2^p + 3 where p is prime.

Terms

a(0) =7a(1) =11a(2) =35a(3) =131a(4) =2051a(5) =8195a(6) =131075a(7) =524291a(8) =8388611a(9) =536870915a(10) =2147483651a(11) =137438953475