Largest prime substring in 4^n (0 if none).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =5a(5) =2a(6) =409a(7) =163a(8) =6553a(9) =2621a(10) =857a(11) =419a(12) =1677721a(13) =71a(14) =43a(15) =107a(16) =4294967a(17) =171798691a(18) =6871947673a(19) =779069a(20) =511627a(21) =4398046511a(22) =18604441a(23) =703687441a(24) =28147497671a(25) =11258999a(26) =59962737049a(28) =759403792793