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Let $m$ be a positive integer such that $m$ has exactly $8$ positive divisors. How many distinct prime factors does $m$ have?

 Oct 20, 2024
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Suppose the prime factorization of m is m=pk11pk22pknn. 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).

 Oct 20, 2024

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