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I am new to xgboost and not so good at math.

I want to use a self defined objective function, here is the expression:

obj = (y * y_pred).sum() / (y_pred).abs().sum()

y is a n-dimensional vector.

I have some experience for tensorflow, in tensorflow, I just need to define the function, it will calculate the derivatives for me automatically.

But in xgboost, the customized obj function needs to return the first and second derivatives.

I dont know how to take derivatives for abs(). Is there good method can take derivatives? Can you help on this?

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