Rene, Thank you very much for your help, it works very well! (too bad my measurements aren't better.. :-) ) One thing: I looked up the documentation for mnexcm, but I couldn't find the options like "SET ERR","FIX" etc. It would be very helpful if you add some comments about the flags you set in the code and the options you set. Maybe some words about how to set a parameter fixed during an optimization would be helpful , too. Is there a function in the TMinuit class that returns the values of the fit-parameters ? (I bet there is one, but I couldn't find it) thanks again, Matthias P.S: I found a fault in the code I sent you. Please change the first line in the function "func" into: Double_t value=((par[0]*par[0])/(x*x)-1)/(par[1]+par[2]*y-par[3]*y*y); So all I did was to change the variable from x/y int y. (Now the function represents the correct fitting function).
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