Definition:Semiprime

Definition

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

The first few semiprimes are:

$4, 6, 9, 10, 14, 15, 21, 22, 25, 26, \ldots$ This sequence is A001358 in the On-Line Encyclopedia of Integer Sequences (N. J. A. Sloane (Ed.), 2008).

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.