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# Combinatorics

Combinatorics is the branch of mathematics concerned with Counting, Combinations and Permutations of Sets.

## Formulas

For choosing $$k$$ items from a set of $$n$$.

Order matters (Permutations) Order does not matter (Combinations)
Without repetition $$\frac{n!}{(n-k)!}$$ $$\frac{n!}{k!(n-k)!}$$
With repetition $$n^k$$ $$\frac{(k+n-1)!}{k!(n-1)!}$$