95

Although the two outputs look quite similar, what is the advantage of using $f^\prime$ instead of $f'$? By the way, here is my code:

\documentclass‎{article‎}‎
\begin{document}‎‎‎
\[‎ ‎f^‎\prime ‎(x)=y‎ ‎\]‎
\[‎ ‎f'‎ ‎(x)=y‎ ‎\]‎‎
\end{document}
  • 39
    There's no advantage in using $f^\prime$; it's just more awkward to type than $f'$ and the result is exactly the same. – egreg Dec 15 '12 at 17:14
106

TL;DR: ' is a shorthand for ^{\prime}.


' is defined in latex.ltx as active math character:

\def\active@math@prime{^\bgroup\prim@s}
{\catcode`\'=\active \global\let'\active@math@prime}
\def\prim@s{%
  \prime\futurelet\@let@token\pr@m@s}
\def\pr@m@s{%
  \ifx'\@let@token
    \expandafter\pr@@@s
  \else
    \ifx^\@let@token
      \expandafter\expandafter\expandafter\pr@@@t
    \else
      \egroup
    \fi
  \fi}
\def\pr@@@s#1{\prim@s}
\def\pr@@@t#1#2{#2\egroup}

The active ' looks for following ' and puts them together as superscript, a''' becomes a^{\prime\prime\prime}. Thus using ' makes the input easier to write.

13

Sometimes you may want to pass LaTeX code as an argument to another program. In that case the code is typically wrapped in quotes. Using quote to mean a prime will confuse the second program. For example to type TeX in graphical output of MATLAB one may use something like

str='$$F^\prime$$'
text(0,0,str,'Interpreter','latex')

to print $F'$ at location $(0,0)$. Using $F'$ in the code however becomes problematic.

  • 1
    use double quotes or escapes for the String? – MaxNoe Jan 26 '15 at 10:21
  • 2
    @MaxNoe Double quotes don't work in Matlab. String escaping is done with a double quote. So you'd have to write things like 'The symbol f'' is used to represent the first derivative of a function', which looks wrong at a first glance. All things considered, f^\prime doesn't look so bad. – Federico Poloni Feb 9 '15 at 22:18
4

Tl;DR: You can use \prime with additional superscripts.

There is certainly one advantage to using \prime under a particular situation. Suppose you have a map \pi which necessitates the use of another map \pi', which at first seems to be appropriately named.

That is, until you have to pullback something with respect to \pi'. Now you can either write (\pi')^* or \pi^{\prime,*}. The latter looks, arguably, a little better.

But both look horrible.

  • 7
    \pi'^{,*} is perfectly equivalent to \pi^{\prime,*} – egreg May 16 '17 at 15:11
  • @egreg \pi'^{,*} fails with error "double superscript"; \pi^{\prime,*} doesn't (using scrartcl and amsmath). So I prefer the latter … ;-) – CL. Feb 6 at 11:32
  • @CL. Works for me with a minimal document. Some more details would be necessary. – egreg Feb 6 at 12:08
  • @egreg Figured it out. It's the breqn package that breaks it. MWE. – CL. Feb 6 at 13:10
  • 1
    @CL. Besides equations, breqn breaks so many things… – egreg Feb 6 at 13:27
1

In fact, there's a slight difference when you have two superscripts. For instance:

{\bar{e}_k}^{\prime\dag}
{\bar{e}_k}^{'\dag}

The first one fits better.

  • Sure, but that's related to ' being the same as ^{\prime}, so if you have foo^{'\dag} your prime symbol is superscripted twice. {\bar{e}_k}'^{\dag} will give identical output as far as my eye can tell, but I agree your input is more elegant. Also fiddling with the my input led me to a double superscript error, which is frankly what I expected to begin with, so using foo^{\prime\dag} is definitely a nice idea here, I think – Au101 Jul 19 '16 at 7:32
  • 6
    You should type \bar{e}_k'^{\dag} (the braces around \bar{e}_k do nothing). – egreg Jul 19 '16 at 13:54
0

Although ' is a shorthand for ^\prime as noted in answers above, this can get in your way when dealing with multiple superscripts in combination with subscripts:

\pi_{s+r}^{\ast\prime} = 
\pi^{\ast\prime}_{s+r} \ne 
\pi^{\ast}'_{s+r}      \ne 
\pi_{s+r}^{\ast}'
0

I give a practical difference for Emacs users.

If you write $x^{\prime\prime}$ it is all right, if you write $x''$ you completely mess up the AUCTeX syntax coloring.

  • Which version opf AUCTEX, I have no issues with $x''$ – daleif Jan 10 at 12:21
  • "12.1.1" I am using the default latex configuration of the latest release version of Spacemacs, so I guess many other people share my latex setting – Nisba Jan 10 at 12:25
  • Interesting, I still use 11.90. Just had a PhD student come by yesterday, who also had problems with AUCTEX 12.1.1, which went away when he downgraded (he installed the Emacs + auctex for Windows there is on CTAN, instead). Seems like I'd better stay of 12.1 for while. Perhaps you should mention this on the auctex mailiing this such that they are aware of the issue. – daleif Jan 10 at 13:44
  • 1
    I reported the issue – Nisba Jan 10 at 14:11

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