a(n) is the (2^n)-th powerful number.

Terms

a(0) =1a(1) =4a(2) =9a(3) =32a(4) =121a(5) =392a(6) =1352a(7) =5000a(8) =18432a(9) =69192a(10) =265837a(11) =1024144a(12) =3968064a(13) =15523600a(14) =60972500a(15) =240413400a(16) =950612224a(17) =3767130288a(18) =14959246864a(19) =59495990724a(20) =236902199076a(21) =944193944097