**Function (mathematics) - Wikipedia**
https://en.wikipedia.org/wiki/Function_(mathematics)

Intuitively, a function is a process that associates each element of a set X, to a single element of a set Y.. Formally, a function f from a set X to a set Y is defined by a set G of ordered pairs (x, y) with x ∈ X, y ∈ Y, such that every element of X is the first component of exactly one ordered pair in G.

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