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A multiset is a collection of unordered objects.

Unlike sets, elements of multisets need not be distinct.

The multiplicity of an element in a multiset is defined to be the number of times the element appears in the multiset. Thus, the multiplicity of the element in the multiset is 3. Furthermore, the multiplicity of the element is 0, is 1, and is 2.

Sets are special instances of multisets in which the multiplicity of an element is either 0 or 1.

Multiset coefficient

The number of multisets of size taken from a set of cardinality is denoted by the multiset coefficient, denoted by , and is equal to: