A prime number is any positive integer greater than zero that has only two proper divisorsNumber that when divided into another number leaves no remainder. , 1 and the number itself. Thus the lowest prime number is 2, which is also the only even prime;[a]One is not a prime number because it has only one proper divisor, itself. numbers greater than 1 that are not prime are known as composite numbers, because they can be expressed as the product of two other numbers smaller than themselves;[1] for instance, 8 can be expressed as 
. (The prime factor of 8 is 
)
Prime numbers are fundamental to mathematics, as all whole numbers can be constructed from them; anything that can be shown to be true about prime numbers is therefore true for all numbers.[2]