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

Terms

a(0) =3a(1) =5a(2) =11a(3) =23a(4) =59a(5) =137a(6) =313a(7) =727a(8) =1621a(9) =3673a(10) =8167a(11) =17881a(12) =38891a(13) =84047a(14) =180511a(15) =386117a(16) =821647a(17) =1742539a(18) =3681149a(19) =7754081a(20) =16290073a(21) =34136059a(22) =71378603a(23) =148948141a(24) =310248251a(25) =645155227a(26) =1339484207a(27) =2777105131a(28) =5750079077