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as the radius of a sphere containing the same volume $\VP=4\pi \rP^3/3$, which allows to re-write the equation   of continuity in the usual form   \begin{equation}  \dxdycz{\mP}{\rP}{t}=4\pi\rP^2\rho   \;, {\mP}{\rP}{t}=4\pi\rP^2\rho  \end{equation}