Math — Counting & Combinatorics
Permutations, combinations, Pascal's rule and multinomials.
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P(n,r) = \frac{n!}{(n-r)!}
C(n,r) = \binom{n}{r} = \frac{n!}{r!(n-r)!}
\binom{n}{r} = \binom{n-1}{r-1} + \binom{n-1}{r}
\frac{n!}{n_1!\,n_2!\cdots n_k!}