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

Terms

a(0) =1a(1) =2a(2) =6a(3) =18a(4) =52a(5) =130a(6) =310a(7) =718a(8) =1618a(9) =3670a(10) =8160a(11) =17862a(12) =38872a(13) =84016a(14) =180502a(15) =386092a(16) =821640a(17) =1742536a(18) =3681130a(19) =7754076a(20) =16290046a(21) =34136028a(22) =71378568a(23) =148948138a(24) =310248240a(25) =645155196a(26) =1339484196a(27) =2777105128a(28) =5750079046a(29) =11891268400