a(n) = 2^(prime(n) + 1) + 1.

Terms

a(0) =9a(1) =17a(2) =65a(3) =257a(4) =4097a(5) =16385a(6) =262145a(7) =1048577a(8) =16777217a(9) =1073741825a(10) =4294967297a(11) =274877906945