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3 1 $begingroup$ Let $f colon A to A$ be a function such that $f circ f=f$ . If $f$ is one-to-one then prove that $f$ is also onto. I know in my head that the func. $f$ is $f(x)=x$ , but I can't develop a proof for the above statement. functions share | cite | improve this question edited 33 mins ago user649511 asked 5 hours ago user649511 user649511 23 4 New contributor