Number of connected 2-element antichains on a labeled n-set.

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =18a(5) =75a(6) =270a(7) =903a(8) =2898a(9) =9075a(10) =27990a(11) =85503a(12) =259578a(13) =784875a(14) =2366910a(15) =7125303a(16) =21425058a(17) =64373475a(18) =193317030a(19) =580344303a(20) =1741819338a(21) =5227030875a(22) =15684238350a(23) =47059006503a(24) =141189602418a(25) =423593973075