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