I am currently taking an electrodynamics course and we are getting prepared with the maths needed.
In the section involving Dirac $\delta$-function, my lecturer writes out such an equation:
$\nabla\nabla\frac{1}{|\vec{r}-\vec{r'}|}=-\frac{\vec{\vec{I}}}{|\vec{r}-\vec{r'}|^3}+3\frac{(\vec{r}-\vec{r'})(\vec{r}-\vec{r'})}{|\vec{r}-\vec{r'}|^5}-\frac{4\pi}{3}\vec{\vec{I}}\delta(\vec{r}-\vec{r'})$
I never studied tensor formally, and this equation seems a total stranger to me.
How is this equation derived?
