Primes of the form 2^k - 3.

Terms

a(0) =5a(1) =13a(2) =29a(3) =61a(4) =509a(5) =1021a(6) =4093a(7) =16381a(8) =1048573a(9) =4194301a(10) =16777213a(11) =536870909