       Re: Mathematica bug??

• To: mathgroup at smc.vnet.net
• Subject: [mg100552] Re: [mg100540] Mathematica bug??
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Mon, 8 Jun 2009 02:04:06 -0400 (EDT)
• Organization: Mathematics & Statistics, Univ. of Mass./Amherst
• References: <200906070902.FAA28100@smc.vnet.net>
• Reply-to: murray at math.umass.edu

```First, you should be very cautious about categorically, and so
dramatically, entitling a post as "Mathematica bug".

Second, such a title is so general as to be unhelpful to potential readers.

Third, you don't say which version of Mathematica you're using. I tried
this in Mathematica 7.0.1 and got the correct inverse:

{{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 // InputForm
{{1/Sqrt[1 - 2/r], 0, 0, 0}, {0, Sqrt[1 - 2/r], 0, 0},
{0, 0, r^(-1), 0}, {0, 0, 0, Csc[\[Theta]]/r}}

But then I also tried it in Mathematica 5.2, and still I got the correct
inverse.

I suspect you had already assigned a value to r or Theta somewhere
earlier in the evaluation steps.  Or else just mistyped something.

oshaughn 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?
>

--
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

```

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