Numbers of closed lambda terms of natural size n.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =3a(5) =6a(6) =17a(7) =41a(8) =116a(9) =313a(10) =895a(11) =2550a(12) =7450a(13) =21881a(14) =65168a(15) =195370a(16) =591007a(17) =1798718a(18) =5510023a(19) =16966529a(20) =52506837a(21) =163200904a(22) =509323732a(23) =1595311747a(24) =5013746254a(25) =15805787496a(26) =49969942138a(27) =158396065350a(28) =503317495573