Combinatorics 

permutation combination variation

Scenarios

• choose $k$ elements from a total of $n$ elements
• distribute $k$ elements on $n$ fields
Ordered Unordered
repeat $\displaystyle n^k$ $\displaystyle \binom{n+k-1}{k}$
distinct $\displaystyle \frac{n!}{(n-k)!}$ $\displaystyle \binom{n}{k}$
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