&mimetex{{{ \bigtriangleup \equiv \left( \frac{\partial^2}{\partial x^2} +\frac{\partial^2}{\partial y^2} +\frac{\partial^2}{\partial z^2}\right) \\ = \overrightarrow{\bigtriangledown}^2 \\ = \mathrm{div}\cdot\mathrm{grad}. }}};