# matrix in a matrix

I would like to draw a matrix with top left, top right, bottom left and bottom right blocks. The top left should be a 3x3 matrix with numerical entries, the top right is 0, the bottom left is 0 and the bottom right is just a "single" entry uJ. Here is my attempt.

$\left[ \begin{array}{c|c} [\begin{array{c|c|c} 0 & 0 & 2\mathrm tr(MM^{*}) \\ 0 & 0 & -ua \\ u & -ua & 0 \end{array}] & 0\\ \hline 0 & uJ \end{array}\right].$

I would like to emphasize that the non-top-left entries are not just single entries, but blocks, so I do not want to just make a 4 by 4 matrix with vlines and hlines and additional 0's.

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Welcome to TeX.SE. While code snippets are useful in explanations, it is always best to compose a fully compilable MWE that illustrates the problem including the \documentclass and the appropriate packages so that those trying to help don't have to recreate it. – Peter Grill Apr 3 '12 at 17:51

$\left[ \begin{array}{c@{}c@{}c} \left[\begin{array}{cc} a_{11} & a_{12} \\ a_{21} & a_{22} \\ \end{array}\right] & \mathbf{0} & \mathbf{0} \\ \mathbf{0} & \left[\begin{array}{ccc} b_{11} & b_{12} & b_{13}\\ b_{21} & b_{22} & b_{23}\\ b_{31} & b_{32} & b_{33}\\ \end{array}\right] & \mathbf{0}\\ \mathbf{0} & \mathbf{0} & \left[ \begin{array}{cc} c_{11} & c_{12} \\ c_{21} & c_{22} \\ \end{array}\right] \\ \end{array}\right]$