Matlab stretched exponential fit

Matlab stretched exponential fit DEFAULT

A Novel Method for Curvefitting the Stretched Exponential Function to Experimental Data

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lsqnonlin and stretched exponential function

Hi all,

I am currrently trying to fit my data with lsqnonlin instead lsqcurvefit for a comparative reason and I am facing some troubles.

Starting with the easiest example in, I modify the function as:

fun = @(r) r(1).*exp(-d.*r(2)).^r(3)-y

This function yields x = [1.0376 3.1962 0.4104].

From here, I calculate my errors in the way:

[x,resnorm,residual,exitflag,output,lambda,J]= lsqnonlin(fun,x0);

N = length(y(:,1));

[Q,R] = qr(J,0);

mse = sum(abs(residual).^2)/(size(J,1)-size(J,2));

Rinv = inv(R);

Sigma_var = Rinv*Rinv'*mse;

x_er = full(sqrt(diag(Sigma_var)));

Here, the values I get are not making any sense to me x_er = [0.02701 6458034.8422 829226.8633]; especially for x(2) and x(3).

Fit and errors are totally fine if I fit similar data in the form: (i) r(1).*exp(-d.*r(2)) or (ii) r(1).*exp(-d.*r(2))+r(3).*exp(-d.*r(4));

Could you please help me? Am I missing something?

Thanks a lot.

Best wishes


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How do I fit an exponential curve to my data?

You are now following this question

Say, I have the following data: x=[1,2,4,6,8],y=[100,140,160,170,175]. How do I fit an exponential curve of the form y=a-b*exp(-c*x) to my data? Is there any Matlab function to do that? Thanks in advance.
Andrei Bobrov
[EDIT] Please read aboutfit and try:
g = fittype('a-b*exp(-c*x)');
f0 = fit(x,y,g,'StartPoint',[[ones(size(x)), -exp(-x)]\y; 1]);

More Answers (4)

Tamara Schapitz
I got this warning messsage:
Warning: Rank deficient, rank = 1, tol = 4.019437e-14.
and it only plots the data, but not the fit... What am I doing wrong?

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Stretched exponential fit matlab

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Plot exponential signal in Matlab

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