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Re: Unwanted Recursion
*To*: mathgroup at smc.vnet.net
*Subject*: [mg120066] Re: Unwanted Recursion
*From*: Bill Rowe <readnews at sbcglobal.net>
*Date*: Thu, 7 Jul 2011 07:32:32 -0400 (EDT)
On 7/6/11 at 5:39 AM, casorati at f-m.fm (Casorati) wrote:
>I'm trying to define an anti-commutative algebra in Mathematica.
>Here I will show only a minimal example to demonstrate my point. The
>product of two elements a^b should be give by p[a,b] so I make p
>flat:
Unless you are inventing new syntax a^b is not the product of a
and b. It is a raised to the b power.
>SetAttributes[p, {Flat}]
>But now I want p[a] to reduce to a. Such a rule would make other
>rules much simpler but when I define
>p[a_] := a
>the expression p[a] causes an infinite recursion. I get the message
>$IterationLimit::itlim: Iteration limit of 4096 exceeded.
While I cannot explain precisely why this message occurs I can
confirm I get the same result when I set the attributes for p to
be Flat. However, if I do not set the attributes of p then
things work fine as demonstrated by:
In[1]:= p[a_] := a
In[2]:= p[a]
Out[2]= a
and given that p has been defined with a single argument, it
makes no sense for it to also have the attribute Flat.
If you were to define p to take an arbitrary number of arguments
and give it the attribute Flat, i.e.,
In[6]:= Clear[p]
In[7]:= SetAttributes[p, {Flat}]
In[8]:= p[a__] := a
In[9]:= p[a, b]
Out[9]= Sequence[a,b]
the error message doesn't occur.
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