Precalculus (6th Edition)

A function $f$ is a one-to-one function if different values of the domain correspond to different values of the range. ------- Given that $f(x)=c$, for some constant c, we take any two different values from the domain, say 0 and 1, and find that $f(0)=c,$ $f(1)=c$, that is, that they have same function values. So, f is not a one-to-one function.