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$$ | \{ \{ \} \} | = 0. $$

Is this true or false?

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    $\begingroup$ The set $\{\{\}\}$ has exactly one element, namely $\{\}$, so its cardinality is $1$. $\endgroup$
    – user26486
    Jun 22, 2015 at 1:11

1 Answer 1

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It's not. That set contains exactly one element: {}, so its cardinality is 1, not 0.

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    $\begingroup$ Thank you for your answer. $\endgroup$
    – Betty
    Jun 22, 2015 at 1:20

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