Combinatorics is a branch of mathematics focused on counting, arrangement, and combination of elements within sets, often under specific constraints. It plays a crucial role in fields like computer science, probability, and optimization by providing foundational techniques for solving complex problems involving discrete structures.
The factorial of a non-negative integer n, denoted as n!, is the product of all positive integers less than or equal to n. It is a fundamental concept in combinatorics, used to calculate permutations and combinations, and has applications in various mathematical and scientific fields.