2^p + 11 where p is prime.

Terms

a(0) =15a(1) =19a(2) =43a(3) =139a(4) =2059a(5) =8203a(6) =131083a(7) =524299a(8) =8388619a(9) =536870923a(10) =2147483659a(11) =137438953483