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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =18a(5) =64a(6) =227a(7) =856a(8) =3280a(9) =12885a(10) =51342a(11) =207544a(12) =847886a(13) =3497384a(14) =14541132a(15) =60884173a(16) =256480895a(17) =1086310549a(18) =4623128656a(19) =19759964149a(20) =84784735379a(21) =365066645854