a(n) = Sum_{d|n} 5^(d-1).

Terms

a(0) =1a(1) =6a(2) =26a(3) =131a(4) =626a(5) =3156a(6) =15626a(7) =78256a(8) =390651a(9) =1953756a(10) =9765626a(11) =48831406a(12) =244140626a(13) =1220718756a(14) =6103516276a(15) =30517656381a(16) =152587890626a(17) =762939846906