How to fit regression

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John fredson on 20 May 2022
Commented: Bjorn Gustavsson on 20 May 2022
I have a equation y = Ae^-Bx, how to polyfit it and find A and B?

Bjorn Gustavsson on 20 May 2022
This is not a polynomial, therefore you cannot use polyfit. You can seamlessly use any least-square fitting routine to fit your 2 parameters. For example something like this:
mod_fcn = @(pars,x) pars(1)*exp(-pars(2)*x); % Describes your model y = A*exp(-B*x)
err_fcn = @(pars,x,y,sigma_y,m_fcn) sum((y(:)-m_fcn(pars,x(:))).^2./sigma_y^2);% general sum-of-squares function
par0 = [1 1]; % initial guess
parbest = fminsearch(@(pars) err_fcn(pars,x,y,ones(size(y))),par0); % least-square fitting
You can sometimes get far better performance with lsqnonlin. Check the help and documentation for that function and fminsearch.
HTH
Bjorn Gustavsson on 20 May 2022
Yes, if you mean your topspin.txt file. Just use the time in the first column for x (or more neatly replace x with t in my code-snippet) and set y to the rotational speed in the second column.

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