The unique primitive prime factor of 2^n-1 for the n in A161508.

Terms

a(0) =3a(1) =7a(2) =5a(3) =31a(4) =127a(5) =17a(6) =73a(7) =11a(8) =13a(9) =8191a(10) =43a(11) =151a(12) =257a(13) =131071a(14) =19a(15) =524287a(16) =41a(17) =337a(18) =683a(19) =241a(20) =2731a(21) =262657a(22) =331a(23) =2147483647a(24) =65537a(25) =599479a(26) =43691a(27) =174763a(28) =61681a(29) =5419