Objective Function and Derivatives The input argument fun refers to an IML module that specifies a function that returns f, a vector of length m for least-squares subroutines or a scalar for other ...

You take the derivative of y with respect to x. You are finding the sensitivity to the change in the function with respect to a change in the argument.

Morris Marden, Logarithmic Derivative of an Entire Function, Proceedings of the American Mathematical Society, Vol. 28, No. 2 (May, 1971), pp. 513-518 ...