lsqnonlin and Jacobian misunderstanding: what is the Jacobian definition ?
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Hello !
I use the lsqnonlin Matlab function to fit a curve, called f, to my experimental points (coordinates x_i and y_i). Thus, we have to make simple :
[optimum_result,resnorm,residual,exitflag,output,lambda,jacobian] = lsqnonlin( y_i - f(a,x_i) ) where a is my fit parameter.
I'm wondering what is the definition of the jacobian returned by Matlab :
- the square of the Jacobian returned by lsqnonlin = the second derivative of the residual squared (calculated at the optimum, means the best fit parameter found). Here my residual is : y_i - f(a,x_i). it is the definition found here http://www.ligo-wa.caltech.edu/~ehirose/work/andri_matlab_tools/fitting/MatlabJacobianDef.pdf
OR
- the Jacobian returned by lsqnonlin = the derivative of the residual (calculated at the optimum). It is why I have understand reading Matlab help.
If the answer is the derivative of the residual (calculated at the optimum), I have a misunderstanding. In fact, at the optimum, the sum of my residual vector squared have to be minimum. So the sum of my jacobian, a derivative, has to be equal to (or close to) zero. yes or not ? In Matlab it is not equal to zero, it is why I have a misunderstanding.
Thanks.
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Accepted Answer
Alan Weiss
on 5 Aug 2013
You have a slight misunderstanding of what a Jacobian is for a sum-of-squares problem. The definition is here in the documentation.
In detail, the objective function is
sum((F(x,xdata) - ydata).^2)
The Jacobian is
J(i,j) = partial(F(x,xdata)(i))/partial(x)(j)
There is no reason to think that J is near zero at a solution. The gradient of the objective function is something like
2J'*F
and if F is near zero then the gradient of the objective is near zero, but J is not necessarily small.
Alan Weiss
MATLAB mathematical toolbox documentation
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More Answers (2)
Joffray Guillory
on 5 Aug 2013
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Alan Weiss
on 5 Aug 2013
It is immaterial whether or not we subtract ydata. For your example,
J(i) = sin(xdata(i))
whether or not ydata is included. In this example, J is a vector of length(xdata) components.
I hope this clarifies the computation.
Alan Weiss
MATLAB mathematical toolbox documentation
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