Re: [Help-gsl] subscribe

[saadaoui]

Thank you for your help,
The main point where I am having problems is in what you said as: ">treating
> other *independent* parameters as fixed at this point in parameter
space." I cannot do that since I have hidden parameters in my histogram
and fixing them is not a solution since I don't know what their values
are!
Hassan



> The definition of a gradient is simple: it's a vector consisting of
> derivatives in each parameter.
>
> So, if you have f(p1, p2, ..., pm), your gradient will be:
> {df/dp1; df/dp2; ...; df/dpm}
>
> You just need to take numerical derivatives in each of the parameters
> (taking a numerical derivative in p1 is no different from p2, given your
> function is smooth in all parameters), and construct a vector out of
> them - the GSL docs on multidimensional minimization address that. In
> the gradient, by definition, you have individual derivatives, treating
> other *independent* parameters as fixed at this point in parameter space.
>
> If some parameters depend on others, eliminate them from the
> minimization routine - solve equations for them each iteration, but
> don't treat them as parameters you are fitting with.
>
> address@hidden wrote:
>> Thanks for the reply,
>> I thought about it, but I don't think it works, because doing a
>> numerical
>> differentiation will be only possible for one free parameter, not for 5
>> parameters. How can I do that since I am not fixing any of the 5
>> parameters? Any comments are welcome?
>> Hassan
>>
>>
>>
>>> Actually, on second thought, this might work, perhaps?
>>>
>>> You can numerically compute your f(x,p1,p2,...). You have a bin of
>>> values you're fitting to - f_x1, f_x2, ..., f_xN (i.e. samples of
>>> another function at some points).
>>>
>>> If you take a function to minimize as sum_{1}^{N}
>>> (f(x_i,p1,p2,...)-f_xi)^2, you have a function C(p1,p2,...).
>>>
>>> Now, if THAT is smooth as a function of p1, p2, ..., pm, you can easily
>>> compute derivatives of it in p1, p2, ..., pm numerically using GSL, so
>>> you don't need any explicit knowledge about your histogram functions'
>>> derivatives.
>>>
>>> Or did I mis-understand something else?
>>>
>>> Max Belushkin wrote:
>>>> Ah, I see. Well, unless there's some clever way of numerically
>>>> computing
>>>> the gradient of that, I'd go with James' suggestion and use a simplex
>>>> method...
>>>>
>>>> address@hidden wrote:
>>>>> My model function is attached in a dvi file to this email. I am
>>>>> completely
>>>>> not sure what do you mean by (.... the actual function you're
>>>>> "fitting"
>>>>> doesn't matter, since chi squared is a sum over the whole range(s) of
>>>>> the
>>>>> squares of the function values minus the data bin (i.e. experimental
>>>>> data)....). Can you clarify,please?  Any library that I try to use
>>>>> invloves derivatives, abut I don't know the derivatives of my
>>>>> histogram
>>>>> par of the functiont?!
>>>>> Thank you for your response.
>>>>> Hassan
>>>>>
>>>>>
>>>>>
>>>>>>      I'm not sure I understand what you're trying to do 100%, but,
>>>>>> generally, for any regression, you choose a function to minimize -
>>>>>> i.e.
>>>>>> chi squared. That takes some parameters over which you're
>>>>>> minimizing,
>>>>>> and, if one is lucky, is continuous in those parameters - then you
>>>>>> need
>>>>>> to take the derivatives in parameters, the actual function you're
>>>>>> "fitting" doesn't matter, since chi squared is a sum over the whole
>>>>>> range(s) of the squares of the function values minus the data bin
>>>>>> (i.e.
>>>>>> experimental data). Well, I lost the experimental errors in there.
>>>>>>
>>>>>>    So, how are you composing your function to be minimized? Is it
>>>>>> somehow  discretized in terms of the parameters?
>>>>>>
>>>>>>    I think a bit more information on how you're trying to get to the
>>>>>> end
>>>>>> result would be helpful.
>>>>>>
>>>>>> address@hidden wrote:
>>>>>>> Hello,
>>>>>>> I was looking for a non-linear fitting algorithm, and the manual
>>>>>>> only
>>>>>>> covers Algorithms with Derivatives, but unfortunatly this does not
>>>>>>> apply
>>>>>>> for code as I have a histogram convolved with a Gaussian as my
>>>>>>> fitting
>>>>>>> function. Is there any Algorithm without Derivatives that I may use
>>>>>>> in
>>>>>>> GSL, and if not what other software that maight be helpful, knowing
>>>>>>> that
>>>>>>> I
>>>>>>> only program in C. Thank you in advance for your response.
>>>>>>> Sincerely,
>>>>>>> Hassan
>>>>>>> ----------------------------------------------
>>>>>>> Hassan Saadaoui,
>>>>>>> PhD candidate,
>>>>>>> TRIUMF
>>>>>>> 4004 Wesbrook Mall
>>>>>>> Vancouver, B.C.
>>>>>>> Canada V6T 2A3
>>>>>>> 1-604-222-1047, ext. 6194
>>>>>>> ----------------------------------------------
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> Help-gsl mailing list
>>>>>>> address@hidden
>>>>>>> http://lists.gnu.org/mailman/listinfo/help-gsl
>>>>>
>>>>> Sincerely,
>>>>> Hassan
>>>>>
>>>>> ----------------------------------------------
>>>>> Hassan Saadaoui
>>>>> TRIUMF
>>>>> 4004 Wesbrook Mall
>>>>> Vancouver, B.C.
>>>>> Canada V6T 2A3
>>>>> 1-604-222-1047, ext. 6194
>>>>> ----------------------------------------------
>>>>
>>>> _______________________________________________
>>>> Help-gsl mailing list
>>>> address@hidden
>>>> http://lists.gnu.org/mailman/listinfo/help-gsl
>>>
>>>
>>> _______________________________________________
>>> Help-gsl mailing list
>>> address@hidden
>>> http://lists.gnu.org/mailman/listinfo/help-gsl
>>>
>>
>>
>> Sincerely,
>> Hassan
>>
>> ----------------------------------------------
>> Hassan Saadaoui
>> TRIUMF
>> 4004 Wesbrook Mall
>> Vancouver, B.C.
>> Canada V6T 2A3
>> 1-604-222-1047, ext. 6194
>> ----------------------------------------------
>>
>


Sincerely,
Hassan

----------------------------------------------
Hassan Saadaoui
TRIUMF
4004 Wesbrook Mall
Vancouver, B.C.
Canada V6T 2A3
1-604-222-1047, ext. 6194
----------------------------------------------