[SciPy-User] scipy.optimize.leastsq question

Ralph Kube ralphkube@googlemail....
Tue Jun 29 02:28:31 CDT 2010


Thank you, I found the error this way. The Jacobian is indeed very
hard to compute, and the leastsq routine computes a zero Jacobian.
The albedo function I want to minimize does not change for values
of h \approx 10**-8, but on scales h \approx 10**-3.
I now use the fmin function and working with other functions that do not
require any information about the derivative. They seem more appropriate
to my problem.

Cheers, Ralph

Den 28.06.10 17.13, skrev Sebastian Walter:
> there may be others who have more experience with scipy.optimize.leastsq.
>
>> From the mathematical point of view you should be certain that your
> function is continuously differentiable or at least
> (Lipschitz-)continuous.
> This is because  scipy.optimize.leastsq  uses the Levenberg-Marquardt
> algorithm which requires the Jacobian J(x) = dF/dx.
>
> You do not provide an analytic Jacobian for scipy.optimize.leastsq.
> That means that scipy.optimize.leastsq uses some finite differences
> approximation to approximate the Jacobian J(x).
> It can happen that this finite differences approximation is so poor
> that no descent direction for the residual can be found.
>
> So the first thing I would check is if the Jacobian J(x) makes sense.
> You should be able to extract it from
> scipy.optimize.leastsq's output infodict['fjac'].
>
> Then I'd check if
> F(x + h*v) - F(x)/h, for h \approx 10**-8
>
> gives the same vector as   dot(J(x),v)
> if this doesn't match at all, then your Jacobian is wrong resp. your
> function is not continuously differentiable.
>
> Hope this helps a little,
> Sebastian
>
>
>
> On Mon, Jun 28, 2010 at 2:36 PM, Ralph Kube<ralphkube@googlemail.com>  wrote:
>> Hello people,
>> I am having a problem using the leastsq routine. My goal is to
>> determine three parameters r_i, r_s and ppw so that the residuals
>> to a model function a(r_i, r_s, ppw) to a measurement are minimal.
>> When I call the leastsq routine with a good guess of starting values, it
>> iterates 6 times without changing the vales of the initial parameters
>> and then exits without an error.
>> The function a is very complicated and expensive to evaluate. Some
>> evaluation is done by using the subprocess module of python. Can this
>> pose a problem for the leastsq routine?
>>
>>
>> This is in the main routine:
>>
>> import numpy as N
>>
>> for t_idx, t in enumerate(time_var):
>>
>>         r_i = 300.
>>         r_s = 1.0
>>         ppw=1e-6
>>         sza = 70.
>>         wl = N.arange(300., 3001., 1.)
>>
>>         albedo_true = compute_albedo(r_i, r_s, ppw, sza, wl)
>>         # This emulates the measurement data
>>         albedo_meas = albedo_true + 0.01*N.random.randn(len(wl))
>>
>>         print 'Optimizing albedo'
>>         p0 = [2.*r_i, 1.4*r_s, 4.*ppw]
>>         plsq2 = leastsq(albedo_residual, p0, args=(albedo_meas, sza,
>> wl))
>>         print '... done: ', plsq2[0][0], plsq2[0][1], plsq2[0][2]
>>         albedo_model = compute_albedo(plsq2[0][0], plsq2[0][1], plsq2[0][2],
>> sza, wl)
>>
>> The residual function:
>> def albedo_residual(p, y, sza, wvl):
>>         r_i, r_s, ppw = p
>>         albedo = compute_albedo(r_i, r_s, ppw, sza, wvl)
>>         err = albedo - y
>>         print 'Albedo for  r_i = %4.0f, r_s = %4.2f, ppw = %3.2e \
>>                 Residual squared: %5f' % (r_i, r_s, ppw, N.sum(err**2))
>>
>>         return err
>>
>> The definition of the function a(r_i, r_s, ppw)
>> def compute_albedo(radius_ice, radius_soot, ppw, sza, wvl):
>>
>> The output is:
>> Optimizing albedo
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> Albedo for r_i =  600, r_s = 1.40, ppw = 4.00e-06               Residual squared:
>> 0.973819
>> ... done:  600.0 1.4 4e-06
>>
>> To check for errors, I implemented the example code from
>> http://www.tau.ac.il/~kineret/amit/scipy_tutorial/ in my code and it
>> runs successfully.
>>
>> I would be glad for any suggestion.
>>
>>
>> Cheers, Ralph
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-- 

Cheers, Ralph


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