Largest prime substring in 2^n (or 0 if none exist).

Terms

a(0) =0a(1) =2a(2) =0a(3) =0a(4) =0a(5) =3a(6) =0a(7) =2a(8) =5a(9) =5a(10) =2a(11) =2a(12) =409a(13) =19a(14) =163a(15) =7a(16) =6553a(17) =131a(18) =2621a(19) =5a(20) =857a(21) =971a(22) =419a(23) =83a(24) =1677721a(25) =5443a(26) =71a(27) =1342177a(28) =43a(29) =709