 A composite number with three distinct prime factors is a sphenic number.In some applications, it is necessary to differentiate between composite numbers with an odd number of distinct prime factors and those with an even number of distinct prime factors.For the latter 

(where μ is the Möbius function and "x" is half the total of prime factors), while for the former

However, for prime numbers, the function also returns −1 and formula_3.