Re: Solve real roots only ?

• To: mathgroup at smc.vnet.net
• Subject: [mg43600] Re: [mg43585] Solve real roots only ?
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Tue, 23 Sep 2003 04:01:45 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```RealOnly is not for this purpose  (in fact it's a fairly useless
package for people who do not wish to see complex number and are
satisfied with simply not getting any answer when these are needed).
Solve can't find real roots only. In any case, since your equation
involves a parameter it does not make sense to speak of a "real root".
Whether a root is real or not depends on the parameter, thus a I is of
course real if a is purely imaginary. And by the way, there are real
form without using I.

Andrzej Kozlowski
Yokohama, Japan
http://www.mimuw.edu.pl/~akoz/
http://platon.c.u-tokyo.ac.jp/andrzej/

On Monday, September 22, 2003, at 04:14 PM, Pavel Pokorny wrote:

>
>   Dear Mathematica friends,
>
> how can I tell Solve to find real roots only?
> I tried RealOnly but it makes this little program to crash!
>
>    Needs["Miscellaneous`RealOnly`"];
>    f = a - (1-x) Sqrt[20 x - 20000 y];
>    g = (a(1-y)- 1000 y (1-x))/(10-x);
>    s = Solve [ f==0 && g==0, {x,y}];
>    xs = x /. s;
>    xx = Series [xs, {a,0,2}];
>
> Is it my fault?
>
> Thanks
>
> --
> Pavel Pokorny
> Math Dept, Prague Institute of Chemical Technology
> http://www.vscht.cz/mat/Pavel.Pokorny
>
>
>

```

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