factorial moment generating function

例句：

1. 1.The number 5! is the factorial of 5.
   5!是5的阶乘。
2. 2. The factorial of a number n!is the product of all positive integers less than or equal to n.
   一个数n的阶乘是所有小于或等于n的正整数的积。
3. 3.The factorial of a prime number p! is p * (p - 1) * (p - 2) * ... * 1.
   质数p的阶乘是p乘以(p-1)乘以(p-2)乘以...乘以1。