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re: Specifying rules

• To: mathgroup at yoda.physics.unc.edu
• Subject: re: Specifying rules
• From: Marnix Van Daele <Marnix.VanDaele at rug.ac.be>
• Date: Wed, 15 Jun 1994 17:58:15 +0200 (MET DST)

On Mon, 13 Jun 1994 Paul=Rubin%Management%Business at banyan.cl.msu.edu wrote:

> Ron I. Herman <m1rih00 at frb.gov> wrote:
> 
> >Is there any way to specify rules for symbols?
> >For example, is there a way to tell Mathematica
> >that a > 0 and b < 0 such that Positive[a b] will
> >yield false?
> 
> Yes, using upvalues, but I'm not confident that it works across the board.  
> For your specific example:
> 
> In[]:=  Clear[a, b];
> In[]:=  a /: Sign[ a ] = 1;
> In[]:=  b /: Sign[ b ] = -1;
> 
> The two preceding lines tell Mma to evaluate the Sign function differently 
> when applied to a and b.  The "a /:" part associates that definition with a 
> rather than Sign (which is protected).  If you clear a, the upvalue goes with 
> it.
> 
> In[]:=  Positive[ a b ]
> Out[]=  False
> In[]:=  Positive[ a ]
> Out[]=  True
> In[]:=  Negative[ b ]
2> Out[]=  True
> 
> It worked this time.  It seems to me, though, that I've seen other examples 
> where it doesn't work so well.  (I tried up-setting NumberQ[a] to True, so 
>2 that Mma would think a was a number, and caused a recursion fault with a 
> later reference to a.)  Off-hand, I can't remember the failure stories I've 
> read.  So caveat emptor.
> 
> 2**************************************************************************
> * Paul A. Rubin                                  Phone: (517) 336-3509   *
> * Department of Management                       Fax:   (517) 336-1111   *
> * Eli Broad Graduate School of Management        Net:   RUBIN at MSU.EDU    *
> * Michigan State University                                              *
> * East Lansing, MI  48824-1122  (USA)                                    *
> **************************************************************************
> Mathematicians are like Frenchmen:  whenever you say something to them,
> they translate it into their own language, and at once it is something
> entirely different.                                    J. W. v. GOETHE
> 
> 
> 

Hello, I have a problem with complex numbers !

I want to tell Mma that a is real, such that
Im[a+I] will give 1,
but I do not get this result.

I have tried

a /: Im[a]=0


Can anyone help me ?

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Marnix Van Daele
Universiteit Gent
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tel. 32-(0)9-264.48.09
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