Math, cells above a bmatrix

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.

-

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}


-
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.

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

-
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}


-
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