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Range

From AoPSWiki

Let A and B be any sets and let f:A\to B be any function between them, so that A is the domain of f and B is the codomain. Then \{b\in B\mid \mathrm{there\ is\ some\ } a\in A\mathrm{\ such\ that\ } f(a)=b\} is called the range or image of f.

Thus, if we have f: \mathbb{R} \to \mathbb{R} given by f(x) = x^2, the range of f is the set of nonnegative real numbers.

A function is a surjection exactly when the range is equal to the codomain.

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