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MathGroup Archive 1997

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Re: Re: nested * and ** (rules for commutative quantities)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7787] Re: [mg7749] Re: [mg7658] nested * and ** (rules for commutative quantities)
  • From: mabi at solidmr.kun.nl (Marlies Brinksma)
  • Date: Tue, 8 Jul 1997 22:41:04 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Dear Carl,

You wrote: 

> On Thu, 26 Jun 1997, Marlies Brinksma wrote:
> 
> > ...
> > 
> > I have a set of operators (let's call them Op[index_,arg2_]).
> > Two of these operators commute when their indices are different and they are 
> > noncommutative otherwise. All operators commute with any scalar quantity.
> > 
> > I would like to define some rules such that expressions like:
> > 
> > Op[3,a] ** Op[2,v] ** 4 ** Op[6,s] ** Op[2,x] ** Op[1,t]
> > 
> > will be automatically changed to:
> > 
> > Times[4, Op[1,t],NonCommutativeMultiply[Op[2,v],Op[2,x]],Op[3,a],Op[6,s]]
> >                                                        ^
> > ...                                                    |
> 
> Hi Marlies,
> 
> I'm not sure of all the rules that you want to impose, since you don't
> explain why Op[1,t] is not within NonCommutativeMultiply. Also, you don't
> mention what kinds of objects other than numbers might occur in addition
> to your Op functions. At any rate, here is a short definition which might
> do what you want.

I think you've missed one ] right behind Op[2,x]... ;)
Op[3,a] and Op[6,s] are also not in NonCommutativeMultiply[]. The reason for 
this (see above) is that 2 operators with different indices commute. So only 
the operators with indices 2 do not commute with eachother, but commute with 
the rest. That's why I would like to have them as parameters of Times[] (which 
has the attribute Orderless) instead of NonCommutativeMultiply[].

Thanks anyway you for having a look at my posting! I appreciate that very much.

Marlies


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