a(n) = 3*8^n + 1.

Terms

a(0) =4a(1) =25a(2) =193a(3) =1537a(4) =12289a(5) =98305a(6) =786433a(7) =6291457a(8) =50331649a(9) =402653185a(10) =3221225473a(11) =25769803777a(12) =206158430209