A permutation is a way of ordering a set of objects or rearranging an already ordered set of objects. Important topics concerning permutations include: 
- Ordering and Replacement
- Permutations of N objects

Permutations 

University of Texas at Austin

Combinatorics is the study of how to count the various combinations, permutations, subgroups, etc. that can be formed from a given set of objects/people/etc. Knowledge of combinatorics is vital for an understanding of probability because when calculating the probability of an event one needs to know the number of potential ways an event can occur. 

Combinatorics

Morin, D. (2016). Probability for the Enthusiastic Beginner. Las Vegas, Nevada: CreateSpace Independent Publishing Platform.

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