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# exp(-b/x) fit, inf problem when fitting

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Basil Eldeeb on 25 Feb 2021
Commented: Basil Eldeeb on 2 Mar 2021
I am trying to fit some data with the model: exp(-b/x); When x goes to zero, y should go to zero as well since anything power negative infinity is zero. However Matlab sees the infinity and terminates everything. Here is my code:
vv=data(:,1);
ii=data(:,2);
g = fittype('exp(-b/x)');
f0 = fit(vv,ii,g);
xx = linspace(-1,1);
plot(vv,ii,'o',xx,f0(xx),'r-');
grid('on')
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Mathieu NOE on 26 Feb 2021
not sure it's really a good model... data = readmatrix('data.txt');
x = data(:,1);
y = data(:,2);
% exponential fit method
% model : y = exp(-b/x)
f = @(b,x) exp(b./x);
obj_fun = @(params) norm(f(params(1), x)-y);
sol = fminsearch(obj_fun, -0.1);
b_sol = sol(1)
y_fit = f(b_sol, x);
figure
plot(x,y,'r',x,y_fit,'-.k');
legend('data','exp fit');

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### Accepted Answer

Matt J on 25 Feb 2021
Edited: Matt J on 25 Feb 2021
When x goes to zero, y should go to zero as well since anything power negative infinity is zero.
Only if b>=0.
g = fittype('exp(-b/x)', 'options', fitoptions('Lower',0) );
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Basil Eldeeb on 2 Mar 2021
I appreciate it, thank you for the help!.

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