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  1. MHB Apprentice

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    #1
    Let f be an application from E to E (E≠∅) such that f∘f∘f = f
    Prove f is an injection ⇔ f is a surjection


    I honestly have no idea how to start and I'd love to know the answer before my math exam tomorrow morning

  2. MHB Master
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    Euge's Avatar
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    #2
    Suppose $f$ is an injection. Show that for all $e\in E$, $f(f(e))= e$. This will prove $f$ is a surjection.

    Now suppose $f$ is a surjection and $f(x) = f(y)$. Let $z,w\in E$ such that $f(z) = x$ and $f(w) = y$. Then $$x = f(z) = f(f(f(z))) = f(f(x)) = f(f(y)) = f(f(f(w))) = f(w) = y $$

    Hence, $f$ is an injection.

  3. MHB Apprentice

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    #3 Thread Author
    Hello Euge ,
    I never would've thought of that ,thank you so much for your help !

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