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### Power law curve fit through a specific point

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Hi all,

I have some data in matlab that I would like to fit with a power law
model.  The complication is that I want to force the model to pass
through a given point ( i.e. x = 37, y = 0).  Maybe this is not
possible using a power law since 37^a will never equal exaclty zero
for any constant a.  It is possible that I need to use a non-linear
least square approach but  I`m not quite sure how to accomplish this,
does anyone have any suggestions?

Thanks,

Anders
 0

Anders <anottrot@ucsd.edu> wrote in message <e1061df0-1c5b-45a3-a61d-6daf5c29f2bc@i4g2000prf.googlegroups.com>...
> Hi all,
>
> I have some data in matlab that I would like to fit with a power law
> model.  The complication is that I want to force the model to pass
> through a given point ( i.e. x = 37, y = 0).  Maybe this is not
> possible using a power law since 37^a will never equal exaclty zero
> for any constant a.  It is possible that I need to use a non-linear
> least square approach but  I`m not quite sure how to accomplish this,
> does anyone have any suggestions?
>
> Thanks,
>
> Anders

Hi

Perhaps you want to transform your data so that a power law is guaranteed to pass through your point of interest?

e.g. subtract 37 from all your x values, and then take a look at whether a power function looks sensible?

>> docsearch least squares

will find you the functions  for this kind of fitting

Ross
 0

On 26 Jul, 09:30, Anders <anott...@ucsd.edu> wrote:
> Hi all,
>
> I have some data in matlab that I would like to fit with a power law
> model. =A0The complication is that I want to force the model to pass
> through a given point ( i.e. x =3D 37, y =3D 0). =A0Maybe this is not
> possible using a power law since 37^a will never equal exaclty zero
> for any constant a. =A0It is possible that I need to use a non-linear
> least square approach but =A0I`m not quite sure how to accomplish this,
> does anyone have any suggestions?

The question makes no sense. If y =3D f(x), then 37^a is a constant,
given a. Are you sure you don't want to fit y =3D 37^x or something
like that?

Rune
 0

2 Replies
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5/17/2013 4:46:18 PM