[Tex/LaTex] Spacing around \cdot when used as a wildcard

spacing

I have to use \cdot as a wildcard indicator in an expression like: K(\cdot,y).

My concern is: is the spacing around the \cdot correct? should I force something in either one side or the other of it?

Best Answer

For what it's worth, this is converted from comments. I'm no expert on mathematical typography, so these are if anything suggestions.

My first idea was

K({\cdot},y)

which produces the same spacing as

K(\cdot,y)

because neither ( nor , is treated as an ordinary character as 5 and x would be in 5 \cdot x.

If you rather want to reproduce the spacing from 5 \cdot x you can use empty groups to fake them.

K({}\cdot{},y)

does look more like something like

K(x,y)

You can, of course, specify any space around the \cdot with \mspace or the primitive \mkern which both take dimensions in mu (math unit: mu = 1/18 em):

{{\mkern 2mu\cdot\mkern 2mu}}

I recommend to define a macro for this kind of wildcard/placeholder so that, if you ever change your opinion about the correct spacing you can just change the macro's definition (aka “The LaTeX way”).

\newcommand*\wc{{\mkern 2mu\cdot\mkern 2mu}}

Code

\documentclass{article}
\newcommand*\wc{{}\cdot{}}
\begin{document}\noindent
\( K (  \cdot  , y ) \) \\
\( K ({ \cdot} , y ) \) \\
\( K ({}\cdot{}, y ) \) \\
\( K (  \wc    , y ) \) \\
\( K (        x, y ) \)
\end{document}

Output

Related Solutions

[Tex/LaTex] How to change default for spacing around binary relations

This answer doesn't encourage modifying the spaces, since TeX does a great job already. It merely discusses the concepts of spacing around math elements.

Spacing around math operators are discussed in the mathmode documentation (section 11 Space, p 28). Here's an example:

enter image description here

\documentclass{article}
\newcommand{\func}{f(x) = x^2 + 3x_0 \cdot \sin x}% Function
\begin{document}
\[
  \renewcommand{\arraystretch}{1.5}
  \begin{array}{rl}
    \mbox{default} & \func \\
    \verb|\thinmuskip=0mu| & \setlength{\thinmuskip}{0mu} \func \\
    \verb|\medmuskip=0mu| & \setlength{\medmuskip}{0mu} \func \\
    \verb|\thickmuskip=0mu| & \setlength{\thickmuskip}{0mu} \func \\
    \mbox{all set to zero} & \setlength{\thinmuskip}{0mu}\setlength{\medmuskip}{0mu}
      \setlength{\thickmuskip}{0mu} \func
  \end{array}
\]
\end{document}

To manually adjust the amount of space around a symbol, you can change it to an ordinal symbol by wrapping it in braces - for example {=} - and then add the space your want around it: \,{=}\,. Here's another example:

enter image description here

\documentclass{article}
\begin{document}
\[
  \begin{array}{rc}
      \mbox{default} & x = x \\
      \verb|\mathbin| & x \mathbin{=} x \\
      \verb|\mathrel| & x \mathrel{=} x \\
      \verb|\mathord| & x \mathord{=} x \\
      \verb|{ }| & x {=} x \\
      \verb|\,{ }\,| & x\,{=}\,x \\
      \verb|\;{ }\;| & x\;{=}\;x \\
      \verb|\:{ }\:| & x\:{=}\:x
  \end{array}
\]
\end{document}

Negative spaces, although not shown is obtainable via \!. Of course, you can also use other spacing macros like \hspace.

Also see What is the difference between \mathbin vs. \mathrel? for a discussion between the difference in spacing around binary and relational operators.

[Tex/LaTex] Spacing around \cdot

Since you want to use the blank next either between an opening atom (the open parenthesis) and a relation atom (\mid) or between a relation atom and a closing atom, you can define \blank to be a relation symbol:

\documentclass{article}

\newcommand{\blank}{\mathrel{\;\cdot\;}}

\begin{document}

$C(\blank \mid F_{t-1})$

$C(x \mid \blank)$

\end{document}

The trick is that TeX doesn't add space in cases

Open Rel Rel

Rel Rel Close