A339066 Number of unlabeled loopless multigraphs with n edges rooted at two indistinguishable vertices.

1, 3, 12, 44, 171, 664, 2688, 11133, 47682, 210275, 955940, 4473128, 21532160, 106504216, 540824997, 2816636171, 15031261538, 82123830645, 458979942506, 2621982351176, 15298840540234, 91112889589166, 553492059017778, 3427579611162937, 21625096669854023, 138927108066308515

Offset: 0

Author

Andrew Howroyd, Nov 22 2020

Keywords

• nonn

Examples

			The a(1) = 3 cases correspond to a single edge which can be attached to zero, one or both of the roots.
		

Crossrefs

Cf. A050535, A007717 (one root), A339043, A339064, A339065.

Programs

Mathematica

seq[n_] := G[2n, x + O[x]^n, {1, 1}] + G[2n, x + O[x]^n, {2}] // CoefficientList[#/2, x]&;
seq[15] (* Jean-François Alcover, Dec 02 2020, after Andrew Howroyd's code for G in A339065 *)

PARI

\\ See A339065 for G.
seq(n)={my(A=O(x*x^n)); Vec((G(2*n, x+A, [1, 1]) + G(2*n, x+A, [2]))/2)}