I recently started learning LaTeX and though I found answers for almost all of my questions by looking online (and especially here), there is still one more thing I would like to be able to do but can't for the article I am writing:

I would like to be able to specify the dimensions and the "subdimensions" below a matrix. To be clearer, let us take the following example:

$ D = \underbracket{\begin{pmatrix} D_1 & 0 & 0 & & & \\ 0 & \ddots & & & {\textrm{\huge 0}} & \\ 0 & 0 & D_n & & & \\ & & & & & \\ & \textrm{\huge 0} & & & {\textrm{\huge 0}} & \\ & & & & & \\ \end{pmatrix}}_N $

this gives a nice matrix whith the dimension N specified as a bracket under the matrix. That is fine, however I would like to add a second bracket to specify the smaller dimension n of the inner block matrix right below the matrix (and possibly above the first main bracket) which is shorter, i.e. I would like it to extend from 1 to n and not all the way below the matrix.

I am open to any solution, but the simpler the better for me :)

3

1 Answer

The following is not really eye-candy, but it's probably what you're after:

enter image description here

\documentclass{article} \usepackage{mathtools}% \begin{document} \[ D = \underbracket{\begin{pmatrix} D_1 & 0 & 0 & & & \\ 0 & \ddots & & & {\textrm{\huge 0}} & \\ 0 & 0 & D_n & & & \\ & & & & & \\ & \textrm{\huge 0} & & & {\textrm{\huge 0}} & \\ & & & & & \\ \end{pmatrix}}_N \] \[ D= \underbracket[.4pt]{\left(\begin{array}{@{}c@{\quad}c} \underbracket[.4pt]{\begin{array}{ccc} D_1 & \vphantom{\ddots}0 & 0 \\ 0 & \ddots & 0 \\ 0 & \vphantom{\ddots}0 & D_n \end{array}}_{n} & \text{\huge 0} \\ \\ \text{\huge 0} & \text{\huge 0} \end{array}\right)}_{N} \] \end{document}

The use of \vphantom ensure that the row height for the "inner matrix" is similar across all rows (without actually printing \ddots).

I went with a more traditional approach (using array), rather than pmatrix. The advantage is that you have control over the alignment.