Proving the Theorem of Wigner by an Exercise in Simple Geometry

Abstract

The leading idea of the paper is to treat the theorem of Wigner with methods inspired by geometry. The exercise mentionned in the title has two functions: On the one hand it can serve as a pedagogical text in order to make the reader acquainted with the essential features of the theorem and its proof. On the other hand it will turn out to be the core of the general proof.