ASU 235 All Soviet Union MO 1977 polygon without self-intersections
Source:
July 6, 2019
combinatorial geometrygeometry
Problem Statement
Given a closed broken line without self-intersections in a plane. Not a triple of its vertices belongs to one straight line. Let us call "special" a couple of line's segments if the one's extension intersects another. Prove that there is even number of special pairs.