Let k<<n denote that k<n and k∣n. Let f:{1,2,...,2013}→{1,2,...,M} be such that, if n≤2013 and k<<n, then f(k)<<f(n). What’s the least possible value of M?
Problem
Source: IV International Festival of Young Mathematicians Sozopol 2013, Theme for 10-12 grade
Tags: number theory, function