A semiprime is a natural number that is the product of two prime numbers. This tag is intended for questions about, related to, or involving semiprime numbers.
A semiprime, also called a 2-almost prime, biprime or pq-number, is a composite number that is the product of two (possibly equal) primes. The first few are 4, 6, 9, 10, 14, 15, 21, 22, ... (OEIS A001358).
Semiprimes are highly useful in the area of cryptography and number theory, most notably in public key cryptography, where they are used by RSA. It relies on the fact that finding two large primes and multiplying them together (resulting in a semiprime) is computationally simple, whereas finding the original factors appears to be difficult.
