LaTeX forum ⇒ Math & Science ⇒ Diagonal overbraces in a matrix

[benbean]

I have looked on the web for an answer to this but have had no luck yet so hopefully someone can offer some help here.
I have the matrix below:

\begin{pmatrix}
x_{1,1} & x_{1,2} & \cdots & x_{1,j} \\
x_{2,1} & x_{2,2} & \cdots & x_{2,j} \\
\vdots & \vdots & \ddots & \vdots \\
x_{i,1} & x_{i,2} & \cdots & x_{i,j}
\end{pmatrix}

Is it possible to group the elements x_{1,1}, x_{2,2}, \ddots, x_{i,j} in a diagonal curly brace? Possibly in a form like \overbrace or \underbrace?

If this is possible could I then have this brace with text?

Thanks,
benbean

[localghost]

I'm not aware of a solution with the known commands. But you can use pgf/tikZ and some of its libraries.

Code: [Select all] [Expand/Collapse] [Download] (untitled.tex) Open in Overleaf

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,decorations.pathreplacing}
 
\begin{document}
   \begin{tikzpicture}[decoration={brace,amplitude=5pt}]
     \matrix (magic) [matrix of math nodes,left delimiter=(,right delimiter=)] {
       x_{1,1} & x_{1,2} & \cdots & x_{1,j} \\
       x_{2,1} & x_{2,2} & \cdots & x_{2,j} \\
       \vdots & \vdots & \ddots & \vdots \\
       x_{i,1} & x_{i,2} & \cdots & x_{i,j} \\
     };
     \draw[decorate,red] (magic-1-1.north) -- (magic-4-4.north) node[above=5pt,midway,sloped] {label};
   \end{tikzpicture}
\end{document}

The pstricks package offers similar capabilities.


Best regards and welcome to the board
Thorsten

[benbean]

Thanks a lot Thorsten exactly what I had in mind!

Just a small query - Is it possible to extend the curly brace so as to enclose the two end elements, namely the x_{1,1} and x_{i,j} elements? The solution you posted seems to have the brace stop over the 'midpoint' of each element.

I am very new to LaTeX so the learning curve is looking steep at the minute .

Thanks again
b

[gmedina]

Hi,

you can fine-tune the coordinates; a little example based on localghost's code:

Code: [Select all] [Expand/Collapse] [Download] (untitled.tex) Open in Overleaf

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix,decorations.pathreplacing,calc}
 
\begin{document}
   \begin{tikzpicture}[decoration={brace,amplitude=5pt}]
     \matrix (magic) [matrix of math nodes,left delimiter=(,right delimiter=)] {
       x_{1,1} & x_{1,2} & \cdots & x_{1,j} \\
       x_{2,1} & x_{2,2} & \cdots & x_{2,j} \\
       \vdots & \vdots & \ddots & \vdots \\
       x_{i,1} & x_{i,2} & \cdots & x_{i,j} \\
     };
     \draw[decorate,red] ($(magic-1-1.north west)+(0.2,0.15)$) -- (magic-4-4.north east) node[above=5pt,midway,sloped] {label};
   \end{tikzpicture}
\end{document}

[benbean]

Thanks gmedina, I am able to get what I am after with your suggestion.

Thank you guys for all your help. I was at a loss until I came here
benbean

[benbean]

This is a bit off topic but how would I write this matrix in an equation style? If it is defined as A = <matrix>, how do I get it so that the "A = " part is centered inline with the matrix?

Any help is greatly received,
benbean

[localghost]

This is a bit off topic but how would I write this matrix in an equation style? If it is defined as A = <matrix>, how do I get it so that the "A = " part is centered inline with the matrix? [...]

I hope that you only mean the alignment with respect to the equal sign and not how you can get it into a math environment in general. Since the origin of the tikzpicture environment is put at the lower left corner by default, it needs an additional option.

Code: [Select all] [Expand/Collapse] [Download] (untitled.tex) Open in Overleaf

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{matrix,decorations.pathreplacing,calc}
 
\begin{document}
\[
   \boldsymbol{A}=
   \begin{tikzpicture}[decoration={brace,amplitude=5pt},baseline=(current bounding box.west)]
     \matrix (magic) [matrix of math nodes,left delimiter=(,right delimiter=)] {
       x_{1,1} & x_{1,2} & \cdots & x_{1,j} \\
       x_{2,1} & x_{2,2} & \cdots & x_{2,j} \\
       \vdots & \vdots & \ddots & \vdots \\
       x_{i,1} & x_{i,2} & \cdots & x_{i,j} \\
     };
     \draw[decorate,red] ($(magic-1-1.north west)+(0.2,0.15)$) -- (magic-4-4.north east) node[above=5pt,midway,sloped] {label};
   \end{tikzpicture}
\]
\end{document}

[benbean]

Yes it was only the alignment I was trying to fix. Your code worked again so many thanks again! It's all looking how I intended, thanks for all your help.

b