They are numbers that are the product of two different prime numbers.
If the primes are p and q, the four factors are 1, p, q and pq.
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Numbers with exactly three factors are squares of prime numbers.
4, 9 and 25.
Numbers with three factors are squares of primes. So: 4, 9, 25
All numbers that are the square of primes have exactly 3 factors.
4, 9, 25, 49