content_image/319721.jpg content_image/319722.jpg
In (3.17), we used the equality Re z ∇ $_{y}$z = 1 2 ∇ y ( | z | 2 ) . 

<img src='content_image/319721.jpg'>

In (3.18), we used the equality Re z$_{x}$$_{j}$ z$_{x}$$_{j}$$_{y}$$_{i}$ = 1 2 ( ∣ z$_{x}$$_{j}$ ∣ 2 ) y$_{i}$ . 

<img src='content_image/319722.jpg'>

