In the text below, I am unable to understand how Eq 2.5 represents the Laplace Equation because I don't see any partial derivatives etc. Does Eq 2.5 somehow reduce to $\nabla^2 u=0$? The snippet is from https://www.stat.uchicago.edu/~lekheng/work/psapm.pdf (pg 4). It would be great if someone could help me understand this form of the Laplace equation.
