4
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If I have a specific set of some elements like:

$$X = \{1,2,3,4,5,6\}$$

How can one make an expression for the number of elements of the set $X$?

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0
24
$\begingroup$

You're looking for the cardinality of a set, which is usually denoted by vertical bars:

$$| X |$$

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13
$\begingroup$

It's common enough to write $\#X$, too.

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3
$\begingroup$

It is commonly denoted with vertical or double vertical lines: $|X|$ or $||X||$. Once I also saw an accent: $\bar{X}$.

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  • 4
    $\begingroup$ Never saw the double vars (rather used form norms in Banach spaces) One also finds $\sharp X$. $\endgroup$
    – Bernard
    Mar 24 '18 at 22:16
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    $\begingroup$ I think it is $\#X$ rather than $\sharp X$. $\endgroup$
    – Umberto P.
    Mar 25 '18 at 12:51
2
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I have aso seen $n(X)$ (in Herstein I believe).

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  • $\begingroup$ This isn't uncommon in lower-level math courses. The Finite Math book I am teaching out of right now uses it. $\endgroup$ Mar 25 '18 at 13:30

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