LaTeX Forum

\documentclass[a4paper]{article}

%% Language and font encodings
\usepackage[english]{babel}
\usepackage[utf8x]{inputenc}
\usepackage[T1]{fontenc}

%% Sets page size and margins
\usepackage[a4paper,top=3cm,bottom=2cm,left=3cm,right=3cm,marginparwidth=1.75cm]{geometry}

%% Useful packages
\usepackage{amsmath}

\begin{document}
\begin{equation} \label{eq:SpTensor}
\sigma_{ij}=\begin{pmatrix} 
\sigma_\text{xx} & \tau_\text{xy} & \tau_\text{xz} \\ \tau_\text{yx} & \sigma_\text{yy} & \tau_\text{yz} \\ \tau_\text{zx} & \tau_\text{zy} & \sigma_\text{zz} 
\end{pmatrix} 
\end{equation}
\end{document}

\documentclass[]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[doublespacing]{setspace}

\usepackage{amsmath}

\begin{document}
\begin{equation} \label{eq:SpTensor}
\sigma_{ij}=\begin{pmatrix}
\sigma_\text{xx} & \tau_\text{xy} & \tau_\text{xz} \\ \tau_\text{yx} & \sigma_\text{yy} & \tau_\text{yz} \\ \tau_\text{zx} & \tau_\text{zy} & \sigma_\text{zz} 
\end{pmatrix}
\end{equation}
\singlespacing
\begin{equation} %\label{eq:SpTensor}
\sigma_{ij}=\begin{pmatrix}
\sigma_\text{xx} & \tau_\text{xy} & \tau_\text{xz} \\ \tau_\text{yx} & \sigma_\text{yy} & \tau_\text{yz} \\ \tau_\text{zx} & \tau_\text{zy} & \sigma_\text{zz} 
\end{pmatrix}
\end{equation}
\end{document}

\usepackage{etoolbox}
\AtBeginEnvironment{equation}{\singlespacing}

\documentclass[]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[doublespacing]{setspace}

\usepackage{amsmath}

\usepackage{etoolbox}
\AtBeginEnvironment{equation}{\singlespacing}
\begin{document}
\begin{equation} \label{eq:SpTensor}
\sigma_{ij}=\begin{pmatrix}
\sigma_\text{xx} & \tau_\text{xy} & \tau_\text{xz} \\ \tau_\text{yx} & \sigma_\text{yy} & \tau_\text{yz} \\ \tau_\text{zx} & \tau_\text{zy} & \sigma_\text{zz} 
\end{pmatrix}
\end{equation}
\end{document}