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  • $\begingroup$ thx. I have already noticed that your first suggested example gives the wrong answer. Will your second example decrease the calculation rate? I have a very large amount of functions and need to do rather complex calculations. Is it possible not to replace all the functions like 'f[x,y][Conjugate]' by 'Evaluate[ComplexExpand[f[x, y][Conjugate]]]'? $\endgroup$ Commented Dec 12, 2018 at 8:24
  • $\begingroup$ @Chipa-Chipa Then I suggest that you do not use a pure function in the middle of Derivative, but use the explicit form of the function you want to go further. $\endgroup$ Commented Dec 12, 2018 at 8:27
  • $\begingroup$ Do you mean that I should make the replacement only when I define the derivative? $\endgroup$ Commented Dec 12, 2018 at 8:31
  • $\begingroup$ @Chipa-Chipa I mean to obtain the explicit expression of the function, before you put it into its derivative calculation. $\endgroup$ Commented Dec 12, 2018 at 8:33
  • 1
    $\begingroup$ I think the problem with your original way was a misplaced Evaluate. Try myConjugate = # /. {I -> -I, -I -> I} &; h[x_, y_] := Derivative[0, 1][Evaluate[myConjugate[f[#, #2]]/g[#, #2]] &][x, y] $\endgroup$ Commented Jan 11, 2019 at 19:54