Surjection
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A surjection is a function which takes each value in its codomain at some value in its domain. That is, the range (or image) of the function is equal to its codomain. (For every function, the range is a subset of the codomain.) In adjectival form, we say that a function is surjective or onto.
For instance, the function 
defined by 
is surjective because every integer is one more than some other integer, but the function 
defined by is not surjective because there exists a natural number which is not one more than any other natural number.
See also
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