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Strange Solve result after previous bad input

• To: mathgroup at smc.vnet.net
• Subject: [mg100937] Strange Solve result after previous bad input
• From: Alain Cochard <alain at geophysik.uni-muenchen.de>
• Date: Thu, 18 Jun 2009 06:19:12 -0400 (EDT)
• Organization: Universites Paris VI/Paris VII - France

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?  

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

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.)

Thanks
AC

• Follow-Ups:
  • Re: Strange Solve result after previous bad input
    • From: DrMajorBob <btreat1@austin.rr.com>
  • Re: Strange Solve result after previous bad input
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
  • Re: Strange Solve result after previous bad input
    • From: George Woodrow III <georgevw3@mac.com>