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🔢 Factorial Calculator

Compute n! (factorial), n!! (double factorial), and !n (subfactorial) with BigInt precision, digit counts, and trailing-zero analysis.

n! = n × (n−1) × … × 2 × 1, the factorial of n. 0! = 1.

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Free online factorial calculator supporting n! (factorial), n!! (double factorial), and !n (subfactorial/derangements). Uses BigInt for exact arbitrary-precision results with scientific notation, digit count, trailing-zero analysis via the Legendre formula, and step-by-step expansion for small values.