Number of unlabeled planar trees (also called plane trees) with n nodes.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =3a(6) =6a(7) =14a(8) =34a(9) =95a(10) =280a(11) =854a(12) =2694a(13) =8714a(14) =28640a(15) =95640a(16) =323396a(17) =1105335a(18) =3813798a(19) =13269146a(20) =46509358a(21) =164107650a(22) =582538732a(23) =2079165208a(24) =7457847082a(25) =26873059986a(26) =97239032056a(27) =353218528324