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Re: Mathematica bug??

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100586] Re: Mathematica bug??
  • From: pfalloon <pfalloon at gmail.com>
  • Date: Mon, 8 Jun 2009 03:06:33 -0400 (EDT)
  • References: <h0fvlb$rct$1@smc.vnet.net>

On Jun 7, 7:01 pm, oshaughn <oshau... at gravity.psu.edu> wrote:
> {{Sqrt[1 - 2/r], 0, 0, 0}, {0, 1/Sqrt[1 - 2/r], 0, 0}, {0, 0, r,
>    0}, {0, 0, 0, r Sin[\[Theta]]}} // Inverse
>
> gives
>
> {{0, 0, 0, 0}, {0, 0, 0, 0}, {0, 0, 1/r, 0}, {0, 0, 0, Csc[\[Theta]]/
>   r}}
>
> with no warnings.  This is a *diagonal* matrix that should be
> trivially invertible.
>
> Does anyone know why Inverse has catastrophically failed here?

Hi,
Perhaps this is a bug from a previous version. It works fine for me in
7.0 on Windows:

In[25]:= $Version
Out[25]= 7.0 for Microsoft Windows (32-bit) (February 18, 2009)

In[26]:= mat = DiagonalMatrix[{Sqrt[1-2/r], 1/Sqrt[1-2/r], r, r Sin[\
[Theta]]}]
Out[26]= {{Sqrt[1-2/r],0,0,0},{0,1/Sqrt[1-2/r],0,0},{0,0,r,0},{0,0,0,r
Sin[\[Theta]]}}

In[27]:= minv = Inverse[mat]
Out[27]= {{1/Sqrt[1-2/r],0,0,0},{0,Sqrt[1-2/r],0,0},{0,0,1/r,0},
{0,0,0,Csc[\[Theta]]/r}}

In[28]:= mat.minv == IdentityMatrix[Length[mat]]
Out[28]= True


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