Tell me more ×
TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. It's 100% free, no registration required.
up vote 6 down vote favorite
3

When you write the differential at a point, you write f`(x)|_(x = k). What is the correct LaTeX command for the "|" symbol? Is it just a pipe, or is there a separate mathematical symbol for it?

My LaTeX for the above is:

\[f^\prime(x)|_{x = 1} = 5\]

Is this formally correct?

share|improve this question
3  
Why not simply f'(1)=5? – egreg Jul 21 '12 at 10:14

2 Answers

up vote 9 down vote accepted

This is not a differential, but a derivative; they are different things. Since f' denotes the derivative of f, which is a function of its own, the best notation for the value at 1 is

f'(1)

If you want to use the heavier notation

f'(x)|x=1

then

$f'(x)|_{x=1}$

is perfectly good. You may want to define a command for this:

\newcommand{\at}[2][]{#1|_{#2}}

to be used as

$f'(x)\at{x=1}$

or, if you need a larger bar to cover a larger function symbol,

$f'(x)\at[\big]{x=1}$

The optional argument can be one of \big, \Big, \bigg or \Bigg. I wouldn't use an automatically growing bar, because it may give bad results.

share|improve this answer
Shouldn't it read \newcommand*{\at}[2][]{#1|_{#2}} ? – cgnieder Jul 21 '12 at 10:41
@cgnieder Of course! – egreg Jul 21 '12 at 10:46
It has to be noted that you can understand the Jacobian as a differential and denote it by f' as well. Although you are right that in the real one-dimensional case one usually does not use the word differential. – canaaerus Jul 21 '12 at 10:48
@canaaerus And why not using $f'(P)$ (P denoting the point where the Jacobian or Fréchet derivative is computed) as well? Of course it depends on local traditions what notation to use. – egreg Jul 21 '12 at 10:56
1  
@asymptotically Writing f' and f^\prime in math mode is equivalent and the former is much better both for typing and for reading the source. The second derivative can be written f'' instead of f^{\prime\prime} and here it's easy to see which one is better. – egreg Jul 21 '12 at 11:08
show 4 more comments
up vote 4 down vote

Basically just using | is mostly ok, but I have defined a special purpose command for this, that automatically scales the line vertically:

\newcommand\at[2]{\left.#1\right|_{#2}}

So you can write \at{f'}{x=1}. Alternatively there is also a \vert command, which is probably a bit more semantic, but is actually just a synonym for |.

Of course this makes sense mostly for bigger expressions like

\at{\frac{df}{dt}}{t=1}

Concerning the spacing see egregs comment, although I didn't notice this before.

share|improve this answer
I tried both, and they produced the same output as "|", except that the \at command had an extra pixel to the left. And the \rvert command was undefined, but \vert produced the same as "|". Is the "|" semantically correct or should I use something different? – asymptotically Jul 21 '12 at 10:19
You're introducing unwanted spacings at the left (1.2pt, which is quite noticeable): probably {\left.\kern-\nulldelimiterspace#1\right|_{#2}} (with an additional pair of braces) is better. Also \right| and \right\rvert are equivalent; I wouldn't use \rvert in the \left-\right free version, but | which is an ordinary symbol, while \rvert is a "closing atom". – egreg Jul 21 '12 at 10:20

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.