Permutations and Combinations

Order matters

Permutations

_{n} P_{r} = \frac{n!}{(n - r)!}

With repeated items

\frac{n!}{n_{1}! n_{2}! \dots n_{k}!}

Around a circle

(n - 1)!

Combinations

_{n} C_{r} = \frac{n!}{r! (n - r)!}

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