Yet another proof from the Book: the Gauss theorem on regular polygons

Abstract

This note is purely expository. The statement of the Gauss theorem on the constructibility of regular polygons by means of compass and ruler is simple and well-known. However, its proofs given in most textbooks rely upon much unmotivated material and are far from being economic. In this note a short elementary proof of the Gauss theorem is presented. The note is accessible for students familiar with polynomials and complex numbers, and could be an interesting easy reading for professional mathematicians.