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St. Petersburg MO 2017 Grade 11 P7

Source: St. Petersburg MO 2017 Grade 11 P7

May 3, 2018
combinatoricsnumber theory

Problem Statement

Given a convex polygon with vertices at lattice points on a plane containing origin OO. Let V1V_1 be the set of vectors going from OO to the vertices of the polygon, and V2V_2 be the set of vectors going from OO to the lattice points that lie inside or on the boundary of the polygon (thus, V1V_1 is contained in V2V_2.) Two grasshoppers jump on the whole plane: each jump of the first grasshopper shift its position by a vector from the set V1V_1, and the second by the set V2V_2. Prove that there exists positive integer cc that the following statement is true: if both grasshoppers can jump from OO to some point AA and the second grasshopper needs nn jumps to do it, then the first grasshopper can use at most n+cn+c jumps to do so.