Funes el memorioso

\documentclass[12pt]{article}

\usepackage[papersize={35mm, 45mm}, text={30mm, 40mm}]{geometry}
\usepackage{concrete}
\usepackage{amsmath}

\DeclareMathOperator*{\AAbf}{\ensuremath{\mathbf{A}}}
\DeclareMathOperator*{\xxbf}{\ensuremath{\mathbf{x}}}
\DeclareMathOperator*{\bbbf}{\ensuremath{\mathbf{b}}}

\begin{document}
\begin{equation*}
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\xxbf_{\scriptscriptstyle n\times 1} =
\bbbf_{\scriptscriptstyle m\times 1}
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\begin{equation*}
\AAbf_{m\times n} \medspace
\xxbf_{n\times 1} =
\bbbf_{m\times 1}
\end{equation*}

\end{document}