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

• To: mathgroup at smc.vnet.net
• Subject: [mg56083] Re: Wierd problems with Mathematica - inversion of a 12x12 matrix
• From: "Carl K. Woll" <carlw at u.washington.edu>
• Date: Fri, 15 Apr 2005 04:47:19 -0400 (EDT)
• Organization: University of Washington
• References: <d3lrsq$sef$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

"Madhusudan Singh" <spammers-go-here at spam.invalid> wrote in message 
news:d3lrsq$sef$1 at smc.vnet.net...
> 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.
>

One possibility is that the front end is having difficulty formatting the 
answer, presumably because the answer contains so many terms. If that is so, 
assign the inverse to a variable, and prevent the front end from displaying 
the answer by ending the statement with a semicolon. For example, if mat is 
your matrix, then do something like

inv = Inverse[mat];

If the text version of Mathematica can find the inverse, then presumably the 
above statement should also take a couple minutes. You can then check how 
big the answer is by doing something like

ByteCount[inv]

or

LeafCount[inv]

If that doesn't help, then providing more details of your particular problem 
may elicit some more helpful responses.

Carl Woll