Hi, I tried to use nls() to fit a complex-valued (non linear) function that looks like this: y = A + B / (1 + C * (i*x*D)^E) where x is the real-valued independent variable, A,B,C,D,E are real-valued parameters and i is the imaginary unit. I had the followin error (my translation into english): "Error in numericDeriv(form[[3L]], names(ind), env) : missing value or infinity generated during model calculation" And a lot of warnings (my translation into english): "In numericDeriv(form[[3L]], names(ind), env) : imaginary parts removed during the conversion" If I got it right R is non able to handle regression problems on complex-valueed function, correct? If yes, is there some workaround? For example using MSExcel I fit the real and imaginary parts separately, and than I minimize the sum of the SSE using the solver function. Quite rude, but it works... thank you for your help Andrea