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What is the best/the usual way to typeset a placeholder for a variable in an equation?

Specifically, I want to do something like g = f(x, .) (which of course means that g is defined by g(y) = f(x,y)). I tried \cdot, but it somehow does not look right (and the spacing needs to be adjusted either). On the other hand, a \bullet seems to be too fat.

Any other suggestions?

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Sometimes a stroke is used: \[ g(-) = f(x,-) \]. –  Gonzalo Medina Mar 6 '12 at 22:55
    
Why not a normal stop $g(y) = f(x,.)$? –  Yiannis Lazarides Mar 6 '12 at 23:01
    
OK, the <code>g=f(x,\cdot)</code> does not look too bad, I mixed something up in my memory. What did not look good was something like <code>M \otimes \cdot</code> denoting the functor given by tensoring. But in my particular case, the <code>\cdot</code> is already taken for a left action on modules... I will consider the "normal" stop-dot and the asterisk –  michael88 Mar 6 '12 at 23:09
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3 Answers

up vote 16 down vote accepted

Here are some considerations, since this question is open-ended:

enter image description here

\documentclass{article}
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\newcommand{\parm}{\mathord{\color{black!33}\bullet}}%
\begin{document}
\[
  f(x,.) \quad f(x,\cdot) \quad f(x,\ast) \quad f(x,\star) \quad 
  f(x,\bullet) \quad f(x,\parm)
\]
\end{document}​

I've included \parm which is a 33% black \bullet, perhaps a little more subtle than the full-fledged solid \bullet. Whatever you choose, best to define a command that sets such a "placeholder". If you're after something "fancy", I would suggest you peruse the tables of symbols contained in Scott Pakin's Comprehensive LaTeX Symbol List.

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4  
In any case I suggest to enclose the symbol in braces (equivalent to use \mathord) so it's surely treated as an ordinary math atom. For \cdot one could consider {\cdot\,} to give it more room; the same for the period. –  egreg Mar 7 '12 at 0:16
    
I think a resized textbullet is best. That is, a textbullet reduced in size to maybe 80% looks best. –  Seamus Mar 7 '12 at 17:36
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How about using a ?, or putting it in a \fbox.. If you still want to use the \cdot, you could use \makebox to reserve enough space.

enter image description here

\documentclass{article}
\newcommand*{\X}{\textbf{?}}%
\newcommand*{\Y}{\textbf{\fbox{?}}}%
\newcommand*{\Z}{\makebox[1ex]{\textbf{$\cdot$}}}%
\begin{document}
    $g(y) = f(x,\X) = f(x,\Y)= f(x,\Z)$
\end{document}
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Based on your last comment, I suggest to use a minus sign, that's widely used in category theory:

\newcommand{\blank}{{-}}

The additional braces mask its nature of binary operator, so it's treated as an ordinary symbol. Using a personal command allows you to change the symbol by only modifying its definition.

\documentclass{article}
\newcommand{\blank}{{-}}
\begin{document}
$f(x,\blank)$

$M\otimes_{R}\blank$
\end{document}

enter image description here

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