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

Terms

a(0) =5a(1) =-10a(2) =45a(3) =-160a(4) =629a(5) =-2590a(6) =11165a(7) =-48910a(8) =217045a(9) =-976258a(10) =4438925a(11) =-20346440a(12) =93900245a(13) =-435959830a(14) =2034505661a(15) =-9536767660a(16) =44878791365a(17) =-211927519090