content_image/319889.jpg content_image/319892.jpg content_image/319893.jpg content_image/319894.jpg content_image/319895.jpg content_image/319890.jpg content_image/319891.jpg
In (3.19), we used the equality Re z ∇ $_{x}$z = 1 2 ∇ x ( | z | 2 ) . 

<img src='content_image/319889.jpg'>

In (3.20), we used the equality Re z ∇ $_{y}$z = 1 2 ( ∇ y | z | 2 ) and Re z ∇ $_{x}$z = 1 2 ( ∇ x | z | 2 ) . 

Hence, we can rewrite (3.13) 

where 

<img src='content_image/319892.jpg'>

<img src='content_image/319893.jpg'>

<img src='content_image/319894.jpg'>

<img src='content_image/319895.jpg'>

<img src='content_image/319890.jpg'>

<img src='content_image/319891.jpg'>

