Typeset a piecewise matrix?

Question

I need to typeset the following:

But I cannot seem to get the dotted line or the centered r_{i, j}. Can anyone show a minimal working example?

You should try with this first: How to create vertical and horizontal dotted lines in a matrix? — Werner, Dec 24 '17 at 17:35

score 6 · Answer 1 · answered Dec 24 '17 at 17:50

For the dotted line, you can use the arydshln package and set the dash length and spacing to appropriate values:

\usepackage{arydshln}    
\setlength{\dashlinedash}{0.5pt}
\setlength{\dashlinegap}{1pt}

For the centered r_{i,j}, you can nest a vertical array inside a horizontal array.

\documentclass{article}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{arydshln}

\setlength{\dashlinedash}{0.5pt}
\setlength{\dashlinegap}{1pt}
\begin{document}
    \begin{equation*}
    R_{a,b}(\theta)=\left[\begin{array}{l:l}
    r_{i,j} & \begin{array}{l}
    r_{a,a}=\cos(\theta) \\
    r_{b,b}=\cos(\theta) \\
    r_{a,b}=-\sin(\theta) \\
    r_{b,a}=\sin(\theta) \\
    r_{j,j}=1,\quad j\ne a,\quad j\ne b \\
    r_{i,j}=0,\quad \mathrm{elsewhere} \\
    \end{array}
    \end{array}\right] \\
    \end{equation*}
\end{document}

score 6 · Accepted Answer · answered Dec 24 '17 at 19:33

Another approach, a little bit simpler

\documentclass{article}
\usepackage{amsmath}
\usepackage{arydshln}

\begin{document}

\[
R_{a,b}(\theta)=\left[\quad r_{i,j}\left.
\begin{array}{r;{2pt/2pt}l}
    &   r_{a,a}=\cos(\theta) \\
    &   r_{b,b}=\cos(\theta) \\
    &   r_{a,b}=-\sin(\theta) \\
    &   r_{b,a}=\sin(\theta) \\
    &   r_{j,j}=1,\quad j\ne a,\quad j\ne b \\
    &   r_{i,j}=0,\quad \mathrm{elsewhere} \\
\end{array}
\,\right.\right]
\]

\end{document}

score 3 · Answer 3 · answered Dec 24 '17 at 19:16

A solution with pstricks for the dotted vertical line, and an alignedat environment inside a bmatrix, which allows for the alignment of the = signs:

\documentclass{article}

\usepackage{mathtools}
\usepackage{pst-node}
\usepackage{auto-pst-pdf} %% to compile with pdflatex

\begin{document}

\begin{equation*}
  \begin{pspicture}
    R_{a,b}(θ)=\begin{bmatrix}
      r_{i,j} & \begin{alignedat}{2}
        & \pnode[-1.3ex, 1ex]{B} & r_{a,a} & =\cos(θ) \\
        & & r_{b,b} & =\cos(θ) \\
        & & r_{a,b} & =-\sin(θ) \\
        & & r_{b,a} & =\sin(θ) \\
        & & r_{j,j} & =1,\quad j\ne a,\quad j\ne b \\
        & \pnode[-1.3ex, -0.25ex]{E} & r_{i,j} & =0,\quad \mathrm{elsewhere} \\
      \end{alignedat}
    \end{bmatrix}
    \ncline[linestyle = dotted, dotsep = 1.5pt]{B}{E}
  \end{pspicture}
\end{equation*}

\end{document}

score 3 · Answer 4 · answered Feb 15 '22 at 20:37

It's easy with {bNiceArray} of nicematrix.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
[
R_{a,b}(\theta)=
\begin{bNiceArray}{c:l@{}l}
\Block{*-1}{r_{i,j}} 
  & r_{a,a} & =\cos(\theta) \
  & r_{b,b} & =\cos(\theta) \
  & r_{a,b} & =-\sin(\theta) \
  & r_{b,a} & =\sin(\theta) \
  & r_{j,j} & =1,\quad j\ne a,\quad j\ne b \
  & r_{i,j} & =0,\quad \mathrm{elsewhere} \
\end{bNiceArray}
]
\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).

Typeset a piecewise matrix?

4 Answers4