Largest prime substring in 7^n (0 if none).

Terms

a(0) =0a(1) =7a(2) =0a(3) =43a(4) =401a(5) =7a(6) =7649a(7) =823a(8) =4801a(9) =3607a(10) =82475249a(11) =7326743a(12) =41287201a(13) =89a(14) =23072849a(15) =7561a(16) =232930569601a(20) =297612001a(21) =558545864083a(22) =2988049a(23) =3400809163