Indices of prime Mersenne numbers (A001348).

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =6a(5) =7a(6) =8a(7) =11a(8) =18a(9) =24a(10) =28a(11) =31a(12) =98a(13) =111a(14) =207a(15) =328a(16) =339a(17) =455a(18) =583a(19) =602a(20) =1196a(21) =1226a(22) =1357a(23) =2254a(24) =2435a(25) =2591a(26) =4624a(27) =8384a(28) =10489a(29) =12331