Index set

In mathematics, an index set is a set whose members label (or index) members of another set. For instance, if the elements of a set $A$ may be indexed or labeled by means of the elements of a set $J$ , then $J$ is an index set. The indexing consists of a surjective function from $J$ onto $A$ , and the indexed collection is typically called an indexed family, often written as ${A j} j \in J$ .

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