The sum

(1) 
where runs through the residues Relatively Prime to , which is important in the representation of numbers by the
sums of squares. If (i.e., and ' are Relatively Prime), then

(2) 
For argument 1,

(3) 
where is the Möbius Function, and for general ,

(4) 
See also Möbius Function, Weyl's Criterion
References
Vardi, I. Computational Recreations in Mathematica. Redwood City, CA: AddisonWesley, p. 254, 1991.
© 19969 Eric W. Weisstein
19990525