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MathGroup Archive 2007

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Re: Re: Logical Expression

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73459] Re: [mg73445] Re: Logical Expression
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Sat, 17 Feb 2007 05:03:10 -0500 (EST)
  • References: <er3ii6$gnh$1@smc.vnet.net> <200702161021.FAA09907@smc.vnet.net>

But the point of it all is that LogicalExpand knows nothing at all  
about numbers and you should not expect it to do that. For example:


LogicalExpand[a=="dog"&&a!="dog"]


False


because this has nothing to do with "dog" being or not being a number  
(it is a string). That is all the LogicalEpxand does: it deal with  
the logical structure of logical statements. Equality and non- 
equality is a general relationship but inequality is not.
That is why it is not reasonable to expect LogicalExpand to deal with  
inequalities, which make sense only for numbers (or, more generally,  
for ordered sets).

On the other hand, the other functions all deal either with numbers  
or (Simplify) more general entities that include numbers, so it makes  
sense for them to deal with inequalities.

Andrzej Kozlowski



On 16 Feb 2007, at 11:21, Martin Schoenecker wrote:

>> "something" is a real number. What do you expect if it is a complex
>> number, of a symbolic expression?
>
> Thank you all for your comments!
>
> As I am dealing with symbols, I expect nothing else:  a==0 means it
> should be zero (which is a real number), a>0 means it should be  
> greater
> than zero (which implies that it should be real), so the  
> assumptions are
> already there.
>
> Simplify (or Reduce, or Resolve...) does the job.
>
> Regards,
> Martin
>



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