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

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




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

Carl Woll 

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