[Tex/LaTex] Derivative at a point

symbols

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?

Best Answer

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.

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