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$A$ is a set and so is $B$.

$f$ is a function $A \to B$.

I have a math question that asks about $|f(A)|$. What does the notation $|\cdot |$ mean?

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If the elements of $ B $ are sets, then the double bars could mean cardinality (i.e. the size of the set).

If $ B $ is a set of numbers, it probably means absolute value.

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    Ah yes, I misread the question. Here $ f(A) $ refers to the *image* of $ A $ under $ f $ and so the output is always a set. So in this case, the notation must mean cardinality.2012-09-26