Hello.

 

I understand that [math]\frac{d|x|}{dx}=\theta(x)-\theta(-x)[/math] and then [math]\frac{d^2|x|}{dx^2}=2\delta(x)[/math].

 

But i DONT UNDERSTAND why when [math]\phi[/math] is a angular coordinate, then [math]\frac{d^2|\phi|}{d\phi^2}=2(\delta(\phi)-\delta(\pi-\phi))[/math]

Perhaps because the angle phi in your problem is defined upon -pi to pi. So, when crossing pi it is transferred to a negative value. This may be the reason.