\[\nabla^{2}=\frac{1}{h_{\xi}h_{\eta}h_{\phi}}\left(\frac{\partial}{\partial\xi}%
\left(\frac{h_{\eta}h_{\phi}}{h_{\xi}}\frac{\partial}{\partial\xi}\right)+%
\frac{\partial}{\partial\eta}\left(\frac{h_{\xi}h_{\phi}}{h_{\eta}}\frac{%
\partial}{\partial\eta}\right)+\frac{\partial}{\partial\phi}\left(\frac{h_{\xi%
}h_{\eta}}{h_{\phi}}\frac{\partial}{\partial\phi}\right)\right)=\frac{1}{c^{2}%
(\xi^{2}-\eta^{2})}\left(\frac{\partial}{\partial\xi}\left((\xi^{2}-1)\frac{%
\partial}{\partial\xi}\right)+\frac{\partial}{\partial\eta}\left((1-\eta^{2})%
\frac{\partial}{\partial\eta}\right)+\frac{\xi^{2}-\eta^{2}}{(\xi^{2}-1)(1-%
\eta^{2})}\frac{{\partial}^{2}}{{\partial\phi}^{2}}\right).\]