The numbers of closed simply typable lambda terms of natural size n.

Terms

a(0) =0a(1) =0a(2) =1a(3) =1a(4) =2a(5) =5a(6) =13a(7) =27a(8) =74a(9) =198a(10) =508a(11) =1371a(12) =3809a(13) =10477a(14) =29116a(15) =82419a(16) =233748a(17) =666201a(18) =1914668a(19) =5528622a(20) =16019330a(21) =46642245a(22) =136326126a(23) =399652720a(24) =1175422931a(25) =3467251920a(26) =10258152021