# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a339066 Showing 1-1 of 1 %I A339066 #10 Feb 26 2023 13:42:06 %S A339066 1,3,12,44,171,664,2688,11133,47682,210275,955940,4473128,21532160, %T A339066 106504216,540824997,2816636171,15031261538,82123830645,458979942506, %U A339066 2621982351176,15298840540234,91112889589166,553492059017778,3427579611162937,21625096669854023,138927108066308515 %N A339066 Number of unlabeled loopless multigraphs with n edges rooted at two indistinguishable vertices. %e A339066 The a(1) = 3 cases correspond to a single edge which can be attached to zero, one or both of the roots. %t A339066 seq[n_] := G[2n, x + O[x]^n, {1, 1}] + G[2n, x + O[x]^n, {2}] // CoefficientList[#/2, x]&; %t A339066 seq[15] (* _Jean-François Alcover_, Dec 02 2020, after _Andrew Howroyd_'s code for G in A339065 *) %o A339066 (PARI) \\ See A339065 for G. %o A339066 seq(n)={my(A=O(x*x^n)); Vec((G(2*n, x+A, [1, 1]) + G(2*n, x+A, [2]))/2)} %Y A339066 Cf. A050535, A007717 (one root), A339043, A339064, A339065. %K A339066 nonn %O A339066 0,2 %A A339066 _Andrew Howroyd_, Nov 22 2020 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE