Factorial factor is an operation symbol invented by the indexist Keston Kaman in 1808. This is a mathematical term, written as! . Specifically defined as: the factorial of any positive integer is the product of all positive integers less than or equal to this integer. In addition, we define that the factorial of 0 is 1 is an empty product. For the factorial of any natural number m, we write m! .

The concrete representation method is: for any positive integer m, m! = 1 * 2 * 3 * 4 * ...* n, or we can express it in another recursive way: m! =m*(m- 1)！ . For example, we count 6! =6*5*4*3*2* 1=720。

The concept of factorial can be applied to many fields of mathematics and computer, such as combinatorics, algebra and mathematical analysis. In the case of positive integers, m! The factorial of can be called the permutation number of m.