a(n) = 8*5^n-1.

Terms

a(0) =7a(1) =39a(2) =199a(3) =999a(4) =4999a(5) =24999a(6) =124999a(7) =624999a(8) =3124999a(9) =15624999a(10) =78124999a(11) =390624999a(12) =1953124999a(13) =9765624999a(14) =48828124999a(15) =244140624999