Non linear Fitting using a Rational Function

Hello,

I have some experimental data and I am trying to fit this data with a rational function, i.e. in the numerator there is a quadratic polynomial function and in the denominator there is a 4th degree polynomial function.

I am using the Levenberg-Marquardt to do the fit and the MatLab can not fit when I put my equation in the "Custom Equations" in the ToolBox Curve Fitting.

The rational function was derived from the combination of three ODE, these equations describe the physical phenomenon.

Can I need to put the equations and the experimental data here?

Thank you very much,

Eduardo

[Matt J]

Elaborate on "Matlab can not fit...".

Do you mean that it gives you an error message when you try? Or, do you just mean that it returns a bad fit?

[Eduardo]

The error mensage is:

"Fit computation did not converge:

Maximum number of function evaluations exceeded. Increasing MaxFunEvals (in fit options) may allow for a better fit, or the current equation may not be a good model for the data.

Fit found when optimization terminated:

Warnings during fitting: Levenberg-Marquardt and Gauss-Newton algorithms do not handle bound constraints; switching to trust-region algorithm instead."