[Bug-gsl] [bug #42830] Bug in gsl_bspline_knot constructor

[Foivos Diakogiannis]

URL:
  <http://savannah.gnu.org/bugs/?42830>

                 Summary: Bug in gsl_bspline_knot constructor
                 Project: GNU Scientific Library
            Submitted by: foivos
            Submitted on: Wed 23 Jul 2014 04:53:31 AM GMT
                Category: None
                Severity: 3 - Normal
        Operating System: Ubuntu 12.04 LTS
                  Status: None
             Assigned to: None
             Open/Closed: Open
                 Release: gsl 1.16
         Discussion Lock: Any

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Details:

Dear all,

I found the following problem which I think is a bug on the construction of
B-spline basis from non uniform breakpts

The constructor:

gsl_bspline_knot(gsl_vector * breakpts, gsl_bspline_workspace *bw)

does not  always check correctly for increasing values in the supplied break
points vector breakpts. Specifically (sometimes) it runs with no warning, and
gives values for B-spline basis that are inconsistent (e.g. negative values
for the basis functions B_i(x)).

I  attach: 
a) A plot of the calculated B-spline basis B_i(x) that were created for a knot
vector with non increasing values.
b) A program that demonstrates this. Specifically, I have two choices of
breakpoints, one that the program compiles and runs and produces inconsistent
results, and another that the program exits on run time, complaining for non
increasing order of knot vector.

GSL version 1.16
Operating system: Ubuntu 12.04 LTS
Compiler version: g++ 4.7.3
File compiled with command:
g++ -std=c++11 test_bspline_order.cpp -o test_bspline_order.xxx -lgsl
-lgslcblas

Hardware: Toshiba stallite pro C850 (PSCBXA-010005)
CPU: Intel i5 processor 3210M (2.5 GHz - 3.1 GHz (Turbo, 2 Cores) 1333MHz FSB,
256 KB per core L2 cache, 3MB L3 cache)
Chipset: Intel HM76



Thank you all for the great help you provide.

All the best,  



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File Attachments:


-------------------------------------------------------
Date: Wed 23 Jul 2014 04:53:31 AM GMT  Name: bsplines_order_fix.png  Size:
29kB   By: foivos

<http://savannah.gnu.org/bugs/download.php?file_id=31756>
-------------------------------------------------------
Date: Wed 23 Jul 2014 04:53:31 AM GMT  Name: test_bspline_order.cpp  Size: 1kB
  By: foivos

<http://savannah.gnu.org/bugs/download.php?file_id=31757>

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