a(n) is the number of unlabeled connected graphs without endpoints with n edges.

Terms

a(0) =1a(1) =0a(2) =0a(3) =1a(4) =1a(5) =2a(6) =5a(7) =9a(8) =22a(9) =60a(10) =164a(11) =494a(12) =1624a(13) =5602a(14) =20600a(15) =79813a(16) =323806a(17) =1371025a(18) =6034105a(19) =27513424a(20) =129641411a(21) =629862824a(22) =3149541908a(23) =16183100922a(24) =85328280263a(25) =461123500894