MathNet 2018-part-04 — 181 problems (EN–VI) Question 178

Determine the increasing functions $f : \mathbb{N} \to \mathbb{N}$ having the property that the number $\frac{f(x) + f(y)}{1 + f(x + y)}$ is a non-negative integer, for all $x, y \in \mathbb{N}$.