The matter which density depends on pressure $\begin{array}{l}p\end{array}$ or temperature $\begin{array}{l}T\end{array}$ :

(1)	$\begin{array}{l}\displaystyle \rho = \rho(p, T)\end{array}$

which means

(2)	$\begin{array}{l}\displaystyle c = \frac{1}{\rho} \frac{\partial \rho}{\partial p} > 0\end{array}$

(3)	$\begin{array}{l}\displaystyle \alpha = \frac{1}{\rho} \frac{\partial \rho}{\partial T} > 0\end{array}$

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See also