Semi-sums (average) of any two distinct Mersenne primes (A000668).

Terms

a(0) =5a(1) =17a(2) =19a(3) =65a(4) =67a(5) =79a(6) =4097a(7) =4099a(8) =4111a(9) =4159a(10) =65537a(11) =65539a(12) =65551a(13) =65599a(14) =69631a(15) =262145a(16) =262147a(17) =262159a(18) =262207a(19) =266239a(20) =327679a(21) =1073741825a(22) =1073741827a(23) =1073741839a(24) =1073741887a(25) =1073745919a(26) =1073807359a(27) =1074003967