rPermutations

r-Permutations

INFO

The number of ways to arrange $r$ objects out of $n$
Rearrangement or ordering of $n$ distinct objects so that each object appears exactly once

$$P(n,r)=nPr=n(n-1)(n-2)\dots (n-r+1)=\frac{n!}{(n-r)!}$$

IMPORTANT

$0!=1$

---

Multisets

INFO

A multiset is a collection that allows for repeated elements.
Example: $\{1,1,2,3\}$ is a multiset.

Multisets are essential when counting permutations of objects that are not all distinct.