The only way to write 42 as the product of primes (except to change the order of the factors) is 2 × 3 × 7. We call 2 × 3 × 7 the prime factorization of 42. It turns out that every counting number (natural number) has a unique prime factorization, different from any other counting number. This fact is called the Fundamental Theorem of Arithmetic. Fundamental theorem of arithmetic
In order to maintain this property of unique prime factorizations, it is necessary that the number one, 1, be categorized as neither prime nor composite. Otherwise a prime factorization could have any number of factors of 1, and the factorization would no longer be unique.
Prime factorizations can help us with divisibility, simplifying fractions, and finding common denominators for fractions.
