       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:= Solve[{a==x + y, b==-x + y},{x,y}]
>
> 		    a - b       a + b
>      Out= {{x -> -----, y -> -----}}
> 		      2           2
>
> Here, there is '=' instead of '==' in the 2nd eq:
>
>      In:= Solve[{a==x + y, b=-x + y},{x,y}]
>
>      Solve::eqf: -x + y is not a well-formed equation.
>
>      Out= Solve[{a == x + y, -x + y}, {x, y}]
>
> and then, subsequent properly input systems will still fail:
>
>      In:= Solve[{a==x + y, b==-x + y},{x,y}]
>
>      Solve::svars: Equations may not give solutions for all "solve" variables.
>
>      Out= {{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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