G.f.: Sum_{n>=0} x^n * (1+x)^(2^n).

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =15a(5) =49a(6) =210a(7) =1191a(8) =8981a(9) =90405a(10) =1219297a(11) =22105506a(12) =540476679a(13) =17875316557a(14) =802011318369