a(n) = (1/(4*n)) * Sum_{d|n} 5^(n/d-1) * phi(5*d).

Terms

a(0) =1a(1) =3a(2) =9a(3) =33a(4) =126a(5) =527a(6) =2233a(7) =9783a(8) =43409a(9) =195378a(10) =887785a(11) =4069297a(12) =18780049a(13) =87194199a(14) =406901134a(15) =1907353533a(16) =8975758273a(17) =42385547227a(18) =200773540297a(19) =953674414158