I have 2 questions on 2 subsequent pages in a book on PDE, so I'll post them as a one question.
The first is formula (1) in the firs snippet, namely what is $u_{x_i}$ there and how we have obtained by integration by parts this formula (1), most importantly the $-$ term in front of $-\int$.
The second question is on the following snippet above Example 2 and word "as required": I think that there should be $\psi(2)-\psi(1)$ and not $\psi(1)-\psi(1)$. Anyway, I do not follow all the three equalities following the words "But we easily calculate". There seem to be some typos in bounds which I'm unable to correct. Can someone kindly help me ?
