Number of pairs of sets of cardinality at least 3.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =0a(6) =20a(7) =70a(8) =182a(9) =420a(10) =912a(11) =1914a(12) =3938a(13) =8008a(14) =16172a(15) =32526a(16) =65262a(17) =130764a(18) =261800a(19) =523906a(20) =1048154a(21) =2096688a(22) =4193796a(23) =8388054a(24) =16776614a(25) =33553780a(26) =67108160a(27) =134216970