A factorial, denoted as $n!$, is the product of all positive integers from 1 to n. It plays a vital role in counting and arranging objects, making it essential for various combinatorial concepts. The factorial function is the backbone for permutations and combinations, leading to the derivation of more complex counting principles such as multinomial coefficients and Stirling numbers. Understanding how to calculate and apply factorials allows for deeper insight into structures like derangements and provides the foundation for combinatorial proofs.
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