paragraph_text: <td><math>{1 \over r^2}{\partial \left( r^2 A_r \right) \over \partial r} + {1 \over r\sin\vartheta}{\partial \over \partial \vartheta} \left(  A_\vartheta\sin\vartheta \right) + {1 \over r\sin\vartheta}{\partial A_\varphi \over \partial \varphi}</math>