Let be $K(x,y)=\frac{2x_n}{n\alpha(n)}\frac{1}{\vert x-y\vert^n}$ where n is the dimension and $\alpha(n)$ is the volume o unitary sphere in $R^n$, $x=(x_1,...,x_n)\in R^n$ and $y=(y_1,...,y_{n-1},0)\in\partial R^n_+$. Show that:
$\int_{\partial R^n_+}K(x,y)dy=1$
Thanks in advance!