\begin{equation*}
\fontsize{12.0}{12.0}\selectfont
    c_{px} \equiv  \frac{\omega}{k} 
                    = -\frac{\beta}{k^2+l^2},
   \quad
    c_{py} \equiv  \frac{\omega}{l} 
                    = -\frac{\beta k}{l(k^2+l^2)}. 
\end{equation*}
