From help-octave-request at bevo dot che dot wisc dot edu Mon Jan 26 08:19:07 2004
Subject: Understanding how octave works...
From: Joerg Frochte <joerg dot frochte at uni-essen dot de>
To: help-octave at bevo dot che dot wisc dot edu
Date: Mon, 26 Jan 2004 15:14:15 +0100

Hello,

I often use octave to control results of a program that uses lapack.

I have got to solve the following :

A=[
 0.00e+00  1.00e-01  0.00e+00  5.00e-03  0.00e+00  0.00e+00  0.00e+00
 0.00e+00  1.67e-04  ;
 0.00e+00 -1.00e-01  0.00e+00  5.00e-03 -0.00e+00  0.00e+00 -0.00e+00
 0.00e+00 -1.67e-04  ;
-7.09e-02  4.48e-02  2.51e-03  1.01e-03 -3.18e-03 -5.94e-05  1.13e-04
-7.13e-05  1.50e-05  ;
-7.09e-02 -5.52e-02  2.51e-03  1.52e-03  3.91e-03 -5.94e-05 -1.39e-04
-1.08e-04 -2.80e-05  ;
-3.55e-02  7.24e-02  6.29e-04  2.62e-03 -2.57e-03 -7.43e-06  4.55e-05
-9.30e-05  6.33e-05  ;
 0.00e+00  5.00e-02  0.00e+00  1.25e-03  0.00e+00  0.00e+00  0.00e+00
 0.00e+00  2.08e-05  ;
-3.55e-02 -7.76e-02  6.29e-04  3.01e-03  2.75e-03 -7.43e-06 -4.88e-05
-1.07e-04 -7.78e-05  ;
 0.00e+00 -5.00e-02  0.00e+00  1.25e-03 -0.00e+00  0.00e+00 -0.00e+00
 0.00e+00 -2.08e-05  ;
-3.55e-02 -2.76e-02  6.29e-04  3.80e-04  9.78e-04 -7.43e-06 -1.73e-05
-1.35e-05 -3.50e-06  ;
-3.55e-02  2.24e-02  6.29e-04  2.51e-04 -7.95e-04 -7.43e-06  1.41e-05
-8.91e-06  1.88e-06  ;
-7.09e-02 -5.16e-03  2.51e-03  1.33e-05  3.66e-04 -5.94e-05 -1.30e-05
-9.44e-07 -2.29e-08  ;
]

b=[  0.00e+00 0.00e+00 7.29e-02 8.51e-02 3.46e-02 0.00e+00 4.39e-02 0.00e+00
4.10e-02 3.80e-02 7.92e-02 ]


octave:16> A\b'
ans =

   -1.1090e+00
    3.7483e-04
   -1.6496e-02
   -4.5344e-04
    1.7691e+00
    1.0526e-03
    8.8226e-01
    2.5739e+00
   -2.6411e-01

The result in octave is quite OK, but the result my own program computes
is very unpleasing.

ans = [ -7.12e+11-3.70e-05-6.03e+13 1.82e-03 1.74e+00-1.70e+15 1.48e-01
2.79e+00 6.10e-02 ]

This has something to do with the fact that A is very close to be singular.

octave:21> cond(A)
ans =  2.9251e+18

Nevertheless, the result of octave is more pleasing.
How does octave deal with such a situation?
I have the source-code but I am unable to find a point to start my analysis,
because I do not know how octave is designed.

Could you tell me how octave works in this situation or give me a file and
line where to start in the octave code?

Thanks very much,

Joerg Frochte




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