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Small improvment of the code

edited Mar 25, 2021 at 12:10

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Before drawing frames, you have to add space. For the vertical space, it's not very easy because you can'tI use struts (it would change the size of the PGF/Tikz nodes created bysome nicematrix\vskip for each cell and we will use those nodes after).

Here is a way to do that (you could also have tried to usein arraystretch\noalign).

\documentclass{standalone} \usepackage{nicematrix} \begin{document} $\left(\vcenter{\smallskip \hbox{\(\begin$\begin{NiceArraypNiceArray}{cc|cc}[margin=2pt] \noalign{\vskip\smallskipamount} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end\noalign{NiceArray}\)\vskip\smallskipamount} \smallskip\end{pNiceArray}\right)$   \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document}   \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin$\begin{NiceArraypNiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document} \documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc\vskip\smallskipamount}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \noalign{\vskip\smallskipamount} \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskippNiceArray}\right)$ \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}}   $\left(\vcenter{\smallskip \hbox{\( \begin$\begin{NiceArraypNiceArray}{cc|cc}[margin=2pt,create-medium-nodes] \noalign{\vskip\smallskipamount} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \noalign{\vskip\smallskipamount} \CodeAfter \begin{tikzpicture}[name suffix  = -medium] \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskippNiceArray}\right)$   \end{document}

Before drawing frames, you have to add space. For the vertical space, it's not very easy because you can't use struts (it would change the size of the PGF/Tikz nodes created by nicematrix for each cell and we will use those nodes after).

Here is a way to do that (you could also have tried to use arraystretch).

\documentclass{standalone} \usepackage{nicematrix} \begin{document} $\left(\vcenter{\smallskip \hbox{\(\begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end{NiceArray}\)} \smallskip}\right)$ \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document}   \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document} \documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}}   $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt,create-medium-nodes] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture}[name suffix  = -medium] \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

Before drawing frames, you have to add space. For the vertical space, I use some \vskip in \noalign.

\documentclass{standalone} \usepackage{nicematrix} \begin{document} $\begin{pNiceArray}{cc|cc}[margin=2pt] \noalign{\vskip\smallskipamount} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \noalign{\vskip\smallskipamount} \end{pNiceArray}$   \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\begin{pNiceArray}{cc|cc}[margin=2pt] \noalign{\vskip\smallskipamount} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \noalign{\vskip\smallskipamount} \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{pNiceArray}$ \end{document}

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\begin{pNiceArray}{cc|cc}[margin=2pt,create-medium-nodes] \noalign{\vskip\smallskipamount} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \noalign{\vskip\smallskipamount} \CodeAfter \begin{tikzpicture}[name suffix = -medium] \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{pNiceArray}$   \end{document}

Typo

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edited Mar 25, 2021 at 11:30

F. Pantigny

51.9k
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99

For the construction you want, consider first the following matrix.

\documentclass{standalone} \usepackage{nicematrix} \begin{document} \(\begin{pNiceArray}{cc|cc} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\ \hline 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end{pNiceArray}\) \end{document}

Here is a way to do that (you could also have tried to use arraystretch).

\documentclass{standalone} \usepackage{nicematrix} \begin{document} $\left(\vcenter{\smallskip \hbox{\(\begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end{NiceArray}\)} \smallskip}\right)$ \end{document}

You can now draw the frames with Tikz in the \CodeAfter.

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document} \documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

Maybe you want homogeneity for the four blue frames. In that case, you should use the medium nodes (created by nicematrix).

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt,create-medium-nodes] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture}[name suffix = -medium] \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

For the construction you want, consider first the following matrix.

\documentclass{standalone} \usepackage{nicematrix} \begin{document} \(\begin{pNiceArray}{cc|cc} \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\ \hline 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end{pNiceArray}\) \end{document}

Here is a way to do that (you could also have tried to use arraystretch).

\documentclass{standalone} \usepackage{nicematrix} \begin{document} $\left(\vcenter{\smallskip \hbox{\(\begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \end{NiceArray}\)} \smallskip}\right)$ \end{document}

You can now draw the frames with Tikz in the \CodeAfter.

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document} \documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture} \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

Maybe you want homogeneity for the four blue frames. In that case, you should use the medium nodes (created by nicematrix).

\documentclass{standalone} \usepackage{nicematrix,tikz} \usetikzlibrary{fit} \begin{document} \tikzset{highlight/.style={rectangle, draw=blue, rounded corners = 0.5 mm, inner sep=1pt, fit=#1}} $\left(\vcenter{\smallskip \hbox{\( \begin{NiceArray}{cc|cc}[margin=2pt,create-medium-nodes] \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} & A^2+B^2 & \frac{a^2+b^2}{2} \\ 0 & \frac{a^2+b^2}{2} & \frac{(a^2+b^2)^2}{A^2+B^2} & \frac{2ab}{a+b} \\[4pt] \hline \noalign{\vskip4pt} 2A^2+2B^2 & 0 & \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} \\ \frac{2ab}{a+b} & \frac{(a^2+b^2)^2}{A^2+B^2} & 0 & \frac{a^2+b^2}{2} \\ \CodeAfter \begin{tikzpicture}[name suffix = -medium] \node [highlight=(1-1)(1-2)(2-1)(2-2)] {} ; \node [highlight=(1-3)(1-4)(2-3)(2-4)] {} ; \node [highlight=(3-1)(3-2)(4-1)(4-2)] {} ; \node [highlight=(3-3)(3-4)(4-3)(4-4)] {} ; \end{tikzpicture} \end{NiceArray} \)} \smallskip}\right)$ \end{document}

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answered Mar 24, 2021 at 23:34

F. Pantigny

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The key highlight is defined previously in the document (documentation of nicematrix).