The factorial function is defined as:

We defineF(n) =n(n- 1)(n- 2)(n- 3)^{ ... }(2)(1)

wherenis a non-negative integer.

The factorial function *F*(*n*) is also represented as "*n*!", read "*n*
factorial."

*Examples.*

5! | = | 5(4)(3)(2)(1) = 120 | |

3! | = | 3(2)(1) = 6 | |

10! | = | 10(9)(8)(7)(6)(5)(4)(3)(2)(1) = 3628800 | |

0! | = | 1;;(by definition) | |

6! | = | 6(5)(4)(3)(2)(1) = 720 |

*Example 1*: If 6 children must form a line, in how many ways can they
arrange themselves?

6 different children can stand in the first spot, 5 in the second spot (since 1 is already occupying the first spot), 4 in the third spot (since 2 are already occupying spots), etc.

Thus, they can arrange themselves in 6(5)(4)(3)(2)(1) = 6! = 720 different ways.

*n*! gives the number of ways *n* distinct items can be arranged in an order.

*Example 2*: In how many ways can the letters of the word TRIANGLE be
arranged?

8! = 40320 different ways.