a(0) = 1; a(n) = 4*5^(n-1) for n >= 1.

Terms

a(0) =1a(1) =4a(2) =20a(3) =100a(4) =500a(5) =2500a(6) =12500a(7) =62500a(8) =312500a(9) =1562500a(10) =7812500a(11) =39062500a(12) =195312500a(13) =976562500a(14) =4882812500a(15) =24414062500a(16) =122070312500a(17) =610351562500