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How would one typset an array with cells above a bmatrix, something like

[a] & [b]
\begin{bmatrix}
1 & 2 \\
3 & 4
\end{bmatrix}

Where somehow the [a] and [b] cells would appear aligned and above the bmatrix.

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3 Answers 3

up vote 3 down vote accepted

You can use a modified \bordermatrix. It is described in mathmode.

You can also use blkarray package for convenience, see also this question.

\documentclass{article}
\usepackage{blkarray}
\begin{document}

\[
\begin{blockarray}{cc}
[a] & [b] \\
\begin{block}{[cc]}
1 & 2 \\
3 & 4 \\
\end{block}
\end{blockarray}
\]

\end{document}

enter image description here

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Is there a solution within the math environment? Great solution, but I need it to be within an equation environment. –  ccook Feb 11 '11 at 14:30
    
@ccook: \[ ... \] is the math environment. You can replace it with euqation env. –  Leo Liu Feb 11 '11 at 14:32
    
I should have made less of an assumption. I'm trying to typeset it for wikimedia and it doesn't recognize the blocks :( –  ccook Feb 11 '11 at 14:34
    
@ccook: Wiki uses part of TeX syntax for math formulas. However, wiki does NOT have full TeX abilities. You'll have to use the restricted TeX described in en.wikipedia.org/wiki/Wikipedia:Editing_Math –  Leo Liu Feb 11 '11 at 14:41
    
Thank you. This is definitely the best way to go. –  ccook Feb 11 '11 at 14:55

For wikipedia only, you can use this manual dirty version. It is tested in Wikipedia's sandbox.

<math>
\begin{matrix}
\begin{matrix} [a] & [b] \end{matrix} \\
\begin{bmatrix} 1 & 2 \\ 3 & 4 \end{bmatrix}
\end{matrix}
</math>
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Thank you, this works well for wikimedia. I did have to push the header around with \quad, but as you said, quick and dirty for wikimedia. –  ccook Feb 11 '11 at 14:54

Instead of \[...\] you can use $...$. one example with arrays and one with \multicolumn

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\[
\begin{array}{cc}
[a] & [b]\\
\left[\begin{array}{c}
1\\
3 
\end{array}\right. &
\left.\begin{array}{c}
2\\
4 
\end{array}\right] 
\end{array}
\]

\[
\begin{array}{cc}
[a] & [b]\\
\multicolumn{2}{@{}c@{}}{%
\begin{bmatrix}
~1 & 2~ \\
~3 & 4~
\end{bmatrix}}%
\end{array}
\]

\end{document}

enter image description here

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This also does not work for wikipedia. Even \multicolumn is not supported by mediawiki. –  Leo Liu Feb 11 '11 at 14:54
    
then we do it withpout \multicolumn ... –  Herbert Feb 11 '11 at 15:07

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