Number of unlabeled semiorders on n points and having dimension at most 2.

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =42a(5) =132a(6) =426a(7) =1390a(8) =4544a(9) =14822a(10) =48183a(11) =156118a(12) =504487a(13) =1627000a(14) =5240019a(15) =16861453a(16) =54228190a(17) =174351450a(18) =560481708a(19) =1801653769a(20) =5791301311a(21) =18615976402a(22) =59841686254a(23) =192366897839a(24) =618392292337