Let $m$ be a positive integer such that $m$ has exactly $8$ positive divisors. How many distinct prime factors does $m$ have?
Suppose the prime factorization of m is m=pk11⋅pk22⋯pknn. Then the number of positive divisors is (1+k1)(1+k2)⋯(1+kn).
The ways of writing 8 into product of numbers are 8=(1+1)×(1+3)=(1+1)×(1+1)×(1+1). From this and the fact above, we know that m is either of the form pq3 or pqr, where p, q, r are distinct primes. The number of distinct prime factors of m is either 2 or 3. The given conditions are not sufficient to conclude definitely that the number of distinct prime factors is 2 (resp. 3).