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Re: inverse matrix


From: Abdul Rahman Riza
Subject: Re: inverse matrix
Date: Sun, 28 Oct 2007 12:33:19 +0800

Hi Olaf,

I found the same result compared to your 2.1.73 running on my ocatve 2.9.9 in ubuntu feisty fawn as stated below:

octave:1> a = [1.2500e+02, 1.4595e+01, 5.8551e-08, 4.9222e+02; 1.4595e+01, 1.0320e+01, -2.2343e-12, 1.7402e+02; 5.8551e-08, -2.2343e-12, 5.8688e+03, -1.5056e-11; 4.9222e+02, 1.7402e+02, -1.5056e-11, 8.8034e+03]
a =

   1.2500e+02   1.4595e+01   5.8551e-08   4.9222e+02
   1.4595e+01   1.0320e+01  -2.2343e-12   1.7402e+02
   5.8551e-08  -2.2343e-12   5.8688e+03  -1.5056e-11
   4.9222e+02   1.7402e+02  -1.5056e-11   8.8034e+03

octave:2> [x,rcond]=inverse(a)
x =

   1.0634e-02  -7.5196e-03  -1.0610e-13  -4.4593e-04
  -7.5196e-03   1.5066e-01   7.5071e-14  -2.5578e-03
  -1.0610e-13   7.5071e-14   1.7039e-04   4.4484e-15
  -4.4593e-04  -2.5578e-03   4.4484e-15   1.8909e-04

rcond =  6.5696e-04
octave:3>

Regards,
Riza

On Mon, 2007-09-17 at 14:45 +0200, Ólafur Jens Sigurðsson wrote:
Hi

I came across the following scenario:
For the following matrix

 1.0634e-02   -7.5196e-03   -1.0610e-13   -4.4593e-04
-7.5196e-03    1.5066e-01    7.5071e-14   -2.5578e-03
-1.0610e-13    7.5071e-14     1.7039e-04    4.4484e-15
-4.4593e-04   -2.5578e-03    4.4484e-15    1.8909e-04

the inverse in version 2.9.10 in debian produces rcond = 0 but in 2.1.73 it is rcond = 0.00065696

Does this happen in all machines or is this just me (a AMD Athlon 1344 MHz, cpu family 6, model 8 ... info taken from /proc/cpuinfo running debian unstable)?

If this is a general thing, is there a reason for why the 2.9.10 is worse in this then 2.1.73 (I would consider this to be worse, unless 2.1.73 is just doing wrong calculation).

Regards

Oli

ps, creat the matrix by the following command (set in so you can just cut and paste)

a = [1.2500e+02, 1.4595e+01, 5.8551e-08, 4.9222e+02; 1.4595e+01, 1.0320e+01, -2.2343e-12, 1.7402e+02; 5.8551e-08, -2.2343e-12, 5.8688e+03, -1.5056e-11; 4.9222e+02, 1.7402e+02, -1.5056e-11, 8.8034e+03]
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