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

Terms

a(0) =1a(1) =4a(2) =24a(3) =120a(4) =624a(5) =3096a(6) =15624a(7) =78000a(8) =390600a(9) =1952496a(10) =9765624a(11) =48824880a(12) =244140624a(13) =1220687496a(14) =6103514976a(15) =30517500000a(16) =152587890624a(17) =762939059400