MathDB
Partition of N with ratios close to 2

Source: Brazil National Olympiad 2019 #5

November 14, 2019
algebra

Problem Statement

(a) Prove that given constants a,ba,b with 1<a<2<b1<a<2<b, there is no partition of the set of positive integers into two subsets A0A_0 and A1A_1 such that: if j{0,1}j \in \{0,1\} and m,nm,n are in AjA_j, then either n/m<an/m <a or n/m>bn/m>b. (b) Find all pairs of real numbers (a,b)(a,b) with 1<a<2<b1<a<2<b for which the following property holds: there exists a partition of the set of positive integers into three subsets A0,A1,A2A_0, A_1, A_2 such that if j{0,1,2}j \in \{0,1,2\} and m,nm,n are in AjA_j, then either n/m<an/m <a or n/m>bn/m>b.