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Here is the question $f:X \rightarrow Y$ and $g: Y \rightarrow Z$ are functions and $g \circ f$ is surjective, is $g$ surjective?

My proof: If $g \circ f$ is surjective than $\forall z \in Z \; \exists x \in X \; \mid (g \circ f)(x)=z $ Suppose $f$ is surjective, than $\forall y \in Y \; \exists x \in X\; \mid f(x)=y$. By def. of $(g\circ f)(x)=z$ we have $g(f(x))=z$ and since $f(x)=y$ than we have $g(y)=z$ which implies surjectivity therefore $g$ is surjecive.

Is this the correct way to prove that $g$ is surjective? Or can I not assume surjectivity on $f$

Martin Sleziak
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adam
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1 Answers1

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You should assume $g \circ f$ is surjective, not $f$. Let $z \in Z$. Choose $x \in X$ so that $g(f(x)) = z$. Let $y = f(x)$. Then $g(y) = z$.

Umberto P.
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  • You are assuming surjectivity when you let $y=f(x)$ are you not? – adam May 01 '14 at 15:52
  • No. $x \in X$ has been chosen. Apply $f$ to get $y$. – Umberto P. May 01 '14 at 16:12
  • So basically just omit the part where I assume $f$ is surjective and I'm good? But than how does this show $g$ is surjective if there could be a element in $X$ that is not necessairly in $Y$ take $-3$ from the reals and $y=\sqrt{x}$ I dont see how you can just omit it. – adam May 01 '14 at 16:16