a(n) = 2*8^n + 1.

Terms

a(0) =3a(1) =17a(2) =129a(3) =1025a(4) =8193a(5) =65537a(6) =524289a(7) =4194305a(8) =33554433a(9) =268435457a(10) =2147483649a(11) =17179869185a(12) =137438953473