Re: Re: Problem with Mathematica 6

*To*: mathgroup at smc.vnet.net*Subject*: [mg77124] Re: [mg77087] Re: [mg77065] Problem with Mathematica 6*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>*Date*: Mon, 4 Jun 2007 03:52:17 -0400 (EDT)*References*: <200706020819.EAA29947@smc.vnet.net> <200706031009.GAA02755@smc.vnet.net>

On 3 Jun 2007, at 19:09, Carl Woll wrote: > The problem is that NonCommutativeMultiply is Flat and OneIdentity, > so I > think > > NonCommutativeMultiply[a,b] > > is being interpreted as > NonCommutativeMultiply[NonCommutativeMultiply[a],b] which gets > converted > to NonCommutativeMultiply[Times[a],b] and then > NonCommutativeMultiply[a,b] and so you get an iteration error. To > avoid > this Flat issue, use HoldPattern: > > Unprotect[NonCommutativeMultiply]; > HoldPattern[NonCommutativeMultiply][a___] /; (Length[GetGradeds[a]] <= > 1) := Times[a]; > Protect[NonCommutativeMultiply]; > > Now, it works: > > In[347]:= a ** b > Out[347]= a**b > > Carl Woll > Wolfram Research > I don't think it is quite as simple as that. In[1]:= ClearAttributes[NonCommutativeMultiply, OneIdentity] In[2]:= Attributes[NonCommutativeMultiply] Out[2]= {Flat, Protected} In[3]:= Grading[_] = 0; Fermion[a__] := ((Grading[#1] = 1) &) /@ {a}; Fermion[a, b]; GetGradeds[a___] := GetGradeds[a] = Select[{a}, Grading[#1] != 0 &]; In[7]:= Unprotect[NonCommutativeMultiply]; NonCommutativeMultiply[a___] /; (Length[GetGradeds[a]] <= 1) := Times [a]; Protect[NonCommutativeMultiply]; In[10]:= a ** b and you get exactly the same problem. Also, see my post in this thread that shows that clearly there has been a change in the way pattern works. Namely, running a ** b /. NonCommutativeMultiply[x___] /; (Print[{x}]; Length[{x}] <= 1) :> f[x] gives different output in Mathematica 5.2 and 6.0. To me it seems clear that the behaviour in 6 is an imporovement, but the OneIdentity attribute alone does not seem to be responsible for it. Andrzej Kozlowski

**References**:**Problem with Mathematica 6***From:*"Michael Weyrauch" <michael.weyrauch@gmx.de>

**Re: Problem with Mathematica 6***From:*Carl Woll <carlw@wolfram.com>