Primes of the form 2^r*7^s - 1.

Terms

a(0) =3a(1) =7a(2) =13a(3) =31a(4) =97a(5) =127a(6) =223a(7) =1567a(8) =3583a(9) =4801a(10) =6271a(11) =8191a(12) =19207a(13) =25087a(14) =33613a(15) =76831a(16) =131071a(17) =401407a(18) =524287a(19) =917503a(20) =1229311a(21) =1605631a(22) =3764767a(23) =6588343a(24) =14680063a(25) =184473631a(26) =737894527a(27) =2147483647a(28) =2259801991a(29) =2877292543