Semiprime

A semiprime is a product of two primes. The semiprimes up to 50 are 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, and 49. Semiprime bases only let fractions with denominators being products of their prime factor's powers terminate.

Semiprimes which are not square numbers are called "distinct" (or sometimes squarefree) semiprimes because their prime factors are distinct (different). The distinct semiprimes up to 50 are 6, 10, 14, 15, 21, 22, 26, 33, 34, 35, 38, 39, and 46.