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MathGroup Archive 2005

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Re: Why does Inverse[M] hesitate?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54431] Re: Why does Inverse[M] hesitate?
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Sun, 20 Feb 2005 00:09:53 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

On 2/19/05 at 2:32 AM, skirmantas.janusonis at yale.edu (Skirmantas)
wrote:

>The Inverse function sometimes calculates the inverse of a matrix
>immediately, sometimes it does not. Try this example in Mathematica
>5.1:

>A={{(1-g)-1,1},{-w P(1-g)/C,-1}}//MatrixForm
>B={{0},{-P(w+1)}}//MatrixForm

>I get Out: Inverse[(expanded A)].(expanded B)

>If I do just 
>A={{a,b},{c,d}}
>B={{e},{f}}
>Inverse[A].B

>I get the final correct result.

Right. This is becuase you used MatrixForm which puts a wrapper around the expression.

If you do

A={{(1-g)-1,1},{-w P(1-g)/C,-1}}//MatrixForm
MatrixQ[A]

you will get False indicating A is not a matrix. The same is true of B. Consequently, neither Inverse nor . can work

There are a couple of ways around this issue. First, you could do

(A={{(1-g)-1,1},{-w P(1-g)/C,-1}})//MatrixForm
(B={{0},{-P(w+1)}})//MatrixForm

This forces assignment to A and B before MatrixForm does its thing. As a consequence,
MatrixQ[A] will evaluate as True and Inverse[A].B will do what you expect.

But I think the more elegant way around this issue is to set the default output display to TraditionalForm and simply not use MatrixForm
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