a(n) = 2*(4^n - 1) / A027760(n).

Terms

a(0) =3a(1) =5a(2) =63a(3) =17a(4) =1023a(5) =195a(6) =16383a(7) =4369a(8) =262143a(9) =31775a(10) =4194303a(11) =12291a(12) =67108863a(13) =89478485a(14) =1073741823a(15) =16843009a(16) =17179869183a(17) =172229265a(18) =274877906943a(19) =6663706835a(21) =254959218035a(23) =206208774147