(1) Remember that if is multiplicative, so is

In this case let: (this function is multiplicative since it's the product of two multiplicative functions)

Try to work with that

(by multiplicative I mean weakly multiplicative, not completely multiplicative)

(2) note that:

And now apply Möbius' inversion formula ( http://www.mathhelpforum.com/math-he...n-formula.html)