In combinatorial mathematics, a combination is an un-ordered collection of unique sizes. (An ordered collection is called a permutation.) Given S, the set of all possible unique elements, a combination is a subset of the elements of S. The order of the elements in a combination isn't important (two lists with the same elements in different orders are considered to be the same combination). Also, the elements can't be repeated in a combination (every element appears uniquely once); this is often… (More on Combination)