a(n) = Sum_{k=1..n} floor(n/k) * 5^(k-1).

Terms

a(0) =1a(1) =7a(2) =33a(3) =164a(4) =790a(5) =3946a(6) =19572a(7) =97828a(8) =488479a(9) =2442235a(10) =12207861a(11) =61039267a(12) =305179893a(13) =1525898649a(14) =7629414925a(15) =38147071306a(16) =190734961932a(17) =953674808838