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Re: How to do symbolic matrix manipulations

• To: mathgroup at smc.vnet.net
• Subject: [mg92298] Re: How to do symbolic matrix manipulations
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Sat, 27 Sep 2008 06:48:40 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <gbjmam$3ab$1@smc.vnet.net>

Peng Yu wrote:

> Suppose I have matrices a and b, which does not have any value yet.

Beware that Mathematica is not a strongly-type language and things like 
vector or matrix do not exist per se. A vector can be represented by a 
one dimensional list holding appropriate element, a matrix by a two 
dimensional list; but a generic type vector or matrix does not exist 
natively.

> If I write b * a, mathematica would give me "a b". I'm wondering how to make
> it know that a and b are matrices, so that it would give me "b a" rather
> than "a b".

You could use the non-commutative multiplication operator **.

In[213]:= b ** a

Out[213]= b ** a

If you are going to do extensive symbolic manipulation withe generic 
objects, you may want to define your own matrix type (say expression 
with head matrix, i.g. something like matrix[a]) and 
associate/(re)define properties and operator.

Regards,
-- Jean-Marc