Definition:Semiprime Number

Definition
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 $p, p_1, p_2$ are prime, then:
 * $p^2$ is semiprime.
 * $2 p$ is semiprime.
 * $p_1 p_2$ is semiprime.