A composite number which is the product of two (possibly equal) primes is called semiprime. Such numbers are sometimes also called 2-almost primes. For example:

• 1 is not a semiprime because it is not a composite number or a prime,
• 2 is not a semiprime, as it is a prime,
• 4 is a semiprime, since $4 = 2\cdot 2$
• 8 is not a semiprime, since it is a product of three primes ($8 = 2\cdot 2\cdot 2$ ,
• 2003 is not a semiprime, as it is a prime,
• 2005 is a semiprime, since $2005 = 5\cdot 401$
• 2007 is not a semiprime, since it is a product of three primes ($2007 = 3\cdot 3\cdot 223$ .

The first few semiprimes are $4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, \ldots$ (Sloane's sequence A001358 ). The Moebius function $\mu(n)$ for semiprimes can be only equal to 0 or 1. If we form an integer sequence of values of $\mu(n)$ for semiprimes we get a binary sequence: $0, 1, 0, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, \ldots$ (Sloane's sequence A072165 ).

All the squares of primes are also semiprimes. The first few squares of primes are then $4, 9, 25, 49, 121, 169, 289, 361, 529, 841, 961, 1369, 1681, 1849, 2209, 2809, 3481, 3721, 4489, 5041, \ldots$ (Sloane's sequence A001248 ). The Moebius function $\mu(n)$ for the squares of primes is always equal to 0 as it is equal to 0 for all squares.