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MathGroup Archive 2006

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Re: mathematica newbie question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg66908] Re: mathematica newbie question
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Sat, 3 Jun 2006 03:25:56 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 6/2/06 at 4:09 AM, jwatts1970 at hotmail.com (Jeremy Watts) wrote:

>Just trying to get to grips with Mathematica and matrices.  Why is
>it when I enter :- {0, 0, 0}, {1, -1, -1}, {-1, 1, 1}}^2

>then Mathematica returns :-

>{{0, 0, 0}, {1, 1, 1}, {1, 1, 1}}

>and not {{0,0,0},{0,0,0},{0,0,0}} (the actual square of the matrix)
>as I'd have expected?

>Is it treating what I entered as separate vectors or something, and
>not  an actual matrix?

Because by default Mathematica does an element by element multiply for expressions equivalent to a*b for matrices a,b. To get matrix multiplication you need to use ".". That is

In[15]:=
a={{0,0,0},{1,-1,-1},{-1,1,1}};

In[16]:=
a.a

Out[16]=
{{0, 0, 0}, {0, 0, 0}, {0, 0, 0}}
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