How is undistortPoints solved

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Hello,

I was trying to implement a different function to undistort points (to test potential new distortion models) and I came across the implementation that OpenCV has here. There, in the for loop (here) it actually computes the undistorted point. Can somebody tell me how does it solve the equation system I have tried to identify it to some iterative solutions (least squares, Newton-Rapson...) and I don't see how it does it.

Thank you very much!