a(1) = 2; for n >= 2, n = Sum_{1<=k<n, gcd(k,n)=1} a(k).

Terms

a(0) =2a(1) =1a(2) =2a(3) =0a(4) =4a(5) =-2a(6) =0a(7) =2a(8) =6a(9) =-4a(10) =6a(11) =-4a(12) =-6a(13) =10a(14) =2a(15) =-2a(16) =12a(17) =-10a(18) =-2a(19) =8a(20) =2a(21) =-4a(22) =8a(23) =4a(24) =-16a(25) =8a(26) =10a(27) =-8a(28) =10a(29) =-8