I have a quick question about single/multi-valued functions.

Given the function $\displaystyle f: \mathbb{R} \to \mathbb{Z}$ where f(x) = the digit in the "tenths" position of x in its decimal representation. An example is $\displaystyle \sqrt{2} \approx 1.4142$ means that $\displaystyle f( \sqrt{2} ) = 4$. It is obvious that this function is single-valued. But I can't find an argument to prove that.

It may well be that I'm over-thinking this.

-Dan