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I have to proof by contradiction that $ A \subseteq A $; if $ A \nsubseteq A $ then $ \exists x \in A ( x \notin A ) $ but $ \exists x \in A ( x \notin A ) $ is contradiction (in fact: $ \exists x ( x \in A \wedge x \notin A ) $) therefore $ A \subseteq A $ is true... The process is correct? Thank you all in advance

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Yes, this is fine. – Brian M. Scott Mar 14 '13 at 15:40
Yes its correct – kalpeshmpopat Mar 14 '13 at 15:41
Thank you soo much!!! – mle Mar 14 '13 at 15:48
up vote 1 down vote accepted

Your proof is correct, the way I would write it is as follows:

Suppose there exists a set $A$ such that $A \nsubseteq A$. Then there exists $x \in A$ such that $x \notin A$. But $x \in A$, contradiction. Hence $A \subseteq A$.

The extra formalism isn't necessary unless you are trying to get a better understanding of using the quantifiers, which in this case you have used correctly.

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It looks fine but it is better you clarify both $A$ as $$A_1\subseteq A_2$$ where $A_1=A_2=A$.

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I don't think I agree. This just encourages you to believe that things are different when they are actually the same. The contradiction is clearer if you write "$x\in A$ and $x\notin A$" than if you write "$x\in A_1$, $x\notin A_2$ and $A_1=A_2$". But maybe there are other reasons to prefer this notation. – Matthew Pressland Mar 14 '13 at 15:42
@MattPressland: I know it might make some points you noted but if I wanted to prove like the OP did, I would prefered student find out what $A$ I mean when I write $x\in A$, the first $A$ or the second one. However, they both are the same. – Babak S. Mar 14 '13 at 15:45
That is also reasonable. My feeling to a degree though is that the contradiction arises from the fact that when you write $x\in A$, you can't tell "which $A$" you mean - you have to mean both - so you can't possibly also have $x\notin A$. – Matthew Pressland Mar 14 '13 at 15:48
@MattPressland: Sometimes, I feel need a blackboard here to say what I mean. – Babak S. Mar 14 '13 at 15:54
It's fine, this is just a semantic argument anyway. Different people will find different notation clearer. – Matthew Pressland Mar 14 '13 at 15:57

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