a(n) = size of union of 2^k (mod 10^n), 0 < k <= 5^n.

Terms

a(0) =4a(1) =21a(2) =102a(3) =503a(4) =2504a(5) =12505a(6) =62506a(7) =312507a(8) =1562508a(9) =7812509a(10) =39062510a(11) =195312511a(12) =976562512a(13) =4882812513a(14) =24414062514a(15) =122070312515a(16) =610351562516