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A004102 Number of signed graphs with n nodes. Also number of 2-multigraphs on n nodes.
(Formerly M2874)
20
1, 1, 3, 10, 66, 792, 25506, 2302938, 591901884, 420784762014, 819833163057369, 4382639993148435207, 64588133532185722290294, 2638572375815762804156666529, 300400208094064113266621946833097, 95776892467035669509813163910815022152 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

A 2-multigraph is similar to an ordinary graph except there are 0, 1 or 2 edges between any two nodes (self-loops are not allowed).

REFERENCES

F. Harary and R. W. Robinson, Exposition of the enumeration of point-line-signed graphs, pp. 19 - 33 of Proc. Second Caribbean Conference Combinatorics and Computing (Bridgetown, 1977). Ed. R. C. Read and C. C. Cadogan. University of the West Indies, Cave Hill Campus, Barbados, 1977. vii+223 pp.

R. W. Robinson, personal communication.

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

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

LINKS

Andrew Howroyd, Table of n, a(n) for n = 0..50 (terms 1..22 from R. W. Robinson)

M. Adamaszek, The smallest nonevasive graph property, Disc. Mathem. Graph Theory 34 (2014) 857

Edward A. Bender and E. Rodney Canfield, Enumeration of connected invariant graphs, Journal of Combinatorial Theory, Series B 34.3 (1983): 268-278. See p. 273.

J. Cummings, D. Kral, F. Pfender, K. Sperfeld et al., Monochromatic triangles in three-coloured graphs, arXiv preprint arXiv:1206.1987 [math.CO]. 2012. - From N. J. A. Sloane, Nov 25 2012

Harald Fripertinger, The cycle type of the induced action on 2-subsets

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.

Vladeta Jovovic, Formulae for the number T(n,k) of n-multigraphs on k nodes

R. W. Robinson, Notes - "A Present for Neil Sloane"

R. W. Robinson, Notes - computer printout

R. W. Robinson & N. J. A. Sloane, Correspondence, 1970-1980

FORMULA

Euler transform of A053465. - Andrew Howroyd, Sep 25 2018

MATHEMATICA

permcount[v_] := Module[{m=1, s=0, k=0, t}, For[i=1, i <= Length[v], i++, t = v[[i]]; k = If[i>1 && t == v[[i-1]], k+1, 1]; m *= t*k; s += t]; s!/m];

edges[v_] := Sum[Sum[GCD[v[[i]], v[[j]]], {j, 1, i-1}], {i, 2, Length[v]}] + Sum[Quotient[v[[i]], 2], {i, 1, Length[v]}];

a[n_] := Module[{s = 0}, Do[s += permcount[p]*3^edges[p], {p, IntegerPartitions[n]}]; s/n!];

Array[a, 16, 0] (* Jean-Fran├žois Alcover, Aug 17 2019, after Andrew Howroyd *)

PROG

(PARI)

permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m}

edges(v) = {sum(i=2, #v, sum(j=1, i-1, gcd(v[i], v[j]))) + sum(i=1, #v, v[i]\2)}

a(n) = {my(s=0); forpart(p=n, s+=permcount(p)*3^edges(p)); s/n!} \\ Andrew Howroyd, Sep 25 2018

CROSSREFS

A column of A063841.

Cf. A053465.

Sequence in context: A306187 A009400 A217388 * A072638 A262843 A080526

Adjacent sequences:  A004099 A004100 A004101 * A004103 A004104 A004105

KEYWORD

nonn,nice,easy

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Vladeta Jovovic, Jan 06 2000

a(0)=1 prepended and a(15) added by Andrew Howroyd, Sep 25 2018

STATUS

approved

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Last modified May 14 22:40 EDT 2021. Contains 343909 sequences. (Running on oeis4.)