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MathGroup Archive 2004

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Re: Lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46995] Re: Lists
  • From: astanoff at yahoo.fr (astanoff)
  • Date: Fri, 19 Mar 2004 01:35:48 -0500 (EST)
  • References: <c3bgbb$7pl$1@smc.vnet.net> <c3bqun$f32$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

astanoff wrote:

> Clément wrote:

>> Hello MathGroup,

>> I'm french so i don't speak english very well...

>> I have resolved an equation. So i have a list of solutions.
>> But how can i extract only the reel solutions of this list ?

>> thanks

> --
> Clément,

Something was missing in my solution : to deal with cases such as the 
"casus irreducibilis", one has to "Chop" the small imaginary parts :

In[1]:=
realSolutions[sol_]:=#[[1]]&/@
      Select[{#,#//N//Chop}&/@sol,FreeQ[#[[2]],Complex]&]; 

In[2]:=
(* Example (InputForms inserted for readability) : *)
sol=Solve[x^3-10x+1 == 0];

In[3]:=
sol//InputForm 

Out[3]//InputForm=
{{x -> ((-9 + I*Sqrt[11919])/2)^(1/3)/3^(2/3) + 10/((3*(-9 + 
I*Sqrt[11919]))/2)^(1/3)}, 
 {x -> -((1 + I*Sqrt[3])*((-9 + I*Sqrt[11919])/2)^(1/3))/(2*3^(2/3)) - 
    (5*(1 - I*Sqrt[3]))/((3*(-9 + I*Sqrt[11919]))/2)^(1/3)}, 
 {x -> -((1 - I*Sqrt[3])*((-9 + I*Sqrt[11919])/2)^(1/3))/(2*3^(2/3)) - 
    (5*(1 + I*Sqrt[3]))/((3*(-9 + I*Sqrt[11919]))/2)^(1/3)}}

In[4]:=
realSolutions[sol]//InputForm

Out[4]//InputForm=
{{x -> ((-9 + I*Sqrt[11919])/2)^(1/3)/3^(2/3) + 10/((3*(-9 + 
I*Sqrt[11919]))/2)^(1/3)}, 
 {x -> -((1 + I*Sqrt[3])*((-9 + I*Sqrt[11919])/2)^(1/3))/(2*3^(2/3)) - 
    (5*(1 - I*Sqrt[3]))/((3*(-9 + I*Sqrt[11919]))/2)^(1/3)}, 
 {x -> -((1 - I*Sqrt[3])*((-9 + I*Sqrt[11919])/2)^(1/3))/(2*3^(2/3)) - 
    (5*(1 + I*Sqrt[3]))/((3*(-9 + I*Sqrt[11919]))/2)^(1/3)}}
--
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