a(n) = 4*5^n-1.

Terms

a(0) =3a(1) =19a(2) =99a(3) =499a(4) =2499a(5) =12499a(6) =62499a(7) =312499a(8) =1562499a(9) =7812499a(10) =39062499a(11) =195312499a(12) =976562499a(13) =4882812499a(14) =24414062499a(15) =122070312499a(16) =610351562499