Combinatorics

Combinatorics is a branch of mathematics concerning the study of finite or countable discrete structures.

A permutation of a set of objects is an arrangement of those objects in a particular order; the number of permutations can be counted.

The number of permutations of distinct elements can be calculated when not all elements from a given set are used.

The expression revealing the number of permutations of distinct items can be modified if not all items in a set are distinct.

A combination is a way of selecting several things out of a larger group, where (unlike permutations) order does not matter.