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Semiprimes
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Semiprimes are composite numbers with exactly two prime factors. If the prime factors of a semiprime are distinct, then the semiprime is squarefree, such as 39, which is the product of 3 and 13 (see A006881 for more squarefree semiprimes); these numbers have exactly four divisors (cubes of primes also have four divisors). The other semiprimes are the squares of the prime numbers (see A001248); these numbers have exactly three divisors and no other numbers do. A001358 lists both kinds of semiprimes.
