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MathGroup Archive 2005

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Re: Wierd problems with Mathematica - inversion of a 12x12 matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56092] Re: Wierd problems with Mathematica - inversion of a 12x12 matrix
  • From: "Ray Koopman" <koopman at sfu.ca>
  • Date: Fri, 15 Apr 2005 04:47:35 -0400 (EDT)
  • References: <d3lrsq$sef$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Maybe the inverse has been calculated, and the time is being spent
formatting the output. Try

  Timing[B = Inverse[A];]

or something similar that suppresses the matrix output,
and then look at B an element at a time.

Madhusudan Singh wrote:
> 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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