I'm reading Classic and Multilinear Harmonic analysis vol.2 - Muscalu, Schlag
In page 134, it says,
$$\int_{\partial B(x,\epsilon)}-F(y-x)\frac{\partial u}{\partial\nu}(y)d\sigma(y)=0$$
can be shown by using explicit formula for F, where F is the fundamental solution for Laplacian in the complex plane given by
$$F(x)=-\frac{1}{2\pi}\log|x|$$
So in here what does it mean by explicit formula? And how can I derive that the show that the above equation is true?
Here $B(x,\epsilon)$ is in bounded Lipschitz domain $U$.