} -\frac{\partial}{\partial x_{k}}  \frac{\partial}{\partial x_{i}} \frac{\partial \rho L_0} {\partial \frac{\partial^{0} \rho}{\partial x_{k} \partial x_{i}}} +\frac{\partial}{\partial x_{k}} \frac{\partial \rho L_0} {\partial \frac{\partial \