Fit parameters stay static

i am currently try to fit a set of data with a self-defined function. The function itself contains three parameters which i will further use for my evaluation. Unfortunately, parameter [2] seems to have no affect on the values of the other two parameters. If i change the start value of [2] or even fix it, the other two remain the same. What can cause this behavior? Is the not existing influence of this parameter owed to the form of the fit function in general? Thanks for any help you can provide.
cs Kevin
example.C (604 Bytes)
bunch_of_data.txt (7.1 KB)


I think you are dealing with a fit which is a bit fragile in general.
The mistake in your formula is in the 1/2 factor of the second pow. That is seen as 0, which is the result of the division of two integers, 1 and 2 :slight_smile:
So you wrote
It should be
or even better, simplified
[0] / ((1+x/[1])sqrt(1+[2][2]*x/[1]))


Thank you. Suddenly it worked :slight_smile: cs Kevin

Nothing to say about that :blush:

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