} -\frac{\partial}{\partial x_{k}}  \frac{\partial}{\partial x_{i}} \frac{\partial \beta} {\partial\left( \frac{\partial^{0} \rho}{\partial x_{k} \partial x_{i}} \right)} +\frac{\partial}{\partial x_{k}} \frac{\partial \beta} {\partial\left( \frac{\partial \rho}{\partial x_{k} } \right)} -\frac{\partial \beta}{\partial \rho} +\frac{\beta}{\rho}=0 \