In the context of complex number functions, what do $\Delta u$ and $\Delta v$ mean?
I think it has something to do with partial derivatives and holomorphic or harmonic functions, but I can't find this notation anywhere except some exercises which give me no more clues about it.
Just so that this question has an answer: $\Delta$ is the Laplacian, i.e.
$$ \Delta u = \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2}.$$
(Or however many variables you have.)