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Re: Question on Positive[x]=True

• To: mathgroup at smc.vnet.net
• Subject: [mg7856] Re: Question on Positive[x]=True
• From: bruck at pacificnet.net (Ronald Bruck)
• Date: Thu, 17 Jul 1997 02:01:39 -0400
• Organization: University of Southern California
• Sender: owner-wri-mathgroup at wolfram.com

In article <5pv0ob$id3 at smc.vnet.net>, Raya Firsov-Khanin
<raya at mech.ed.ac.uk> wrote:

:Dear Steve,
:
:I have the following question.  I want to state that a variable 
:x is positive. On page 1170 of the Third edition of The Mathematica book 
:it is said that I can do it by making a definition
:     
:    Positive[x] = True 
:
:However, while trying to do it I get a message : 
:       
:Set::write: Tag Positive in Positive[x] is Protected 
:
:Any suggestions on how one can make variables positive, negative etc? 
:
:Thanks a lot. 
:                  Raya Khanin. 
:                  University of Edinburgh

There seem to be a lot of lacunae in the third edition; unfortunately, this
one ISN'T a lacunae, but a false statement!

Use the ^= construction.  But a word of warning:  Mathematica is rather
brain-dead in this regard; if you set Positive[x] ^= True, and ask
"Positive[x]", well, you get true; excellent; but if you ask "Negative[x]",
you get it back unevaluated.  (Similarly, Sqrt[x^2] doesn't evaluate to x.)

In another post, Tony Nicolas suggests using ==.  But he's missed the point
of what you're trying to do (which is to ASSUME x is positive).

--Ron Bruck

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