A one-to-one function has an inverse. The inverse is another function that undoes the action of the first one, so if we evaluate a function
at some point
to get the number
, evaluating the inverse at
will recover the original input
. In other words,

The process works in the opposite direction, too:

From the given definition of
, we have
, so taking inverses on both sides, we find

Given
, evaluating
at its inverse will recover
, so that

is another way of writing the compound function
. As already discussed, this reduces to
, so
