I was studying the first chapter "Sets and Structures" of the "A Course in Advanced Calculus - Robert S. Borden". I faced a difficulty at the part of the proof of contradiction.(adsbygoogle = window.adsbygoogle || []).push({});

I got confused at what this [itex]B= \{x \in A : x \not\in f(x) \} [/itex] is and

how it's true that [itex]If~y \in A ~\text{is such that}~f(y)=B, \text{where is y? It must be either in}~B~\text{or in} A \setminus B.[/itex]

Can anyone explain what's going on here?

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# Proof by Contradiction?

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