# [Tex/LaTex] Highlight elements in the matrix

framedhighlightingmath-modematrices

I have a matrix and need to highlight some elements in the matrix. I have found how to highlight a single element (I want to put the cell in the square):

$$A = \begin{bmatrix} \fbox{0} & \fbox{0} & 0 \\ \fbox{0} & \fbox{0} & 0 \\ 1 & 1 & 1 \\ \end{bmatrix}$$


But how can I outline elements of the submatrix like in the picture below? I need just a frame where some elements are located. You could use TikZ, and

• the fit library for creating nodes fitting the desired area,
• a style for the highlighted node, so separated from the code and easy to change,
• the TikZ options overlay and remember picture, so you can later refer to those nodes, for example for drawing arrows and annotations later.

To demonstrate why it's useful to have nodes for reference, I created an example, which shows transposing a matrix, highlights a submatrix both in the original matrix and the result, connected by an arrow and annotated: \documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit}
\tikzset{%
highlight/.style={rectangle,rounded corners,fill=red!15,draw,
fill opacity=0.5,thick,inner sep=0pt}
}
\newcommand{\tikzmark}{\tikz[overlay,remember picture,
baseline=(#1.base)] \node (#1) {#2};}
%
\newcommand{\Highlight}[submatrix]{%
\tikz[overlay,remember picture]{
\node[highlight,fit=(left.north west) (right.south east)] (#1) {};}
}
\begin{document}
$M = \left(\begin{array}{*5{c}} \tikzmark{left}{1} & 2 & 3 & 4 & 5 \\ 6 & 7 & 8 & 9 & 10 \\ 11 & 12 & \tikzmark{right}{13} & 14 & 15 \\ 16 & 17 & 18 & 19 & 20 \end{array}\right) \Highlight[first] \qquad M^T = \left(\begin{array}{*5{c}} \tikzmark{left}{1} & 6 & 11 & 16 \\ 2 & 7 & 12 & 17 \\ 3 & 8 & \tikzmark{right}{13} & 18 \\ 4 & 9 & 14 & 19 \\ 5 & 10 & 15 & 20 \end{array}\right)$
\Highlight[second]
%
\tikz[overlay,remember picture] {
\draw[->,thick,red,dashed] (first) -- (second) node [pos=0.66,above] {Transpose};
\node[above of=first] {$N$};
\node[above of=second] {$N^T$};
}
\end{document}​