In mathematics, the range (or image) of a function are the values it hits. It is not to be confused with the codomain of a function, which is a designated set to which all the values of the function belong.
A function is onto (or surjective) if every value in its codomain is hit by the function, or, equivalently, if its range is equal to its codomain. More formally, a function is onto if for every
there exists
such that
.
Let be the function defined by the equation
. By definition, the codomain
of
is
. However, the range of
consists of all nonnegative real numbers. Indeed, let
be a nonnegative real number. Then
, and so
is one of the values hit by
.
Let be the function defined by the equation
. Then, for every real number
, we can see that
, so every real number is hit by
. This means that the codomain and range of
are equal, namely
. Therefore,
is onto.
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