a(n) = Sum_{k=1..n} 5^(gcd(k, n) - 1).

Terms

a(0) =1a(1) =6a(2) =27a(3) =132a(4) =629a(5) =3162a(6) =15631a(7) =78264a(8) =390681a(9) =1953774a(10) =9765635a(11) =48831564a(12) =244140637a(13) =1220718786a(14) =6103516983a(15) =30517656528a(16) =152587890641a(17) =762939850086