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\end{equation}  then the adiabatic sound speed is  \begin{equation}  \left( c^{\rm{ad}}_{s}\right)^2 = \frac{dP}{d\rho} = \gamma \frac{{P}{\rho} \frac{P}{\rho}  \end{equation}  $P = \frac{\left( c^{\rm{ad}}_{s}\right)^2}{\gamma} \rho$