MathDB
Determine f(30)

Source: IMO Longlist 1989, Problem 96

September 18, 2008
algebra unsolvedalgebra

Problem Statement

Let f:NN f : \mathbb{N} \mapsto \mathbb{N} be such that (i) f f is strictly increasing; (ii) f(mn) \equal{} f(m)f(n)   \forall m, n \in \mathbb{N}; and (iii) if mn m \neq n and m^n \equal{} n^m, then f(m) \equal{} n or f(n) \equal{} m. Determine f(30). f(30).