How to initialize the dzdy in practice?

[robot520]

For example, in exercise 3, why you compute dzdx3 as follows:
dzdx3 = ...
- single(res.x3 < 1 & pos) / sum(pos(:)) + ...
+ single(res.x3 > 0 & neg) / sum(neg(:)) ;
In other words, how to obtain the initial project tensor p in practice?

[jizhihang]

I have the same question about it. Why is the dzdx3 initialized with this value?