# Composition of Isometries is Isometry

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## Theorem

Let:

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

be metric spaces.

Let:

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

be isometries.

Then the composite of $\phi$ and $\psi$ is also an isometry.

## Proof

An isometry is a distance-preserving mapping which is also a bijection.

From Composition of Distance-Preserving Mappings is Distance-Preserving, $\psi \circ \phi$ is a distance-preserving mapping.

From Composite of Bijections is Bijection, $\psi \circ \phi$ is a bijection.

$\blacksquare$