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

0,2

Links

Table of n, a(n) for n=0..25.

Example

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

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 *)

Prog

(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)}

Crossrefs

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

Sequence in context: A109437 A331473 A005656 * A260146 A229936 A258626

Adjacent sequences: A339063 A339064 A339065 * A339067 A339068 A339069

Keyword

nonn

Author

Andrew Howroyd, Nov 22 2020

Status

approved