a(n) = -(1/n) * Sum_{d|n} phi(n/d) * (-4)^d.

Terms

a(0) =4a(1) =-6a(2) =24a(3) =-66a(4) =208a(5) =-676a(6) =2344a(7) =-8226a(8) =29144a(9) =-104760a(10) =381304a(11) =-1398476a(12) =5162224a(13) =-19172796a(14) =71582944a(15) =-268439586a(16) =1010580544a(17) =-3817734596a(18) =14467258264a(19) =-54975633768a(20) =209430787824a(21) =-799644629556