Primes of form (p^x - 1)/(p^y - 1), p prime.

Terms

a(0) =3a(1) =5a(2) =7a(3) =13a(4) =17a(5) =31a(6) =73a(7) =127a(8) =257a(9) =307a(10) =757a(11) =1093a(12) =1723a(13) =2801a(14) =3541a(15) =5113a(16) =8011a(17) =8191a(18) =10303a(19) =17293a(20) =19531a(21) =28057a(22) =30103a(23) =30941a(24) =65537a(25) =86143a(26) =88741a(27) =131071a(28) =147073a(29) =262657