Number of unlabeled hereditary semiorders on n points.

Terms

a(0) =1a(1) =2a(2) =5a(3) =14a(4) =42a(5) =132a(6) =428a(7) =1415a(8) =4730a(9) =15901a(10) =53593a(11) =180809a(12) =610157a(13) =2058962a(14) =6947145a(15) =23437854a(16) =79067006a(17) =266717300a(18) =899693960a(19) =3034814143a(20) =10236853534a(21) =34530252629a(22) =116475001757a(23) =392885252033