a(n) = {2^n}_n.

Terms

a(0) =2a(1) =4a(2) =8a(3) =10a(4) =17a(5) =40a(6) =71a(7) =174a(8) =416a(9) =1024a(10) =2714a(11) =7026a(12) =17864a(13) =55584a(14) =160298a(15) =415030a(16) =1675454a(17) =4421056a(18) =12669455a(19) =64706146a(20) =173347946a(21) =460825600a(22) =1205935387a(23) =5791248030a(24) =19093815077a(25) =50365299040a(26) =315389650247a(27) =840487488658