Mathematics-Factorial

Factorial of a non-negative integer, n is denoted as n!. And it is calculated using product of all positive integers less than or equal to n.

Eg ) 5! = 5 * 4 * 3 * 2 * 1 = 120

The notation n! is introduced by Christian Kramb.

Factorial function defined normally by  n! =
 

or it is denoted recursively by n! =   1                 if n = 0
                                                     (n - 1)! * n    if n > 0

Factorial function is denoted by using a symbol  ' ! ' . It means to multiply a series of descending natural numbers.
4! = 4 * 3 * 2 * 1 = 24

Main usage of factorial is in permutation & combination

Half Factorial ::

Factorial of a half ( 1/2 ) is half of the square root of ( pi ) ∏.
eg ) ( 1/2 ) ! = ( 1/2 )√∏

To conclude factorial of a any number is that number times the factorial of 1 less than that number.