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I was doing the following question:

Find a simpler way to specify the set $A=\big\{ X: X\in \Bbb N \cap \{-5,3,7,11\} \cup \{-5, -3, 13\} \big\}$

Am I right to suggest a simpler way of specifying the following sets is

$A=\big\{X:X\in \Bbb N \cup \{-5,-3\} \big\} ~?$

Thank you in advance!

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    Julius is right, the intersection with $\mathbb N$ removes $-5$ from the first finite set but this element is added back, from the union with the second finite set. So you just take the union of the 2 finite sets, disregarding "$\mathbb N\cap$".2012-04-15

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I'd start by noting that ${\bf N}\cap\{{-5,3,7,11\}}=\{{3,7,11\}}$.

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    @Fatz: No, you would have $\{x:x\in\{-5,-3,7,11,13\}\}=\{-5,-3,7,11,13\}$2012-04-15