a(n) is the number of closed BCK (a.k.a. affine) lambda terms of size n.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =3a(5) =9a(6) =30a(7) =81a(8) =242a(9) =838a(10) =2799a(11) =9365a(12) =33616a(13) =122937a(14) =449698a(15) =1696724a(16) =6558855a(17) =25559806a(18) =101294687a(19) =409363758a(20) =1673735259a(21) =6928460475a(22) =29115833976a(23) =123835124242a(24) =532449210893