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I am trying to fit a complex data with a complex function, with the real fitting parameters. I was trying to put there constraints like this, but I got error:

FindFit[data,{1-a/(b-x^2-I*c*x),{Element[{a,b,c},Reals]}},{a,b,c},x]

If I remove constraints, I get in general complex parameters.

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  • $\begingroup$ Are you xs complex too? $\endgroup$ – Dr. belisarius Feb 26 '15 at 15:47
  • $\begingroup$ No, they are real. $\endgroup$ – infinity Feb 26 '15 at 17:11
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The following works quite well:

k = 1 - a/(b - x^2 - I*c*x);
t = Table[{x, k /. {a -> 10, b -> 5, c -> 2}}, {x, 1, 50}];
FindFit[t, {k, (a | b | c) ∈ Reals}, {a, b, c}, x, NormFunction -> (Norm@## &)]

(* {a -> 9.99976, b -> 4.99994, c -> 1.99994} *)
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