Number of n-element posets which are unions of 2 chains.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =26a(6) =75a(7) =225a(8) =711a(9) =2311a(10) =7725a(11) =26313a(12) =91141a(13) =319749a(14) =1134234a(15) =4060128a(16) =14648614a(17) =53208998a(18) =194423568a(19) =714130372a(20) =2635256408a(21) =9764995800a(22) =36320086418a(23) =135548135854a(24) =507434502474