I was reading a research paper, and I got stuck at this partial differentiation.(adsbygoogle = window.adsbygoogle || []).push({});

Please check the image which I have uploaded.

Now, I got stuck at Equation (13).

How partial derivative was done, where does summation gone?

Is it ok to do derivative wrt Pi where summation also includes Pi?

Please provide me solution or any reference. Its very urgent.

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# How to solve this partial derivative which includes a summation?

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