# Brouwer in Dimension Two

The Brouwer Fixed-Point Theorem via Sperner’s Lemma
• Joel H. Shapiro
Chapter
Part of the Universitext book series (UTX)

## Overview

In dimension two the Brouwer Fixed-Point Theorem states that every continuous mapping taking a closed disc into itself has a fixed point. In this chapter we’ll give a proof of this special case of Brouwer’s result, but for triangles rather than discs; closed triangles are homeomorphic to closed discs (Exercise 2.2 below) so our result will be equivalent to Brouwer’s. We’ll base our proof on an apparently unrelated combinatorial lemma due to Emanuel Sperner, which—in dimension two—concerns a certain method of labeling the vertices of “regular” decompositions of triangles into subtriangles. We’ll give two proofs of this special case of Sperner’s Lemma, one of which has come to serve as a basis for algorithms designed to approximate Brouwer fixed points.

## Keywords

Convex Hull Triangle Inequality Interior Vertex Closed Disc Fair Division
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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