       Re: Don't understand behaviour of Solve[]

• To: mathgroup at smc.vnet.net
• Subject: [mg58587] Re: Don't understand behaviour of Solve[]
• From: dh <dh at metrohm.ch>
• Date: Sat, 9 Jul 2005 04:07:48 -0400 (EDT)
• References: <daitlm\$sus\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi Oliver,
Mathematica can not know what you consider to be a parameter and what a
variable. If you do not specify a variable Mathematica takes it for a
parameter. But then you have an overdetermined system without a generic
solution. If you want the solution only for a fraction of variables you
have to tell Mathematica to eliminate the rest by specifying a third argument.
Example:
Solve[{x + y == 3, x + 2 y == 5}, {x, y}]  gives {{x -> 1, y -> 2}}
Solve[{x + y == 3, x + 2 y == 5}, {x}] gives {} because Mathematica takes y now
as a parameter, not  a variable. You have 2 equations for 1 variable
that do not have a generic solution. A solution only exists for a
special value of y.
If you only want a solution for x, you must tell Mathematica by the third
argument:
Solve[{x + y == 3, x + 2 y == 5}, {x}, y]  gives {{x -> 1}}

sincerely, Daniel

Oliver Friedrich wrote:
> Hallo,
>
> I have a problem understanding the general behaviour of the Solve[]
> function.
>
> I have a set of equations with the variables a,b,c,d and k.
>
> Evaluating Solve[set,k] or Solve[set,{a,b}] returns with {}, whereas Solve
> [set,{k,b,c,d}] returns solutions for k,b,c and d.
>
> My question: Why does Solve returns no solution when searching only for k
> for example and why is there a solution for several variables? I thought
> that giving a list of variables just means that I want to search for all of
> them. But the procedure doesn't seem to be independant from one variable to
> another.
>
> What kind of information contains my list of variables except my wish to
> solve for these?
>
> Many thanks
>
> Oliver friedrich
>

```

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