2. Except 1, some numbers except 1 and itself cannot be divisible by other integers, such as 2, 3, 5, 7, 1 1, 13, 17, ..., and so on. Such numbers are called prime numbers. Some numbers can be divisible by 1 and other integers other than itself. This number is called a composite number, such as 4, 6, 8, 9, 10, 12, 14, ... and so on, this is a composite number. 1 is a special number. It is neither a prime number nor a composite number. In this way, all natural numbers are divided into 1, prime numbers and composite numbers. Complex number series-terms such as 4, 6, 8, 9, 10, 12, 14, ... are called complex number series.