# How does one denote the number of elements of a set $X$?

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 $X$?

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

$$| X |$$

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

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

• Never saw the double vars (rather used form norms in Banach spaces) One also finds $\sharp X$. Mar 24 '18 at 22:16
• I think it is $\#X$ rather than $\sharp X$. Mar 25 '18 at 12:51

I have aso seen $n(X)$ (in Herstein I believe).

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