n！ = 1×2×3×...×n .

Factor can also be defined recursively: 0! = 1，n！ =(n- 1)！ ×n .

With "m! ! "Express delivery.

When m is a natural number, it means the product of all positive integers not exceeding m and having the same parity with m, such as:

When m is a negative odd number, it means the reciprocal of the absolute product of all negative odd numbers whose absolute values are less than their absolute values.

When m is a negative even number, m! ! Does not exist.

Extended data:

Gamma function (gamma function)

Define the gamma function:

Using the knowledge of integral, we can prove that γ (s) = (s- 1) × γ (s- 1).

Therefore, when x is an integer n,

In this way, the gamma function is actually a continuation of factorial.

Baidu encyclopedia-factorial