# What is $|X|$ and $1n$?

I'm doing a course in calculus but I'm quite young so haven't learnt all the mathematical terms one should perhaps no before learning calculus, and these 2 terms keep coming up and I have no idea in the world of what they are:

"$|X|$" and "1n".

I am really sorry that this probably seems ridiculously elementary but I really want to know what they are.

What are they?

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For the meaning of absolute value, and information about it, please follow this link. I leave the search for natural logarithm to you. – André Nicolas Aug 15 '12 at 19:48

## 2 Answers

$|x|$ is the absolute value of $x$. $\ln(x)$ (that's ell, not one) is the natural logarithm of $x$.

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That depends on the context. $\lvert X\rvert$ is usually used to denote the cardinality of the set $X$ (esp. since you used capital letter, which are usually used for sets). On the other hand, if $X$ is a real (or complex) number, it is almost surely the absolute value of $X$.

If the symbol you refer to is $\ln$ as Robert guessed, it is the natural (base $e$) logarithm. If it is $1_n$, then I would guess it is the characteristic function of $\{n\}$, the function which takes value $1$ at $n$ and $0$ elsewhere...

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