Curve Fitting Using Outside Parameter

I am trying to find a fit to a set of (X,Y) data where X varies between 0.001 & 1000 and the form of the fit functions is as shown in the FitFun below:

n=-0.3;
  FitOpt=fitoptions('Method','NonlinearLeastSquares',...
                    'lower',[1*max(Y) 0 1],...
                    'upper',[50*max(Y) 3 2])   
  FitFun=fittype('(Y0*(1+(X0*X)^a))^((n-1)/a)',...
      'dependent',{'Y'},'independent',{'X'},...
      'coefficients',{'Y0','X0','a'});

When I run the script I get the message:

Conversion to cellstr from double is not possible.

It looks like the fit function cannot accept n as an outside parameter. How can I solve this problem?

Answer 1

[Torsten]

Use ...'problem','n'... in the FitType structure and set "n" to a value in the call to "fit".

Best wishes

Torsten.