“…Here, part of the chain rule is calculated via finite differencing e.g. for the direct method: [38,44,62,138] direct differentiation [24,111,132,[137][138][139]159] adjoint variable method [6, 20, 21, 26, 44, 60, 72-74, 81, 84, 100, 101, 105, 106, 123, 149, 159] complex step method [20,21,26] automatic differentiation [4,25,63] The complex-step method of sensitivity analysis in which the perturbation is carried out with the imaginary value j x i originates in [95,96,130],…”