Changing column separation in smallmatrix environment

Question

How can I change the width of the space between columns in a smallmatrix environment? I can't find any decent documentation on smallmatrix anywhere.

The array environment has the setcolsep{..} command, but I can't find anything analogous for smallmatrix.

Answer 1

The intercolumn space is \thickspace; you can do a trick

\newenvironment{xsmallmatrix}[1]
  {\renewcommand\thickspace{\kern#1}\smallmatrix}
  {\endsmallmatrix}

Now you can decide for yourself what the separation is (default .2777em):

\begin{xsmallmatrix}{.5em}
a & b\\
c & d
\end{smallmatrix}

If you want a fixed different space, say .5em, define

\newenvironment{xsmallmatrix}
  {\renewcommand\thickspace{\kern.5em}\smallmatrix}
  {\endsmallmatrix}

Beware that \thickspace is used in other places and it might happen in one of the small matrix entries (not very probable, though). A more robust patch might be

\usepackage{etoolbox}
\patchcmd{\smallmatrix}{\thickspace}{\kern.5em}{}{}

and from now on all smallmatrix environments will use .5em as intercolumn space.

Answer 2

Here's a small document, which shows a redefinition of smallmatrix fur supporting an optional argument for the separation. The definition is taken from amsmath.sty, it's just extended by using this argument instead of \thickspace.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\renewenvironment{smallmatrix}[1][.2777em]{\null\,\vcenter\bgroup
  \Let@\restore@math@cr\default@tag
  \baselineskip6\ex@ \lineskip1.5\ex@ \lineskiplimit\lineskip
  \ialign\bgroup\hfil$\m@th\scriptstyle##$\hfil&&\kern#1\hfil
  $\m@th\scriptstyle##$\hfil\crcr
}{%
  \crcr\egroup\egroup\,%
}
\makeatother
\begin{document}
\[
  \begin{smallmatrix}
    a & b\\
    c & d
  \end{smallmatrix}
  \quad
  \begin{smallmatrix}[8pt]
    a & b\\
    c & d
  \end{smallmatrix}
\]
\end{document}

It's using internal macros as the original definition does, which might be changed. While egreg shows a workaround and a patch, this should show a straighforward redefinition starting from the original source code.