I am trying to understand a proof in [Maz’ya, 2011] Sobolev Spaces: With Applications to Elliptic Partial Differential Equations. Springer.
Let $\Omega \subset \mathbb{R}^2$ be given as bellow (Figure 27). For a function $v$ defined on $\Omega$ with first order derivates in $L_2(\Omega)$ I am struggling to understand why the following inequality is true:
I tried integration by parts but didn't manage to arrive at the result. Any hint would be of great help.
