a(1)=5. For n > 1, a(n) = 4*5^(n-1) = A005054(n).

Terms

a(0) =5a(1) =20a(2) =100a(3) =500a(4) =2500a(5) =12500a(6) =62500a(7) =312500a(8) =1562500a(9) =7812500a(10) =39062500a(11) =195312500a(12) =976562500a(13) =4882812500a(14) =24414062500a(15) =122070312500a(16) =610351562500