I was reading a paper of Lu-Wei named "On a Sobolev inequality with remainder terms" (link at AMS site).
and I have quite a few questions regarding that.
(1) In page 78 they got a system of two equations involving $\phi$ and $w_i$ and the claim is $\phi_i=0$ when $\mu\leq p$ and $i>N+2$. Then it's written WLOG assume that $\phi_i<0$ on $(0,r_1)$ and $\phi_i(r_1)=0$ ( why it has to be the case, can't it be negative everywhere, why it has to hit $0$?) Also in the very next line it's written that as $i>N+2$ ,$\phi_i(0)=w_i(0)=\phi_i'(0)=w_i'(0)=0$.I couldn't see how this follows ?
Any help is very much helpful for my better understanding of the material.