Number of non-isomorphic set-systems of weight n with no singletons.

Terms

a(0) =1a(1) =0a(2) =1a(3) =1a(4) =3a(5) =4a(6) =12a(7) =19a(8) =51a(9) =106a(10) =274a(11) =647a(12) =1773a(13) =4664a(14) =13418a(15) =38861a(16) =118690a(17) =370588a(18) =1202924a(19) =4006557a(20) =13764760a(21) =48517672a(22) =175603676a(23) =651026060a(24) =2471150365a(25) =9590103580a(26) =38023295735a(27) =153871104726a(28) =635078474978