MathDB
Many Lattice Points

Source: 2023 China Team Selection Test Round 2 Day 4 Problem 23

April 3, 2023
number theorycombinatoricslattice points

Problem Statement

Given a prime pp and a real number λ(0,1)\lambda \in (0,1). Let ss and tt be positive integers such that st<λp12s \leqslant t < \frac{\lambda p}{12}. SS and TT are sets of ss and tt consecutive positive integers respectively, which satisfy {(x,y)S×T:kxy(modp)}1+λs.\left| \left\{ (x,y) \in S \times T : kx \equiv y \pmod p \right\}\right| \geqslant 1 + \lambda s.Prove that there exists integers aa and bb that 1a1λ1 \leqslant a \leqslant \frac{1}{ \lambda}, btλs\left| b \right| \leqslant \frac{t}{\lambda s} and kab(modp)ka \equiv b \pmod p.