\documentclass[11pt]{article}
\usepackage{geometry}                
\geometry{a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm}
\usepackage{natbib}
\usepackage{color}
\definecolor{mygreen}{RGB}{28,172,0} % color values Red, Green, Blue
\definecolor{mylilas}{RGB}{170,55,241}
\usepackage{epsfig}
\usepackage{amssymb,amsmath}
\usepackage{enumerate}
\usepackage{enumitem}
\usepackage[utf8]{inputenc}
\usepackage{hyperref}
\usepackage{mathtools}


\newcommand{\ssd}{\text{ssd}}
\newcommand{\sS}{\mathsf{S}}
\newcommand{\tot}{\text{tot}}

\begin{document}


\input{symbols}

\section*{Linear stability analysis}

With $h_b = 0$, the linear eigenproblem is

\begin{equation}
 \sA \, \mathsf{\Phi} = \omega \, \sB\, \mathsf{\Phi}\,,
\end{equation}
where

\begin{equation}
\sA \equiv \mathsf{B}(\mathsf{U}\, k + \mathsf{V}\,l) + \mathsf{I}\left(k\,\mathsf{Q}_y - l\,\mathsf{Q}_x\right) + \mathsf{I}\,\delta_{\mathsf{N}\mathsf{N}}\, \mathrm{i} \,r_{ek}\,\kappa^2\,,
\end{equation}
where $\delta_{\mathsf{N}\mathsf{N}} = [0,0,\dots,0,1]\,,$
and 
\begin{equation}
\mathsf{B} \equiv  \mathsf{S} - \mathsf{I} \kappa^2\,. 
\end{equation}
The growth rate is Im$\{\omega\}$.

\end{document}
