4 and 9
Numbers with exactly three factors are squares of prime numbers.
Squares of prime numbers have exactly three factors.
4, 9 and 25.
2 4 6
Squares of prime numbers, like 4, 9 and 25.
4 (factors 1, 2 and 4) and 9 (factors 1, 3 and 9) are two examples.
Numbers with three factors are squares of primes. So: 4, 9, 25
Prime numbers have exactly two factors. 3 has exactly two factors: 1 and 3. So 3 is prime. Composite numbers have more than two factors. 4 is composite because it has three factors: 1, 2, and 4.
All numbers that are the square of primes have exactly 3 factors.
All numbers that have exactly two factors are prime numbers whereas composite numbers have more than two factors.
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.