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Re: Matrix Inverse Issue

• To: mathgroup at smc.vnet.net
• Subject: [mg80634] Re: Matrix Inverse Issue
• From: "Brian Gladman" <brg at nowhere.org>
• Date: Sun, 26 Aug 2007 23:28:42 -0400 (EDT)
• References: <far922$3t8$1@smc.vnet.net>

"Brian Gladman" <brg at nowhere.org> wrote in message 
news:far922$3t8$1 at smc.vnet.net...
>I am having a problem with the matrix inverse for the following matrix in
> Mathematica 6:
>
> A = Table[If[j == i, 1 - i/10, If[j == i + 1, i/10, 0]], {i, 9}, {j, 9}]
>
> {{9/10, 1/10, 0, 0, 0, 0, 0, 0, 0}, {0, 4/5, 1/5, 0, 0, 0, 0, 0,
>  0}, {0, 0, 7/10, 3/10, 0, 0, 0, 0, 0}, {0, 0, 0, 3/5, 2/5, 0, 0, 0,
>  0}, {0, 0, 0, 0, 1/2, 1/2, 0, 0, 0}, {0, 0, 0, 0, 0, 2/5, 3/5, 0,
>  0}, {0, 0, 0, 0, 0, 0, 3/10, 7/10, 0}, {0, 0, 0, 0, 0, 0, 0, 1/5, 4/
>  5}, {0, 0, 0, 0, 0, 0, 0, 0, 1/10}}
>
> A Inverse[A]
>
> {{1, -1/72, 0, 0, 0, 0, 0, 0, 0}, {0, 1, -1/14, 0, 0, 0, 0, 0, 0}, {0,
>   0, 1, -3/14, 0, 0, 0, 0, 0}, {0, 0, 0, 1, -8/15, 0, 0, 0, 0}, {0,
>  0, 0, 0, 1, -5/4, 0, 0, 0}, {0, 0, 0, 0, 0, 1, -3, 0, 0}, {0, 0, 0,
>  0, 0, 0, 1, -49/6, 0}, {0, 0, 0, 0, 0, 0, 0, 1, -32}, {0, 0, 0, 0,
>  0, 0, 0, 0, 1}}
>
> I expected the identity matrix here but I get extra terms.
>
> Am I doing something wrong?
>
>   Brian Gladman

Ok, I now know I need a '.' for matrix multiplication.

    Brian Gladman