Re: [Bug-gsl] What I am doing wrong / gsl_interp_polynomial

[Patrick Alken]

Hello,

  I confirmed that your program is failing for the polynomial 
interpolation, but polynomial interpolation is known to be difficult for 
large datasets. From the manual:

====
Interpolation Type: gsl_interp_polynomial

    Polynomial interpolation. This method should only be used for 
interpolating small numbers of points because polynomial interpolation 
introduces large oscillations, even for well-behaved datasets. The 
number of terms in the interpolating polynomial is equal to the number 
of points.
====

So basically you are trying to fit a degree 1000 polynomial to your 
dataset, which will not be numerically stable - even though your dataset 
is well behaved.

Cubic splines are probably the way to go here, but if you insist on a 
degree 1000 polynomial (even though you shouldn't) I might be able to 
give you some further ideas.

You mentioned that previous versions of GSL worked for you. Can you 
verify if you used exactly this same dataset successfully with a 
previous version of GSL? If so please tell me the version number.

Thanks,
Patrick

On 11/30/2015 10:25 AM, Petrus, Adam (UK) wrote:
> I am attempting to use the polynomial interpolation method with GSL. However I 
> am getting very strange results.
>
> If I run the attached program I get the below result. If I change the method to 
> linear or cspline the interpolation works fine.
>
> Not in previous gsl versions the polynomial interpolation has worked!
>
> [cid:image001.png@01D12B93.B5E118F0]
>
> Is there something I am missing?
>
> Yours aye
>
> Adam
>
>
>
> ********************************************************************
> This email and any attachments are confidential to the intended
> recipient and may also be privileged. If you are not the intended
> recipient please delete it from your system and notify the sender.
> You should not copy it or use it for any purpose nor disclose or
> distribute its contents to any other person.
> ********************************************************************