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

Terms

a(0) =0a(1) =1a(2) =1a(3) =4a(4) =19a(5) =76a(6) =271a(7) =904a(8) =2899a(9) =9076a(10) =27991a(11) =85504a(12) =259579a(13) =784876a(14) =2366911a(15) =7125304a(16) =21425059a(17) =64373476a(18) =193317031a(19) =580344304a(20) =1741819339a(21) =5227030876a(22) =15684238351a(23) =47059006504a(24) =141189602419a(25) =423593973076