Combinations with repetition (multiset selection) (Multiset coefficient: C(n+r−1, r) for r selections from n distinct types)

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Multiset coefficient: C(n+r−1, r) for r selections from n distinct types

What each symbol means

SymbolWhat it stands for
nTypes/categories
rItems selected (with replacement conceptually)

When to use this

Types distinguishable; selections indistinguishable except by type counts; r can be any non-negative integer.