Draw a multiplication of matrices with arrows

Question

Any idea how can I draw this? I thought to use matices, but I have no idea how can I draw the circles around the index.

Answer 1

Perhaps tikzmark library from TikZ is what you need. With it you can create a node at any point in the document and then use it to draw the 'overlaying' diagram.

Something like this:

\documentclass {article}
\usepackage    {lipsum} % for dummy text
\usepackage    {tikz}
\usetikzlibrary{tikzmark}
\tikzset{every node/.style={minimum size=1.75ex}}
\begin{document}
\lipsum[1]
[
 A_{\tikzmarknode[red,draw,circle]{m1}{m}\times\tikzmarknode                  {n1}{n}} \quad\cdot\quad
 B_{\tikzmarknode                 {n2}{n}\times\tikzmarknode[blue,draw,circle]{p1}{p}} \quad=\quad
AB_{\tikzmarknode[red,draw,circle]{m2}{m}\times\tikzmarknode[blue,draw,circle]{p2}{p}}
]
\begin{tikzpicture}[remember picture,overlay]
\draw[red   ,-latex] (m1.south) --++ (0,-1.5) -| (m2.south);
\draw[blue  ,-latex] (p1.south) --++ (0,-1)   -| (p2.south);
\draw[dashed]        (n1.south) --++ (0,-1)   -| (n2.south);
\end{tikzpicture}
\vspace{1.5cm} % we need space for the 'overlayed'` pciture
\lipsum[2]
\end{document}

Edit1: As the picture is 'overlayed', you'll probably need to add some vertical space between it and the following text.

Edit 2: I removed the circles in the n1 and n2 nodes. There weren't such circles in the OP picture.

Answer 2

A solution with pstricks: define the indices components as nodes of the relevant kind, and connect them with the \ncbar node connection.

\documentclass {article}
\usepackage{sansmath}
\usepackage{pst-node}
\begin{document}
Some text. Some text. Some text. Some text. Some text. Some text. Some text. Some text.
\sansmath
[ A_{\circlenode{ml}{m}\times\rnode{nA}{n}} \cdot
 B_{\rule{0pt} {2.5ex}\rnode{nB}{n}\times\circlenode{pl}{p}} =
AB_{\circlenode{mr}{m}\times\circlenode{pr}{p}}
\psset{arrowinset=0.12, linejoin=1, angle=-90, dash=3pt 2pt}
\ncbar[armA=6mm]{->}{ml}{mr}
\ncbar{->}{pl}{pr}
\ncbar[linestyle=dashed, nodesep = 2pt]{-}{nA}{nB}
]
\bigskip
Some more text. Some more text. Some more text. Some more text. Some more text. Some more text.
\end{document}