Okay, suppose that $U=B_{r}(0)$ is an open disk around $0$ and $K=\overline{K_{s}(0)}$ is a closed disk contained in it. Take $t$ with $s < t < r$, then for any $\xi \in K$ we have for $C$ the circle around $0$ in counterclockwise rotation once with radius $t$
$$f(\xi)= \frac{1}{2\pi i}\int_{C} \frac{f(z)}{z-\xi}\mathrm{d}z=\frac{1}{2\pi i}\int_0^{2\pi}\frac{f(t e^{ix})te^{ix}}{\xi-te^{ix}}\mathrm{d}x$$

$$\|f\|_{L^p(U)}^p=\int_{x+iy \in U}|f(x+iy)|^p \mathrm{d}x \mathrm{d}y=\int_{0}^{2 \pi} \int_{0}^r |f(re^{i \varphi})|^pr \mathrm{d}r \mathrm{d}\phi$$

Hi all! Here it says, in the red part, that $\forall i,m,n=1, 2, 3$

$\delta_{i2}\delta_{m1}=0 $ and $\delta_{i3}\delta_{n1}=0 $

But for $\delta_{i2}\delta_{m1}=0 $, when plugging values $i=2$ and $m=1$, we get $\delta_{22}\delta_{11}=1.1=1 $
Similarly for $\delta_{i3}\delta_{n1}=0 $, when plugging values $i=3$ and $n=1$, we get $\delta_{33}\delta_{11}=1.1=1 $

So, $\delta_{i1}\delta_{i1}+\delta_{i2}\delta_{m1}+\delta_{i3}\delta_{n1}=\delta_{ii}+\delta_{i2}\delta_{m1}+\delta_{i3}\delta_{n1}= \delta_{11}+\delta_{22}+\delta_{33}+\delta_{22}\delta_{11}+\delta_{33}\delta_{11}=1+1+1+1+1=5 $