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MathGroup Archive 2006

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Re: Matrix multiplication problem fixed via dot operation.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg64606] Re: Matrix multiplication problem fixed via dot operation.
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Thu, 23 Feb 2006 00:35:56 -0500 (EST)
  • Organization: The University of Western Australia
  • References: <dthh0q$n8d$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <dthh0q$n8d$1 at smc.vnet.net>,
 Chris Young <c1572young at earthlink.net> wrote:

> To answer my own posting, it looks like the "dot" operator is needed 
> when the matrices aren't both square.

No -- the Dot operator is _always_ needed when you want to (matrix) 
multiply two matrices.

> E.g.: a row vector with components a and b times a column vector with 
> components c and d will yield the correct result, the dot-product.
> 
> I'm still puzzled why Mathematica doesn't do the same thing without the dot.

Because if you have two objects of the same dimension and multiply them 
together using Times, you get element-by-element multiplication. For 
example,

  {{a,b},c} {{d,e},f}

Cheers,
Paul

_______________________________________________________________________
Paul Abbott                                      Phone:  61 8 6488 2734
School of Physics, M013                            Fax: +61 8 6488 1014
The University of Western Australia         (CRICOS Provider No 00126G)    
AUSTRALIA                               http://physics.uwa.edu.au/~paul


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