       Re: Matrix multiplication problem fixed via dot operation.

• To: mathgroup at smc.vnet.net
• Subject: [mg64585] Re: [mg64552] Matrix multiplication problem fixed via dot operation.
• From: Kristjan Kannike <kkannike at physic.ut.ee>
• Date: Thu, 23 Feb 2006 00:34:28 -0500 (EST)
• References: <200602221058.FAA23462@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```On Wed, 22 Feb 2006, Chris Young wrote:

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

Beware: though you got no error messages in case of multiplying square
matrices with a space between them (as opposed to a dot), the result is
NOT a dot product (matrix multiplication), but the matrices are multiplied
element by element, e.g. if

C = A B

then

C[[1,1]] equals A[[1,1]] * B[[1,1]], etc. In short, the ordinary
multiplication has the attribute Listable. (It can be very useful, e.g.
when you have a vector variable x and a vector (list) of coefficients a,
to compose them into a x.)

Kristjan Kannike
<http://www.physic.ut.ee/~kkannike/english/>

The same person at different times can seem like a different person;
sometimes that person is you.

-- Peter Norvig

```

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