Number of unlabeled connected leafless loopless multigraphs with n edges.

Terms

a(0) =1a(1) =0a(2) =1a(3) =2a(4) =4a(5) =9a(6) =26a(7) =68a(8) =217a(9) =718a(10) =2553a(11) =9574a(12) =38005a(13) =157306a(14) =679682a(15) =3047699a(16) =14150278a(17) =67844305a(18) =335262807a(19) =1704500229a(20) =8902528600a(21) =47704608478a(22) =261960998230