Prime number (also called prime number) 1. That is to say, among all integers greater than 1, there are no other factors except 1 and itself. This integer is called prime number or prime number (commonly called prime number). In other words, a prime number has only 1 and its own two divisors. 2. A prime number is an integer, and it cannot be expressed as the product of any other two integers except itself and 1. For example, 15 = 3 * 5, then 15 is not a prime number; Another example is 12 = 6 * 2 = 4 * 3, so 12 is not a prime number. On the other hand, 13 cannot be expressed as the product of any other two integers except 13 * 1, so 13 is a prime number. [Edit this paragraph] The concept of prime number is a number. If there are only 1 and its two factors, such a number is called a prime number (or prime number). For example, 2, 3, 5 and 7 are prime numbers, but 4, 6, 8 and 9 are not. The latter is called a composite number or a composite number. From this point of view, integers can be divided into two types, one is called prime number and the other is called composite number. (1 is neither a prime number nor a composite number) The famous Gauss "unique decomposition theorem" says that any integer. It can be written as the product of a series of prime numbers.