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 Original Message (ID '324680') By devil: In Response To 'Re: Re: Re: derivative of amplitude-frequency c...' --------- Yes, you are right. I get the same output. ampl = 2 \[Pi] Abs[f/(1000 + 2 I f \[Pi])] and the derivation of it is the same you said 2 \[Pi] (-((2 I f \[Pi])/(1000 + 2 I f \[Pi])^2) + 1/( 1000 + 2 I f \[Pi])) Derivative[1][Abs][f/(1000 + 2 I f \[Pi])] I figure out that ampl is a real function. Im[ampl] is 0! So thats why Mathematica have no problem to plot it. The derivation is in addition imaginary ... so Mathematica don't know how to plot. Like you said. But I can't explain why the derivation of a function should not exist, when I can plot the original function. There are no bounds in my plot. LogLogPlot[ampl, {f, 0.0000001, 10000000}] And that's why I think the derivation should exist. Could the problem are the bounce by f=0 ? Plot[ampl, {f, -100, 100}] How can I limit the derivation from 0 to inf so that there are no more bounces? Could this be the reason why Mathematica can't solve the derivation ?