A note on Brehm's extension theorem

Abstract

Brehm's extension theorem states that a non-expansive map on a finite subset of a Euclidean space can be extended to a piecewise-linear map on the entire space. In this note, it is verified that the proof of the theorem is constructive provided that the finite subset consists of points with rational coordinates. Additionally, the initial non-expansive map needs to send points with rational coordinates to points with rational coordinates. The two-dimensional case is considered.