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

Terms

a(0) =3a(1) =-3a(2) =11a(3) =-21a(4) =51a(5) =-119a(6) =315a(7) =-831a(8) =2195a(9) =-5883a(10) =16107a(11) =-44357a(12) =122643a(13) =-341487a(14) =956635a(15) =-2690841a(16) =7596483a(17) =-21522347a(18) =61171659a(19) =-174342165a(20) =498112275a(21) =-1426403751a(22) =4093181691a(23) =-11767920107a(24) =33891544419a(25) =-97764009003a(26) =282429537947a(27) =-817028472645