       Re: Non Commutative Multiply

• To: mathgroup at smc.vnet.net
• Subject: [mg128607] Re: Non Commutative Multiply
• From: Roland Franzius <roland.franzius at uos.de>
• Date: Thu, 8 Nov 2012 19:01:50 -0500 (EST)
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```Am 08.11.2012 08:14, schrieb Rodrigo AP:
> Dear group,
>
> I want to multiply two matrices, for example,
>
> A = {{e, f}, {g, h}}
> B = {{a, b}, {c, d}}
>
> Using A.B, Mathematica returns
>
> {{a e + b g, a f + b h}, {c e + d g, c f + d h}}
>
> I would like to get, however, the following result:
>
> {{e a + f c, e b + f d}, {g a + h c, g b + h d}}
>
> since, for me, the entries {a,b,c,d,e,f,g,h} are operators, i.e., they
> are non-commutative.
>
> I could solve this problem clearing the attribute "Orderless" in the
> built-in function Times:
>
> ClearAttributes[Times, Orderless]
>
> I know, however, this can be dangerous. I tried to define a function
> Times2[a_,b_]:=Times[a,b]
> and then use ClearAttributes[Times2, Orderless] but it doesn't work.
>
> How could I solve this problem?

Changing fundamental operators like Times is not a solution. It will
cause a breakdown of the algebra machine.

Use operator  Inner  and

a multiplicative oprator symbol from

http://reference.wolfram.com/mathematica/tutorial/OperatorsWithoutBuiltInMeanings.html

like

CircleTimes, SmallCircle.

SmallCircle[A_?MatrixQ, B_?MatrixQ]:=Inner[SmallCircle[#1,#2]&,A,B,Plus]
SmallCircle[A_,B_, C___]:=SmallCircle[SmallCircle[A,B], C]

Now you can enrich procedures like Simplify, Factor and Expand by the
typical set of rules for the linear expressions with head Plus und Times, eg

Unprotect[Simplify]

Simplify[expression_]:=
expression//.
{SmallCircle[a___,b_?NumericQ,c___]:>b*SmallCircle[a,c],
SmallCircle[a___,b_+c__,d___]:> SmallCircle[a,b,d]+SmallCircle[a,c,d],
SmallCircle[a___,b_?NumericQ c__:1,d___]:>b*SmallCircle[a,b,d]}

Probably it is better to give the class of noncommutative objects an
identity by naming them explicitely in a logical statement as

operators = {P,Q, d_Q, d_P}

(OperatorQ[#]=True&)/@operators

or

OperatorQ[x_]:= MemberQ	[ operators, x]

and formulate the rules in such a way that only products and powers of
operators remain inside the noncommutative operator product.

This makes it easier to extract the typical scalar functions, starting
with Power and in principle has to be applied to the body of all special
complex functions.

--

Roland Franzius

```

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