(easy?) Matrix with wide blocks

Question

I'd like to create a matrix like this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$$ \begin{bmatrix}
    a & b' \\
    b & D
\end{bmatrix} $$
\end{document}

but I'd like to indicate that the size of vector b and put it like on two fields, horizontally or vertically. The same with matrix D which I'd like to put like a 2×2 matrix with a name in the center.

Welcome to TeX.SX! Could you please complete your code in order to make a minimal working example (MWE) that inlcudes the relevant packages as well as the documentclass? Could you please also include a sketch of the output you'd like to achieve? — leandriis, Mar 02 '19 at 14:26
Now it is good that you already provided an MWE, so I upvoted your question, but I can't still get it. Please add a sketch showing what you want. — , Mar 02 '19 at 14:36
Thank you for helping me with my first post. I have some experience at other community where it would automatically compile. Polish one. — lelel, Mar 02 '19 at 14:40
@lelel Into your original code there are a double dollar. See here https://tex.stackexchange.com/questions/503/why-is-preferable-to — Sebastiano, Mar 02 '19 at 17:29

egreg · Accepted Answer · 2022-12-13T23:13:18.390

I pretend that the second item in the first row is as wide as two normal columns. Similarly, the second item in the second row is as tall as two normal rows.

\documentclass{article}
\usepackage{amsmath,mathtools}
\begin{document}
[
\sbox0{$\begin{matrix} D & D\end{matrix}$}
\left[\begin{array}{c|c}
a & \mathmakebox[\wd0]{b'} \
\hline
b & \vphantom{\begin{matrix} D \ D \end{matrix}} D
\end{array}\right]
]
\end{document}

Added December 2022

A much better tool has appeared, namely nicematrix.

\documentclass{article}
\usepackage{amsmath}
\usepackage{nicematrix}
\begin{document}
[
\begin{bNiceArray}{c|w{c}{3em}}[margin]
a & b' \
\hline
\Block{2-1}{b} & \Block{2-1}{D} \
&
\end{bNiceArray}
]
\end{document}

score 1 · Answer 2 · answered Jan 01 '23 at 09:15

An alternative with tabularray:

\documentclass{article}
\usepackage{tabularray}
\UseTblrLibrary{amsmath}
\begin{document}
\[
\begin{+bmatrix}[
    vline{2} = {1}{abovepos = -1},
    vline{2} = {2}{belowpos = -1},
    colspec={cm{3em}},
    rowspec={Q[m]|m{3em}},
    ]
    a & b' \\
    b & D
\end{+bmatrix}
\]
\end{document}

I set abovepos and belowpos to -1 to shorten the vertical line, because I don't like it when it is higher than the parenthesis. If you like it, you can remove that setting, of course.

(easy?) Matrix with wide blocks

2 Answers2

Added December 2022

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