factorial of a vector

例句：

1. 1.The factorial of 5 is 120.
   5的阶乘是120。
2. 2. The factorial of a number is the product of all positive integers up to that number.
   一个数的阶乘是所有小于等于该数的正整数的积。
3. 3. The factorial of a number can be calculated using a formula.
   一个数的阶乘可以使用公式计算。