Problem

Source: OLCOMA Costa Rica National Olympiad, Final Round, 2015 C2 p5

Tags: algebra, inequalities, Functional inequality, functional



Let $f: N^+ \to N^+$ be a function that satisfies that $$kf(n) \le f (kn) \le kf(n)+ k- 1, \,\, \forall k,n \in N^+$$Prove that $$f(a) + f(b) \le f (a + b) \le f(a) + f(b) + 1, \,\, \forall a, b \in N^+$$