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A function from the plane to itself which preserves the distance between any two points is called an isometry. Prove that an isometry must be a bijection.
To prove that an isometry is injective is easy:
For an isometry: [tex]||f(x)-f(y)||=||x-y||[/tex]
If [tex]x\neq y[/tex] then [tex]||x-y||>0[/tex] and therefore [tex]||f(x)-f(y)||>0[/tex] and [tex]f(x)\neq f(y)[/tex].
But to prove that an isometry is surjective... how should I do that?
To prove that an isometry is injective is easy:
For an isometry: [tex]||f(x)-f(y)||=||x-y||[/tex]
If [tex]x\neq y[/tex] then [tex]||x-y||>0[/tex] and therefore [tex]||f(x)-f(y)||>0[/tex] and [tex]f(x)\neq f(y)[/tex].
But to prove that an isometry is surjective... how should I do that?
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