Problem

Source: IMO ShortList 1988, Problem 20, Mongolia 4, Problem 54 of ILL

Tags: combinatorics, partition, Coloring, Extremal combinatorics, Ramsey Theory, IMO Shortlist



Find the least natural number $ n$ such that, if the set $ \{1,2, \ldots, n\}$ is arbitrarily divided into two non-intersecting subsets, then one of the subsets contains 3 distinct numbers such that the product of two of them equals the third.