Hi everyone,

I define this fitting function:

TF1 *Cosine = new TF1(“Cosine”, “[0]

cos(1.e6[1]*x) -1. + [2]”, t0, tmax);

and I get this result from the fit:

```
Minimizer is Minuit / Migrad
Chi2 = 1.350178747317e-10
NDf = 9996
Edm = 2.69445426695e-10
NCalls = 44
= 1.000000108898 +/- 1.667279042214e-09
#omega = 7.865440737302 +/- 2.775765883295e-10
= 1.00000008007 +/- 1.169006249596e-09
```

which is good. I am only interested in the parameter omega and its comparison with the analytic value. If I define instead:

TF1 *Cosine = new TF1(“Cosine”, “[0]

cos(1.e6[1]*x) -1. + [3]”, t0, tmax);

I get the following result (see that I changed the parameter [2] for [3]):

```
Minimizer is Minuit / Migrad
Chi2 = 1.344104546597e-10
NDf = 9995
Edm = 5.90438077721e-10
NCalls = 191
= 1.000000108898 +/- 1.654223854588e-09
#omega = 7.86534073805 +/- 2.762878919271e-10
= -2827.427157079 +/- 4.919957807864e-08
Error in <TFormula::GetParName>: Parameter with index 3 not found !!
= 1.00000008007 +/- 1.159703162969e-09
Error in <TFormula::GetParName>: Parameter with index 3 not found !!
```

I think that ROOT considered another parameter [2] not included in my function for the fit, but I don’t know how it is working. The thing is that this value for omega is closer to the analytic value, which I could say that this fit is better. How is this possible? Is it just a coincidence?

Thank you.

*ROOT Version:* 6.12/04