TheoremBase

Continuity of the Projections and of the Distance Function on a Product Metric Space

Statement

Let R\mathbb{R} be the set of real numbers with the addition, multiplication and order ≤\le of its ordered field structure, regarded as a metric space through the metric dRd_{\mathbb{R}} of The Absolute Value Metric on the Real Line. Then the following hold.

1. (Projections) Let (X,dX)(X,d_X) and (Y,dY)(Y,d_Y) be metric spaces, let dX×Yd_{X\times Y} be the product metric on X×YX\times Y, a metric by claim 1 of The Product Metric is a Metric, and let S⊆X×YS\subseteq X\times Y. Then the map πX:S→X\pi_X:S\to X with πX((x,y))=x\pi_X((x,y))=x is continuous on SS relative to SS, and the map πY:S→Y\pi_Y:S\to Y with πY((x,y))=y\pi_Y((x,y))=y is continuous on SS relative to SS.

2. (Distance function) Let (Z,dZ)(Z,d_Z) be a metric space, let dZ×Zd_{Z\times Z} be the product metric on Z×ZZ\times Z obtained from dZd_Z and dZd_Z, and let T⊆Z×ZT\subseteq Z\times Z. Then the map ρ:T→R\rho:T\to\mathbb{R} with ρ((x,y))=dZ(x,y)\rho((x,y))=d_Z(x,y) is continuous on TT relative to TT, as a map into the metric space (R,dR)(\mathbb{R},d_{\mathbb{R}}).

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