Past exam of the mathematics course of the University of Cambridge / 2014 / iii / Paper 69 / 3 / ii

Use a consistent number $n$ of vertices, with $z_{n+1}=z_1$, and set $m_j=(z_j+z_{j+1})/2$, $h_j=(z_{j+1}-z_j)/2$. Pull back the one-form to $z_j(s)=m_j+s{h}_j$. Since $dz=h_{j}ds$ and $d\bar{z}={\bar{h}}_{j}ds$, the side integrand isFor counterclockwise traversal, put ${\ell }_j=|h_j|$ and let $q_j={\partial }_{n}u$ be the outward normal derivative, while $g_j=u{|}_{S_j}$ is the Dirichlet boundary data. The outward unit normal is $-i{h}_j/{\ell }_j$ in complex notation. Therefore $h_j{u}_z-{\bar{h}}_j{u}_{\bar{z}}=i{\ell }_j{q}_j$, andThere is no tangential-derivative term: it cancels in this particular one-form. The Generalized Stokes theorem and $dW=0$ now give the polygonal global relationThe same zero identity holds with every side traversed clockwise, but then $h_j{u}_z-{\bar{h}}_j{u}_{\bar{z}}=-i{\ell }_j{q}_j$ for outward normals. One must change this sign consistently rather than mix the two orientations.