# Special matrix - don't know how

Now... I know this code is embarrassing but it's the closer I've got, at least as displayed output, to what I'm trying to do

\documentclass{article}
\usepackage{amsmath,blkarray,newpxtext,newpxmath}
\begin{document}
\begin{aligned} j\qquad\qquad\:\:\;\, \\ i\,\left( \begin{blockarray}{lrlr} \begin{block}{lr|lr} a_{11} & \dots & \dots & a_{1n} \\ \vdots & \quad & \quad & \vdots \\ \end{block} \hline \begin{block}{lr|lr} \vdots & \quad & \quad & \vdots \\ a_{n1} & \dots & \dots & a_{nn} \\ \end{block} \end{blockarray} \right) \\ \end{aligned}
\end{document}


The output is the following:

What I'm trying to do is basically what you see in the image above, but with a "a_{ij}" element in the middle instead of the cross. Also the alignment of i and j is made using spaces, which I suppose is not the best solution.

Any suggestions?

Adding this as an additional answer. You could use the multirow package with this code:

\documentclass{article}
\usepackage{amsmath,newpxtext,newpxmath,multirow}
\begin{document}
$i\ \overset{\textstyle{j}}{\left(\begin{array}{ccccc} a_{11} & \cdots & \multirow{2}{*}{\rule[-4ex]{.5pt}{5.5ex}} & \cdots & a_{1n}\\ \vdots & & & & \vdots\\ \multicolumn{2}{c}{\rule[.5 ex]{9ex}{.5pt}} & a_{ij} & \multicolumn{2}{c}{\rule[.5 ex]{9ex}{.5pt}} \\ \vdots & & \multirow{2}{*}{\rule[-2ex]{.5pt}{5ex}} & & \vdots\\ a_{n1} & \cdots & & \cdots & a_{nn} \end{array}\right)}$
\end{document}


Here's the output:

Is this what you have in mind?

$i\overset{\textstyle{j}}{\begin{pmatrix} a_{11} & \cdots & & \cdots & a_{1n}\\ \vdots & & \vdots & & \vdots\\ & \cdots & a_{ij} & \cdots & \\ \vdots & & \vdots & & \vdots\\ a_{n1} & \cdots & & \cdots & a_{nn} \end{pmatrix}}$


You could remove the additional \vdots and \cdots commands if you like. Or add some \ddots for diagonals.

• This solves the i and j alignment problem, but I also need the straight lines instead of dots. Sorry if I didn't made myself clear, and thanks for your help – CAKES Sep 26 '17 at 17:08
• I provided an additional answer with the lines. – Sandy G Sep 26 '17 at 20:04

Here is a simple solution with booktabs, a pmatrix environment and one more column:

\documentclass{article}
\usepackage{mathtools, blkarray, newpxtext, newpxmath}
\usepackage{booktabs}

\begin{document}

\begin{aligned} \MoveEqLeft[4.6]\scriptstyle j \\[-0.5ex] \raisebox{0.4ex}{\scriptstyle i ~}\begin{pmatrix} \!\!\begin{blockarray}{lr@{}c@{}lr} \begin{block}{c@{\enspace}c@{}c@{}c@{\enspace}c} a_{11} & ... & \vrule & ... & a_{1n} \\ ⋮ & \quad &\vrule & \quad & ⋮ \\ \end{block} & & a_{ij} & & \\[-1.5ex] \cmidrule[0.4pt](l){1-2}\cmidrule[0.4pt](r){4-5}\addlinespace[0.6ex] \begin{block}{c@{\enspace}c@{}c@{}c@{\enspace}c} ⋮ & \quad & \vrule & \quad & ⋮ \\ a_{n1} & ... & \vrule & ... & a_{nn}\\[-1.6ex] \end{block} \end{blockarray}\!\! %\right) \\ \end{pmatrix} \end{aligned}

\end{document}


• Ok, now this is exactly what I want! Although I'm having some problems with the newpxpackage (which I've just installed), when I compile it gives me ! LaTeX Error: File 'binhex.tex' not found. <\br> Any idea what is the problem here and how to solve? – CAKES Sep 26 '17 at 17:46
• @CAKES - Which TeX distribution (and which vintage) is installed on your system? – Mico Sep 26 '17 at 18:26
• @Mico - I'm using MacTeX and... what's a vintage? (Sorry if I'm such a newbie) – CAKES Sep 26 '17 at 18:42
• Mico means ‘which year?’. – Bernard Sep 26 '17 at 18:45
• @CAKES - Is it MacTeX2017, MacTeX2016, MacTeX2009, etc? Did you do a full or just a "minimal" installation of MacTeX? – Mico Sep 26 '17 at 18:45

You can do it with blkarray; the only needed tricks are \BAmulticolumn and a definition for a filling rule higher than \hrulefill.

I defined the height the same as the fraction bar.

\documentclass{article}
\usepackage{amsmath,blkarray,newpxtext,newpxmath}

\newcommand{\highhrulefill}{%
\leavevmode
$\begin{blockarray}{ccc|cc} && \BAmulticolumn{2}{c}{j} \\ \begin{block}{c(cc|cc)} & a_{11} & \cdots & \cdots & a_{1n} \\ & \vdots & & & \vdots \\ i & \BAmulticolumn{4}{c}{\highhrulefill \; a_{ij} \; \highhrulefill} \\ & \vdots & & & \vdots \\ & a_{n1} & \cdots & \cdots & a_{nn} \\ \end{block} \end{blockarray}$