content_image/315495.jpg content_image/315496.jpg content_image/315497.jpg content_image/315498.jpg content_image/315499.jpg content_image/315500.jpg content_image/315501.jpg
and 

<img src='content_image/315495.jpg'>

<img src='content_image/315496.jpg'>

<img src='content_image/315497.jpg'>

Since 

and for 0 < α, β < 1 , 

we have 

<img src='content_image/315498.jpg'>

<img src='content_image/315499.jpg'>

<img src='content_image/315500.jpg'>

Then, we see that 

<img src='content_image/315501.jpg'>

Since the signs of the terms of |∇ $_{x}$z | 2 and |∇ $_{y}$z | 2 are different, we need to perform 

