Number of unlabeled PQ-trees with n leaves.

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =9a(5) =29a(6) =105a(7) =390a(8) =1528a(9) =6119a(10) =25140a(11) =104936a(12) =444637a(13) =1905331a(14) =8246619a(15) =35988793a(16) =158199975a(17) =699788234a(18) =3112679085a(19) =13913394416a(20) =62465305846a(21) =281551756181