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MathGroup Archive 2004

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RE: Re: List of solution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47005] RE: [mg46990] Re: List of solution
  • From: "Florian Jaccard" <florian.jaccard at eiaj.ch>
  • Date: Sat, 20 Mar 2004 03:50:17 -0500 (EST)
  • Reply-to: <florian.jaccard at eiaj.ch>
  • Sender: owner-wri-mathgroup at wolfram.com

It is true...

But the Select[%, FreeQ[#, I] &] method doesn't do it better...

But you can use :

Reduce[x^3 - 10x + 1 == 0,x,Reals]

This shoes that the 3 solutions are Real...

So you can obtain the exact solutions like this :

Simplify[ComplexExpand //@ Solve[x^3 - 10x + 1 == 0, x]]

And if you want a automatic "real solve" function, you can for example make
this :


mySolveReal[equ_, var_] :=
  DeleteCases[Simplify[
    ComplexExpand //@ Solve[equ,
      var]], {var -> xx_} /;
    Im[xx] != 0]

and try on a few examples... It works fine on your example !


mySolveReal[x^3-10x+1==0,x]

mySolveReal[x^2-x-10==0,x]

mySolveReal[x^2-x+10==0,x]

Meilleures salutations

Florian Jaccard


-----Message d'origine-----
De : astanoff [mailto:astanoff at yahoo.fr]
Envoyé : ven., 19. mars 2004 07:36
À : mathgroup at smc.vnet.net
Objet : [mg46990] Re: List of solution


Florian Jaccard wrote:

> Hello !

> C'est très simple :

> In[4]:=
> liste = {{x -> 0, y -> 0}, {x -> 1, y -> 0},
>    {x -> I, y -> 1 + 2*I}}

> Out[4]=
> {{x -> 0, y -> 0}, {x -> 1, y -> 0},
>   {x -> I, y -> 1 + 2*I}}

> In[5]:=
> DeleteCases[liste, {x -> xx_, y -> yy_} /;
>    Im[xx] != 0 || Im[yy] != 0]

> Out[5]=
> {{x -> 0, y -> 0}, {x -> 1, y -> 0}}


The case with 3 real roots Solve[x^3 - 10x + 1 == 0] doesn't seem to work
fine...


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