2^p + 5 where p is prime.

Terms

a(0) =9a(1) =13a(2) =37a(3) =133a(4) =2053a(5) =8197a(6) =131077a(7) =524293a(8) =8388613a(9) =536870917a(10) =2147483653a(11) =137438953477