a(n) = 7*(10^n - 1)/9.

Terms

a(0) =0a(1) =7a(2) =77a(3) =777a(4) =7777a(5) =77777a(6) =777777a(7) =7777777a(8) =77777777a(9) =777777777a(10) =7777777777a(11) =77777777777a(12) =777777777777