Ricardo,

Although there is a limit of around 250 characters in the Definition text box, Origin's nonlinear curve fitter can conceivably handle any length fitting function by placing the equation into a text file (*.txt or *.ogs) which is saved to your Origin directory. Here's an example of what I mean:

Example

If the fitting function is:

y = a + x * b; // Note: In this case, set the Form drop-down list to Equations.

It can be put directly into the Definition text box when defining the function. The alternative, which should be applied for large equations, is to enter a line such as this into the Definition text box when defining the function:

run.section(filename.txt, myfunc); // Note: In this case, set the Form drop-down list to Y-Script.

Then, when fitting is performed, it will execute the script contained in the section “myfunc” of the file called filename.txt. The section should contain the actual equation in the following manner:

[myfunc]
y = a + x * b;

I hope this information proves helpful to you.

Sincerely,
Ryan Toomey

[This message has been edited by rtoomey (edited 03-26-99).]