Re: commutativity

• To: mathgroup@smc.vnet.net
• Subject: [mg11835] Re: commutativity
• From: David Annetts <dannetts@laurel.ocs.mq.edu.au>
• Date: Wed, 1 Apr 1998 00:35:41 -0500
• Organization: CRCAMET/Macquarie University
• References: <6ek4j9\$jag@smc.vnet.net> <6fd237\$5u9@smc.vnet.net> <6fo47n\$ock\$7@dragonfly.wolfram.com>

```> Hi,
>
> Daitaro Hagihara wrote:
>
> > In article <6ek4j9\$jag@smc.vnet.net>, John Albert Horst
> > <horst@cme.nist.gov> wrote:
> >
> > > I would like to be able to define a rule called, say, commutativeOff
> > > that would work as follows:
> > >
> > > In[1] := a*b == b*a/.commutativeOff
> > >
> > > Out[1] := False
> > >
> > > As it is,
> > >
> > > In[1] := a*b == b*a
> > >
> > > Out[1] := True
> > >
> > > How can I do this?
> >
> > Working on this topic on and off for a week, I came up with a poor man's
> > non-commutative package.  It uses ordinay symbols to represent basic
> > algebra, and it can be turned on and off by a couple of switches in a
> > considerately unintrusive way.  Just eval all of the following in
> > brackets ([...]), and it starts out with the usual commutative
> > behavior; all math is unchanged.  Then by issuing "commutativeOff"
> > command, things will become completely non-commutative.  To get back to
> > the usual behavior, issue "commutativeOn" command.  The vector
> > differentiation is currently nothing more than a hack.  Even with a
> > future improvement, if any, this is no replacement for full tensor
> > packages out there.

A thousand apologies for the empty message:(

What I intended to say, was that I recall a MathSource package called
NonCommutativeAlgebra, that looks like a Rich man's package

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