- Factorial of a non-negative integer n is the product of all positive integers less than or equal to n. $$n!=n\times(n−1)\times(n−2)\times\dots\times2\times1$$
- Special case: 0!=1
- Recursive Definition: $$n!=n\times(n−1)!$$
- (n+1)! Expansion: $$(n+1)!=(n+1)\times n!$$
- Division Property: $$\frac{n!}{\left(n-r\right)!}=n\times(n−1)\times\dots\times(n−r+1)$$
Factorial Table
| 0! = | 1 |
| 1! = | 1 |
| 2! = | 2 |
| 3! = | 6 |
| 4! = | 24 |
| 5! = | 120 |
| 6! = | 720 |
