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

Terms

a(0) =1a(1) =2a(2) =3a(3) =2a(4) =5a(5) =-1a(6) =7a(7) =5a(8) =7a(9) =-3a(10) =11a(11) =-3a(12) =13a(13) =5a(14) =7a(15) =9a(16) =17a(17) =-15a(18) =19a(19) =7a(20) =11a(21) =3a(22) =23a(23) =-15a(24) =9a(25) =17a(26) =33a(27) =-3a(28) =29a(29) =-55