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3 vertrices have same color, also isosceles, when 3 colors for 13-gon

Source: 2012 Swedish Mathematical Competition p5

April 30, 2021
combinatorial geometryColoringisosceles

Problem Statement

The vertices of a regular 1313-gon are colored in three different colors. Show that there are three vertices which have the same color and are also the vertices of an isosceles triangle.