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In the paper https://doi.org/10.1016/j.physleta.2011.08.072 authors propose amodification of the conventional delayed feedback control algorithm, wheretime-delay is varied continuously to minimize the power of control force.Minimization is realized via gradient-descent method. However, the derivationof the gradient with respect to time-delay is not accurate. In particular, ascalar factor is omitted. The absolute value of the scalar factor is notcrucial, as it only changes the speed of the gradient method. On the otherhand, the factor's sign changes the gradient direction, therefore for negativevalue of the multiplier the gradient-decent becomes gradient-ascent method andfail power minimization. Here the accurate derivation of the gradient ispresented. We obtain an analytical expression for the missing factor and showan example of the Lorenz system where the negative factor occurs. We alsodiscuss a relation between the negativeness of the factor and the odd numberlimitation theorem.

Comment on "Adaptive modification of the delayed feedback control  algorithm with a continuously varying time delay"  https://doi.org/10.1016/j.physleta.2011.08.072