Laplacian

The Laplacian of a function $f(x,y,z)$ is \[\nabla^2 f(x,y,z) = \frac{\partial^2 f(x,y,z)}{\partial x^2} + \frac{\partial^2 f(x,y,z)}{\partial y^2}+\frac{\partial^2 f(x,y,z)}{\partial z^2}\]