Consider the function $f : N \to R$ given by $f(n) = \frac{n^2}{7}$. Prove that the image of $f$ is countably infinite.

Question

Consider the function $f : N \to R$ given by $f(n) = \frac{n^2}{7}$. Prove that the image of $f$ is countably infinite.

I have no idea how to do this so can someone please show me ?

I know I can prove $2 \mathbb Z$ (even integers) is countably infinite by the bijection $g: \mathbb Z \to 2 \mathbb Z$ by $g(n)=2n$. Could I use a similar approach. So let $h:\mathbb N \to \frac{n^2}{7}$ by $h(n) = \frac{n^2}{7}$?