Question:
Lana defines a function $f(x)$ which is given by the formula $$f(x) = x^2,$$ but only on a domain she has specified which consists of finitely many values $x$; she leaves the function undefined for all other $x$.

Given that the range of $f(x)$ is $\{0,1,2,3,4,5,6,7,8,9\}$, what is the maximum number of points that could be in its domain?

Answer:
We know $x$ can be in the domain of $f(x)$ only if $x^2$ is an element of the set $\{0,1,2,3,4,5,6,7,8,9\}$. There are $19$ values of $x$ for which this is true: $$x=0, \pm 1, \pm\sqrt2, \pm\sqrt3, \pm 2, \pm\sqrt 5, \pm\sqrt 6, \pm\sqrt 7, \pm\sqrt 8, \pm 3.$$ Therefore, the domain of $f(x)$ contains at most $\boxed{19}$ points.