fitting 2 variable function to (x-1) form

I have this fit currently which works great:

fitobject = fit([r,eta/eta_c],H,ft,'problem',knownVals)

However, because of the nature of the data, it would make more sense if r was actually in the form of .

When I change the code to this:

fitobject = fit([(r-1),eta/eta_c],H,ft,'problem',knownVals)

My fit is completey messed up, and it looks nothing like the data.

My question is, am I employing the fit correctly?

The form should look like: Cij * (r-1)^i *(eta/eta_c)^j.

I wonder if I missed something in the fitting procedure?

[Walter Roberson]

Possibly it would help to use 'Upper' and 'Lower' (bounds) or 'StartPoint' options.

Answer 1

[Catalytic]

My fit is completey messed up, and it looks nothing like the data.

It shouldn't look like the r and eta data, because that's not the data you fed to the fit, but it should look a lot like r-1 and eta/eta_c.

If I were you, I would just re-define all the r and eta data and forget about it instead of repeatedly having to remember to transform them everywhere they are used in the code. So, I would have

r=r-1;
eta=eta/eta_c;

fitobject = fit([r,eta],H,ft,'problem',knownVals);
plot(fitobject,[r,eta],z)

[Benjamin]

Thanks for catching that mistake!!