What does this gradient-like symbol mean?

Question

If $\nabla \phi$ denotes the gradient of some scalar field $\phi$,

then what does $\nabla^2 (\phi^2)$ mean? I don't think it means taking the gradient of a gradient (of a squared-scalar field), but I'm not sure what it could be.

It's part of a hint that I am supposed to use to show that a vector field $\vec F$ is identically zero in some convex region.

Thanks,

Answer 1

It's the Laplacian. I.e. $\nabla^2 f=\sum \frac{\partial^2}{\partial x_i^2} f$