Factorial number

A factorial number is the product of all positive integers less than or equal to a given number. It is denoted by an exclamation mark (n!) and plays a crucial role in mathematics, particularly in permutations, combinations, and probability theory. For example, 5! equals 5 × 4 × 3 × 2 × 1 = 120. Factorials are often used in algorithms, series expansions, and various counting problems.