:

c:=1:ord:=1:for i from 1 to n do c:=c*a(i)!*i^a(i): if a(i)<>0 then ord:=lcm(ord, i):fi:od:

g:=0:for d from 1 to ord do if ord mod d=0 then g1:=0:for del from 1 to d do if del<=n and d mod del=0 then g1:=g1+del*a(del):fi:od:g:=g+phi(ord/d)*g1*(g1-1):fi:od:

s:=s+2^(g/ord)/c:

od:

print(n, s):

od:

# Vladeta Jovovic, Jun 06 2006

Table[CycleIndex[PairGroup[SymmetricGroup[n], Ordered], t]/.Table[t[i]->1+x^i, {i, 1, n^2}]/.{x->1}, {n, 1, 7}] (* or *)

  Table[GraphPolynomial[n, t, Directed]/.{t->1}, {n, 1, 20}]

(* Geoffrey Critzer, Mar 08 2011 *)

Sequence in context: A166860 A196562 A113597 * A071897 A182012 A013923

Adjacent sequences:  A000270 A000271 A000272 * A000274 A000275 A000276

nonn,core,nice

N. J. A. Sloane

More terms from Vladeta Jovovic, Dec 14 1999

approved