# Composition of Distance-Preserving Mappings is Distance-Preserving

## Theorem

Let:

$\struct{X_1, d_1}$
$\struct{X_2, d_2}$
$\struct{X_3, d_3}$

Let:

$\phi: \struct{X_1, d_1} \to \struct{X_2, d_2}$
$\psi: \struct{X_2, d_2} \to \struct{X_3, d_3}$

Then the composite of $\phi$ and $\psi$ is also a distance-preserving mapping.

## Proof

Let $x,y \in X_1$ then:

 $\displaystyle d_1 \paren {x,y}$ $=$ $\displaystyle d_2 \paren {\map \phi x, \map \phi y }$ $\phi$ is a distance-preserving mapping $\displaystyle$ $=$ $\displaystyle d_3 \paren {\map \psi {\map \phi x}, \map \psi {\map \phi y} }$ $\psi$ is a distance-preserving mapping $\displaystyle$ $=$ $\displaystyle d_3 \paren {\map {\psi \circ \phi} x, \map {\psi \circ \phi} y }$ Definition of composite mappings

By the definition of a distance-preserving mapping then $\psi \circ \phi$ is distance-preserving.

$\blacksquare$