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[Bug-gsl] Bug in gsl_sf_coulomb_wave_F_array
From: |
Hongcheng Ni |
Subject: |
[Bug-gsl] Bug in gsl_sf_coulomb_wave_F_array |
Date: |
Tue, 17 Nov 2015 18:14:39 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 |
Dear GSL developers,
I found a bug in the function gsl_sf_coulomb_wave_F_array which uses the
recurrence relation to obtain F for different l's.
To be short, when eta is large, and l is at least moderately large,
inaccuracy occurs in the implementation of the recurrence such that F
has unphysical spikes at certain x for smaller l's.
My observation is that F is determined directly for the largest l, and F
for smaller l's are determined from the recurrence relation.
When l is large, the number of recurrence is large to obtain F for small
l's.
Therefore, errors in F's get accumulated as we proceed to get F for
small l's.
Attached please find a test program.
The program uses two methods to obtain F for different l's and x's.
The first method obtains F directly, and the second method obtains F
from recurrence relation.
Please compile it using "g++ testCoulomb.cpp -lgsl -lgslcblas".
Now run the program using "./a.out > resultCoulomb".
We can now take a look at the output of the program using gnuplot.
After launching gnuplot, just type plot "resultCoulomb" using 1:2
every ::2::2000 to see the output for l=0 at different x's using
the first method.
And type plot "resultCoulomb" using 1:2 every ::2002::4000 to see
the output for l=0 at different x's using the second method.
Now you can find, F is fine using the first method, but F has unphysical
spikes.
You can see similar behaviors for other l's by "using 1:3", "using 1:4"
etc in gnuplot.
I can of course always stick to the first method, but the second method
is much faster.
So it would be great if you could fix this problem.
Thanks a lot!
--
Best regards,
Hongcheng Ni
Max-Planck-Institut für Physik komplexer Systeme
Nöthnitzer Str. 38, 01187 Dresden, Germany
testCoulomb.cpp
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