a(n) = largest noncomposite factor of 2^(2n+1) - 1.

Terms

a(0) =1a(1) =7a(2) =31a(3) =127a(4) =73a(5) =89a(6) =8191a(7) =151a(8) =131071a(9) =524287a(10) =337a(11) =178481a(12) =1801a(13) =262657a(14) =2089a(15) =2147483647a(16) =599479a(17) =122921a(18) =616318177a(19) =121369a(20) =164511353a(21) =2099863a(22) =23311a(23) =13264529a(25) =131071a(26) =20394401a(27) =201961a(28) =1212847