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Thread: Symmetric Difference

  1. #1
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    Symmetric Difference

    I have a problem which s related to this symmetric propety.
    Show that there exist a unique set $\displaystyle N$ such that $\displaystyle (A \Delta N)=A$ for all $\displaystyle A$. The set $\displaystyle N$ is obviously the empty set. But then how do I prove the uniqueness part of $\displaystyle N$, i.e. if $\displaystyle (A \Delta N1)=A$ and $\displaystyle (A \Delta N2)=A$, then $\displaystyle N1=N2$= the empty set
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  2. #2
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    Quote Originally Posted by santiagos11 View Post
    I have a problem which s related to this symmetric propety.
    Show that there exist a unique set $\displaystyle N$ such that $\displaystyle (A \Delta N)=A$ for all $\displaystyle A$. The set $\displaystyle N$ is obviously the empty set.
    $\displaystyle A\Delta N = A\, \Rightarrow \,\left( {A\backslash N} \right) \cup (N\backslash A) = A\; \Rightarrow \;A \cap N^c \subseteq A\;\& \;N \cap A^c \subseteq A$
    We know that $\displaystyle N = \left( {N \cap A} \right) \cup \left( {N \cap A^c } \right)$.
    But that along with the above means that $\displaystyle N \subseteq A$. WHY?

    Now suppose that $\displaystyle \left( {\exists p} \right)\left[ {p \in N} \right]$ this means that $\displaystyle p \in N \cap A\; \Rightarrow \;p \notin N\backslash A \wedge p \notin A\backslash N$.
    Is that a contradiction?
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  3. #3
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    thank you. You are very good in your arguments.
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  4. #4
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    I have yet another simmilar problem.
    Suppose $\displaystyle A,B$ are sets. Show that there exist a unique set $\displaystyle C$ such that $\displaystyle (A \Delta C)=B$. If $\displaystyle A,B$ are disjoint, then $\displaystyle C=A \cup B$. But if not, then what would C have to be?
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  5. #5
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    never mind. i got it
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