a(n) = (1/9)*(-1+10^n)*(4*n+3).

Terms

a(0) =7a(1) =121a(2) =1665a(3) =21109a(4) =255553a(5) =2999997a(6) =34444441a(7) =388888885a(8) =4333333329a(9) =47777777773a(10) =522222222217