Definition:Semiprime Number

A semiprime is an integer which is the product of two (not necessarily distinct) primes.

The first few semiprimes are:
 * $$4, 6, 9, 10, 14, 15, 21, 22, 25, 26, \ldots$$

Obvious examples: if $$p$$ is prime, then:
 * $$p^2$$ is semiprime;
 * $$2 p$$ is semiprime.
 * $$p_1 p_2$$ is semiprime.