MathDB
A guessing game on polygon and lines

Source: MEMO 2024 I2

August 26, 2024
combinatoricscombinatorial geometrypolygonquestionguessing game

Problem Statement

There is a rectangular sheet of paper on an infinite blackboard. Marvin secretly chooses a convex 20242024-gon PP that lies fully on the piece of paper. Tigerin wants to find the vertices of PP. In each step, Tigerin can draw a line gg on the blackboard that is fully outside the piece of paper, then Marvin replies with the line hh parallel to gg that is the closest to gg which passes through at least one vertex of PP. Prove that there exists a positive integer nn, independent of the choice of the polygon, such that Tigerin can always determine the vertices of PP in at most nn steps.