Number of weakly ordered plane trees with n leaves.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =34a(6) =92a(7) =277a(8) =806a(9) =2500a(10) =7578a(11) =24198a(12) =75370a(13) =243800a(14) =776494a(15) =2545777a(16) =8223352a(17) =27221690a(18) =88984144a(19) =296856400a(20) =979829772a(21) =3287985078a(22) =10934749788a(23) =36912408342a(24) =123519937044a(25) =418650924886