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Re: Transpose[A]*A wrong?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg41193] Re: Transpose[A]*A wrong?
  • From: Petr Kujan <kujanp at fel.cvut.cz>
  • Date: Tue, 6 May 2003 06:07:56 -0400 (EDT)
  • Organization: Czech Technical University
  • References: <b950vl$r49$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Daniel Lidström wrote:

>Hi,
>
>why does Mathematica give the wrong result here?
>
>In[1]:= \!\(A\  = \ {{x\_11, x\_12, x\_13}, {x\_21, x\_22, x\_23}, {x\_31,
>x\_32,
>                x\_33}}\)
>
>Out[1]= {{x  , x  , x  }, {x  , x  , x  }, {x  , x  , x  }}
>           11   12   13     21   22   23     31   32   33
>
>In[2]:= Transpose[A]*A
>
>             2                                  2
>2
>Out[2]= {{x   , x   x  , x   x  }, {x   x  , x   , x   x  }, {x   x  , x   x
>, x   }}
>           11    12  21   13  31     12  21   22    23  32     13  31   23
>32   33
>
>Look at the diagonal elements for example.
>Thanks!
>
>/Daniel
>
>  
>
Hi Daniel,

function - Times[ ] ..  "*" -> array multiply,
function - Dot[ ] .. "." -> matrix multiply.

Now it's matrix multiply:
In[9]:=
\!\(A\  = \ {{x\_11, x\_12, x\_13}, {x\_21, x\_22, x\_23}, {x\_31, x\_32,
        x\_33}}\)

Out[9]=
\!\({{x\_11, x\_12, x\_13}, {x\_21, x\_22, x\_23}, {x\_31, x\_32, x\_33}}\)

In[8]:=
Transpose[A].A

Out[8]=
\!\({{x\_11\%2 + x\_21\%2 + x\_31\%2,
      x\_11\ x\_12 + x\_21\ x\_22 + x\_31\ x\_32,
      x\_11\ x\_13 + x\_21\ x\_23 + x\_31\ x\_33}, {x\_11\ x\_12 +
        x\_21\ x\_22 + x\_31\ x\_32, x\_12\%2 + x\_22\%2 + x\_32\%2,
      x\_12\ x\_13 + x\_22\ x\_23 + x\_32\ x\_33}, {x\_11\ x\_13 +
        x\_21\ x\_23 + x\_31\ x\_33,
      x\_12\ x\_13 + x\_22\ x\_23 + x\_32\ x\_33,
      x\_13\%2 + x\_23\%2 + x\_33\%2}}\)

Petr



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