Question 84
Let $Z_{\ge 0}$ be the set of all nonnegative integers. Find all the functions $f: Z_{\ge 0} \to Z_{\ge 0}$ satisfying the relation
$$
f(f(f(n))) = f(n + 1) + 1 \text{ for all } n \in Z_{\ge 0}.
$$
$$
f(f(f(n))) = f(n + 1) + 1 \text{ for all } n \in Z_{\ge 0}.
$$