Number of unrooted unlabeled 4-cactus graphs on 3n+1 nodes.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =25a(6) =88a(7) =366a(8) =1583a(9) =7336a(10) =34982a(11) =172384a(12) =867638a(13) =4452029a(14) =23194392a(15) =122462546a(16) =653957197a(17) =3527218134a(18) =19192275883a(19) =105248481503a(20) =581223149532