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 A004105 Number of point-self-dual nets with 2n nodes. Also number of directed 2-multigraphs with loops on n nodes. (Formerly M3153) 8
 1, 3, 45, 3411, 1809459, 7071729867, 208517974495911, 47481903377454219975, 85161307642554753639601848, 1221965550839348597865127102714827, 142024245093355901785105779901319683262778, 135056692539998733060710198802224149631056479068139 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 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). Also nonisomorphic relations on 3-state logic. 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 R. W. Robinson and Alois P. Heinz, Table of n, a(n) for n = 0..40 (terms n = 1..13 from R. W. Robinson) Frank Harary, Edgar M. Palmer, Robert W. Robinson, Allen J. Schwenk, Enumeration of graphs with signed points and lines, J. Graph Theory 1 (1977), no. 4, 295-308. R. W. Robinson, Notes - "A Present for Neil Sloane" R. W. Robinson, Notes - computer printout FORMULA a(n) = sum_{1*s_1+2*s_2+...=n}(fix A[s_1, s_2, ...]/ (1^s_1*s_1!*2^s_2*s_2!*...)) where fix A[s_1, s_2, ...] = 3^sum_{i, j>=1} (gcd(i,j)*s_i*s_j). MATHEMATICA Prepend[Table[CycleIndex[Join[PairGroup[SymmetricGroup[n], Ordered], Permutations[Range[n^2-n+1, n^2]], 2], s]/.Table[s[i]->3, {i, 1, n^2-n}], {n, 2, 7}], 1] (* Geoffrey Critzer, Oct 20 2012 *) 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[2*GCD[v[[i]], v[[j]]], {i, 2, Length[v]}, {j, 1, i - 1}] + Total[v]; a[n_] := (s=0; Do[s += permcount[p]*3^edges[p], {p, IntegerPartitions[n]}]; s/n!); Array[a, 15, 0] (* Jean-François Alcover, Jul 08 2018, 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, 2*gcd(v[i], v[j]))) + sum(i=1, #v, v[i])} a(n) = {my(s=0); forpart(p=n, s+=permcount(p)*3^edges(p)); s/n!} \\ Andrew Howroyd, Oct 22 2017 CROSSREFS Cf. A000595, A001374, A053467, A053516. Sequence in context: A198952 A099168 A227379 * A060336 A268196 A057863 Adjacent sequences:  A004102 A004103 A004104 * A004106 A004107 A004108 KEYWORD easy,nonn AUTHOR EXTENSIONS More terms from Vladeta Jovovic, Jan 14 2000 Formula from Christian G. Bower, Jan 06 2004 STATUS approved

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Last modified April 18 22:02 EDT 2021. Contains 343090 sequences. (Running on oeis4.)