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MathGroup Archive 2005

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Wierd problems with Mathematica - inversion of a 12x12 matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56070] Wierd problems with Mathematica - inversion of a 12x12 matrix
  • From: Madhusudan Singh <spammers-go-here at spam.invalid>
  • Date: Thu, 14 Apr 2005 08:56:39 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi

I am solving a problem using f95 that involves solving a 12x12 coupled
linear system. I have written out the matrix A (A x = f), and instead of
calculating the inverse repeatedly for different values of the input 
parameters, I am trying to calculate the symbolic inverse of the matrix,
post multiply it with the RHS and code the resulting 12 equations for x_i 
into fortran.

I believe this is the sensible way to do it, since this system needs to be
solved lots of times (its part of a half-infinite integral) and inverting
such a system numerically would not only be time consuming, but also
obscure possible stability problems.

Now, I am using Mathematica 5.1 (multiprocessor Sun machine) to do it, but
the inverse has been running for the past 40 minutes without any result. A
prior attempt to run it with Mathematica 4.1 (single processor Pentium M
Linux machine) resulted in an aborted process.

Out of a hunch, I just exported the notebook to text, and used "math" (the
text version of Mathematica) to run it through. Less that two minutes
later, I had an answer. The problem is that the answer (in nohup.out) is so
ill formatted (I used MatrixForm with FullSimplify) that it is impossible to
make sense of.

OTOH, the graphical Notebook is still chugging away. Has anyone seen this
odd behaviour before ?

Thanks.


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