2^p + 7 where p is prime.

Terms

a(0) =11a(1) =15a(2) =39a(3) =135a(4) =2055a(5) =8199a(6) =131079a(7) =524295a(8) =8388615a(9) =536870919a(10) =2147483655a(11) =137438953479