mittels
\begin{equation}
\begin{bmatrix}
\sigma_{11} & \sigma_{12} & \sigma_{13}\\
\sigma_{21} & \sigma_{22} & \sigma_{23}\\
\sigma_{31} & \sigma_{32} & \sigma_{33}\\
\end{bmatrix}
Liebe Grüße
\begin{equation}
\begin{bmatrix}
\sigma_{11} & \sigma_{12} & \sigma_{13}\\
\sigma_{21} & \sigma_{22} & \sigma_{23}\\
\sigma_{31} & \sigma_{32} & \sigma_{33}\\
\end{bmatrix}
\documentclass[11pt,ngerman]{article}
\usepackage[T1]{fontenc}
\usepackage{selinput}
\SelectInputMappings{
adieresis={ä},
germandbls={ß}
}
\usepackage{babel}
\usepackage{mathtools}
\begin{document}
\begin{equation}
\begin{pmatrix}
\sigma_{11} & \sigma_{12} & \sigma_{13}\\
\sigma_{21} & \sigma_{22} & \sigma_{23}\\
\sigma_{31} & \sigma_{32} & \sigma_{33}\\
\end{pmatrix}^\top
\end{equation}
\end{document}