MathDB
Fraction between sqrt(n) and sqrt(n+1)

Source: 2016 IMO Shortlist A5

July 19, 2017
IMO ShortlistalgebraFractionsHi

Problem Statement

Consider fractions ab\frac{a}{b} where aa and bb are positive integers. (a) Prove that for every positive integer nn, there exists such a fraction ab\frac{a}{b} such that nabn+1\sqrt{n} \le \frac{a}{b} \le \sqrt{n+1} and bn+1b \le \sqrt{n}+1. (b) Show that there are infinitely many positive integers nn such that no such fraction ab\frac{a}{b} satisfies nabn+1\sqrt{n} \le \frac{a}{b} \le \sqrt{n+1} and bnb \le \sqrt{n}.