Number of unlabeled connected series-parallel posets with n nodes.

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =30a(5) =103a(6) =375a(7) =1400a(8) =5380a(9) =21073a(10) =83950a(11) =338878a(12) =1383576a(13) =5702485a(14) =23696081a(15) =99163323a(16) =417553252a(17) =1767827220a(18) =7520966100a(19) =32135955585a(20) =137849390424a(21) =593407692685