A one/one (monotonic) function f (x) is, by definition, a function with the property that to one x there corresponds one value of f (x ) and, conversely, to
each value of f (x ) there corresponds only one x . The graph y/f (x) of a typical one/one function is shown in Fig. 31. The one/one property can be expressed by saying that no two of the ordered pairs (x , f (x )), which describe the graph
f (x ), have the same second component f (x) with different first components x .