a(n) is the least prime of the form 2^m - 2^n - 1.

Terms

a(0) =2a(1) =5a(2) =3a(3) =7a(4) =47a(5) =31a(6) =191a(7) =127a(8) =16127a(9) =3583a(10) =15359a(11) =6143a(12) =1044479a(13) =8191a(14) =245759a(15) =16744447a(16) =4128767a(17) =131071a(18) =786431a(19) =524287a(20) =274876858367a(21) =14680063a(23) =260046847a(24) =4278190079a(25) =4261412863a(28) =16911433727