a(n) = Sum_{d|n} phi(n/d) * (-3)^(d-1).

Terms

a(0) =1a(1) =-2a(2) =11a(3) =-28a(4) =85a(5) =-238a(6) =735a(7) =-2216a(8) =6585a(9) =-19610a(10) =59059a(11) =-177428a(12) =531453a(13) =-1593606a(14) =4783175a(15) =-14351152a(16) =43046737a(17) =-129134082a(18) =387420507a(19) =-1162281100a(20) =3486785925a(21) =-10460294174a(22) =31381059631a(23) =-94143360856a(24) =282429536825a(25) =-847288078026