If the positive integer n has a repeated prime factor then μ(n) is defined to be zero. If n is square-free, then μ(n) = +1 if n has an even number of prime factors and -1 if n has an odd number of prime factors.
Comparison with the zeta function shows that formally at least .
Möbius inversion formula
We therefore have
giving the Möbius inversion formula
A useful special case is the formula
The Mertens conjecture is that the summatory function
The truth of the Mertens conjecture would imply the Riemann hypothesis. However, computations by Andrew Odlyzko have shown that the Mertens conjecture is false.