Primes of the form q*2^h - 1, where q is a Mersenne prime (A000668).

Terms

a(0) =2a(1) =5a(2) =11a(3) =13a(4) =23a(5) =47a(6) =61a(7) =191a(8) =223a(9) =383a(10) =991a(11) =3583a(12) =3967a(13) =6143a(14) =16381a(15) =63487a(16) =253951a(17) =262111a(18) =786431a(19) =917503a(20) =1048447a(21) =1048573a(22) =4194271a(23) =14680063a(24) =16777183a(25) =67108351a(26) =260046847a(27) =3758096383a(28) =4261412863a(29) =51539607551