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Re: Strange Solve result after previous bad input

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100971] Re: Strange Solve result after previous bad input
  • From: Albert Retey <awnl at gmx-topmail.de>
  • Date: Thu, 18 Jun 2009 20:49:33 -0400 (EDT)
  • References: <h1d4ao$o0q$1@smc.vnet.net>

Alain Cochard wrote:
> This is the proper solution to the correctly input system:
> 
>      Mathematica 6.0 for Linux Itanium (64-bit)
>      Copyright 1988-2007 Wolfram Research, Inc.
> 
>      In[1]:= Solve[{a==x + y, b==-x + y},{x,y}]
> 
> 		    a - b       a + b
>      Out[1]= {{x -> -----, y -> -----}}
> 		      2           2
> 
> Here, there is '=' instead of '==' in the 2nd eq:
> 
>      In[2]:= Solve[{a==x + y, b=-x + y},{x,y}]
> 
>      Solve::eqf: -x + y is not a well-formed equation.
> 
>      Out[2]= Solve[{a == x + y, -x + y}, {x, y}]
> 
> and then, subsequent properly input systems will still fail:
> 
>      In[3]:= Solve[{a==x + y, b==-x + y},{x,y}]
> 
>      Solve::svars: Equations may not give solutions for all "solve" variables.
> 
>      Out[3]= {{x -> a - y}}
> 
> 
> Is this normal?  

yes, absolutley. As elsewhere one is not the same a two:

"=" is short for Set and "==" is short for Equal, very different objects
for Mathematica.

> If so, is there a cure (some "resetting" procedure), apart from
> quiting Mathematica and entering back?

b=. , and "=." is the short form of Unset.

> Also, is there a way of testing that a previous bad input is indeed
> the cause of that strange output? (Because if it happened a long time
> ago in the Mathematica history, one might no longer remember.)

Just evaluate b and when it evaluates to something it is not a good
choice for a parameter in Solve, especially if it's value makes one of
the two equations be True...

hth,

albert


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