OFFSET
0,2
COMMENTS
Euler transform of A007719.
Also the number of non-isomorphic multiset partitions of {1, 1, 2, 2, 3, 3, ..., n, n}. - Gus Wiseman, Jul 18 2018
Number of distinct n X 2n matrices with integer entries and rows sums 2, up to row and column permutations. - Andrew Howroyd, Sep 06 2018
a(n) is the number of unlabeled loopless multigraphs with n edges rooted at one vertex. - Andrew Howroyd, Nov 22 2020
REFERENCES
Huaien Li and David C. Torney, Enumerations of Multigraphs, 2002.
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..50
Huaien Li and David C. Torney, Enumeration of unlabelled multigraphs, Ars Combin. 75 (2005) 171-188. MR2133219.
R. J. Mathar, Statistics on Small Graphs, arXiv:1709.09000 [math.CO] (2017) table 67.
EXAMPLE
a(2) = 7 (here - denotes an edge, = denotes a pair of parallel edges and o is a loop):
oo
o o
o-
o -
=
--
- -
From Gus Wiseman, Jul 18 2018: (Start)
Non-isomorphic representatives of the a(2) = 7 multiset partitions of {1, 1, 2, 2}:
(1122),
(1)(122), (11)(22), (12)(12),
(1)(1)(22), (1)(2)(12),
(1)(1)(2)(2).
(End)
From Gus Wiseman, Jan 08 2024: (Start)
Non-isomorphic representatives of the a(1) = 1 through a(3) = 7 rooted loopless multigraphs (root shown as singleton):
{{1}} {{1},{1,2}} {{1},{1,2},{1,2}}
{{1},{2,3}} {{1},{1,2},{1,3}}
{{1},{1,2},{2,3}}
{{1},{1,2},{3,4}}
{{1},{2,3},{2,3}}
{{1},{2,3},{2,4}}
{{1},{2,3},{4,5}}
(End)
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];
Kq[q_, t_, k_] := SeriesCoefficient[1/Product[g = GCD[t, q[[j]]]; (1 - x^(q[[j]]/g))^g, {j, 1, Length[q]}], {x, 0, k}];
RowSumMats[n_, m_, k_] := Module[{s=0}, Do[s += permcount[q]* SeriesCoefficient[Exp[Sum[Kq[q, t, k]/t x^t, {t, 1, n}]], {x, 0, n}], {q, IntegerPartitions[m]}]; s/m!];
a[n_] := RowSumMats[n, 2n, 2];
Table[an = a[n]; Print["a(", n, ") = ", an]; an, {n, 0, 25}] (* Jean-François Alcover, Oct 27 2018, after Andrew Howroyd *)
PROG
(PARI) \\ See A318951 for RowSumMats
a(n)=RowSumMats(n, 2*n, 2); \\ Andrew Howroyd, Sep 06 2018
(PARI) \\ See A339065 for G.
seq(n)={my(A=O(x*x^n)); Vec(G(2*n, x+A, [1]))} \\ Andrew Howroyd, Nov 22 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
More terms from Vladeta Jovovic, Jan 26 2000
a(0)=1 prepended and a(16)-a(25) added by Max Alekseyev, Jun 21 2011
STATUS
approved