In mathematics, a surjective function (also known as surjection, or onto function) is a function f that maps an element x to every element y; that is, for every y, there is an x such that f(x) = y. In other words, every element of the function’s codomain is the image of at least one element of its domain. It is not required that x be unique; the function f may map one or more elements of X to the same element of Y.

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“Surjective Function.” 2022. Wikipedia, August. https://en.wikipedia.org/w/index.php?title=Surjective_function&oldid=1102796717.