Primes with exactly seven 7's.

Terms

a(0) =77767777a(1) =77777177a(2) =77777377a(3) =77777747a(4) =137777777a(5) =172777777a(6) =177677777a(7) =177776777a(8) =177777377a(9) =177777577a(10) =177777727a(11) =177777773a(12) =177777779a(13) =177797777a(14) =197777777a(15) =272777777a(16) =277177777a(17) =277727777a(18) =277771777a(19) =277775777a(20) =277777177