In mathematics, a semiprime number is a number formed by multiplying two smaller primes. The two primes may be the same or different.
Semiprimes are also called 2-almost primes. For a number which is the product of n prime numbers counted with multiplicity, the number is said to be an n-almost prime.
Semiprimes that are the product of two equal primes are squares of primes. Semiprimes that are the product of two distinct primes are called square-free semiprimes.
The semiprimes up to 100 are: 4, 6, 9, X, 12, 13, 19, 1X, 21, 22, 29, 2X, 2E, 32, 33, 3X, 41, 43, 47, 49, 4X, 52, 55, 59, 62, 65, 6X, 71, 72, 73, 77, 79, 7X, 7E, 8X, 93, 97, 9X, 9E, X1, X2, X3, X9, E1, E2, E9, EX, EE.
Semiprimes cannot be end in the digit 0, because 10 = 2 x 2 x 3 and thus the number would have more than 2 prime factors. Apart from 6, semiprimes cannot end in the digit 6. Therefore, the ending digit of a semiprime can be any of {1, 2, 3, 5, 7, 9, X, E}, except for 4 and 6.