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MathGroup Archive 2006

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Re: solve and Abs

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69397] Re: solve and Abs
  • From: Peter Pein <petsie at dordos.net>
  • Date: Sun, 10 Sep 2006 07:20:34 -0400 (EDT)
  • References: <edtrmv$le6$1@smc.vnet.net>

Richard J. Fateman schrieb:
> Is there a reason (in 5.1) for Solve[x-Abs[x]==0,x] to return {{}}?
> This supposedly means all variables can have all possible values.
> 
> Reduce does better.
> 

Hello,

  the result {{}} means that Solve did not find values for x, which 
fulfill the equation without "special conditions". The help browser 
tells us:
"Solve gives generic solutions only. It discards solutions that are 
valid only when the parameters satisfy special conditions. Reduce gives 
the complete set of solutions. "

The special condition in this case is obviously x>=0.

Greetings,
  Peter


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