# Adding space between matrices

I am having some trouble when changing the order of the matrices. The dot product does not look very well, actually it is like it is embedded; hard to spot. How can add space between the two matrices to the right and make de dot point bigger?

CODE (Credits to Zarko)

\documentclass{article}
\usepackage{blkarray}

\begin{document}
$\renewcommand\arraystretch{1.1} \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{R}}\\ \begin{block}{(cccc)} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{block} \end{blockarray} \approx \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{\Theta}^T}\\ \begin{block}{(cccc)} \theta_1^{(1)} & \theta_2^{(1)} & \dots & \theta_b^{(1)} \\ \theta_1^{(2)} & \theta_2^{(2)} & \dots & \theta_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ \theta_1^{(m)} & \theta_2^{(m)} & \dots & \theta_n^{(m)} \\ \end{block} \end{blockarray}^{\raisebox{-1.5\baselineskip}{T}} \cdot \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{X}}\\ \begin{block}{(cccc)} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{block} \end{blockarray}$
\end{document}

## 2 Answers

\documentclass{article}
\usepackage{blkarray}

\begin{document}
$\renewcommand\arraystretch{1.1} \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{R}}\\ \begin{block}{(cccc)} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{block} \end{blockarray} \approx \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{\Theta}^T}\\ \begin{block}{(cccc)} \theta_1^{(1)} & \theta_2^{(1)} & \dots & \theta_b^{(1)} \\ \theta_1^{(2)} & \theta_2^{(2)} & \dots & \theta_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ \theta_1^{(m)} & \theta_2^{(m)} & \dots & \theta_n^{(m)} \\ \end{block} \end{blockarray}^{\raisebox{-1.5\baselineskip}{T}} \bullet \quad \begin{blockarray}{cccc} \BAmulticolumn{4}{c}{\mathbf{X}}\\ \begin{block}{(cccc)} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{block} \end{blockarray}$
\end{document}

Or you can keep your code as it is and add to \cdot a space, for example: \cdot \quad just to add the space while keeping the dot product small. In this case, the result will be:

You don't need blkarray for the job; as you discovered, it does some tricks for placing inner fences that require explicit space around the big matrix if it is to be used together with other objects. Better using a standard array that encloses the two rows.

\documentclass{article}
\usepackage{amsmath}
\usepackage{array}

\begin{document}

$\renewcommand\arraystretch{1.1} \begin{array}{@{} c *{2} { @{} >{{}}c<{{}} @{} c } @{} } \mathbf{R} && \mathbf{\Theta}^T && \mathbf{X} \\ \begin{pmatrix} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{pmatrix} & \approx & \begin{pmatrix} \theta_1^{(1)} & \theta_2^{(1)} & \dots & \theta_b^{(1)} \\ \theta_1^{(2)} & \theta_2^{(2)} & \dots & \theta_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ \theta_1^{(m)} & \theta_2^{(m)} & \dots & \theta_n^{(m)} \\ \end{pmatrix}^{\textstyle T} & \cdot & \begin{pmatrix} r_1^{(1)} & r_2^{(1)} & \dots & r_b^{(1)} \\ r_1^{(2)} & r_2^{(2)} & \dots & r_n^{(2)} \\ \vdots & \vdots & \ddots & \vdots \\ r_1^{(m)} & r_2^{(m)} & \dots & r_n^{(m)} \\ \end{pmatrix} \end{array}$

\end{document}

The purpose of the outer array preamble is to have no intercolumn space, but the “middle” entries (corresponding to the relations or binary operations) are surrounded by {}, so as to insert the right spacing pertaining to them.

• Thank you very much for such concise and good explanation! Aug 8, 2019 at 10:47