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

Terms

a(0) =2a(1) =5a(2) =17a(3) =47a(4) =127a(5) =307a(6) =709a(7) =1613a(8) =3659a(9) =8147a(10) =17851a(11) =38867a(12) =84011a(13) =180497a(14) =386083a(15) =821603a(16) =1742527a(17) =3681113a(18) =7754017a(19) =16290041a(20) =34136021a(21) =71378551a(22) =148948133a(23) =310248233a(24) =645155191a(25) =1339484149a(26) =2777105117a(27) =5750079043a(28) =11891268397a(29) =24563311217