RE: Real valued parameters/variables
- To: mathgroup at smc.vnet.net
- Subject: [mg34104] RE: [mg34082] Real valued parameters/variables
- From: "Florian Jaccard" <jaccardf at eicn.ch>
- Date: Sat, 4 May 2002 04:28:22 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
One way is to load the package Miscellaneous`RealOnly`
If you load it, you can for example plot y = x^(1/3) even for x<0, wich
would not be as comfortable elsewere.
If you solve an equation, the complex solutions are encountered.
For example :
In[2]:=
<<Miscellaneous`RealOnly`
In[3]:=
Solve[x^3==-1,x]
>From In[3]:=
Nonreal::warning: Nonreal number encountered.
Out[3]=
{{x -> -1},{x -> Nonreal},{x -> Nonreal}}
The problem is that it is not very practical if complex and real variables
have to live in the same notebook.
Some weeks ago, somebody presented the swichable realonly package :
http://www.mathsource.com/Content/Enhancements/MathFunctions/0211-396
It can help avoid the problem related.
If you prefere not to load the package but want to delete the complex
solutions from a list of solutions, you can use for example a fonction like
the following:
In[1]:=
realSolutions[règles_]:=DeleteCases[x/.règles,a_/;Im[a]!= 0]
In[2]:=
realSolutions[Solve[x^3==-1,x]]
Out[2]=
{-1}
To tell Mathematica that something is real, you can proceed like this :
If a parameter, lets say a, has to be real positive, you can use :
Simplify[... , a>0]
For example :
In[6]:=
Simplify[Integrate[Sqrt[(a^2 - x^2)],{x,-a,a}],a > 0]
Out[6]=
a^2*Pi/2
Or, if a has to be real :
In[9]:=
Simplify[Integrate[1/(x^a),{x,1,Infinity}],Element[a,Reals]]
My response is not complete, but I hope it can already help a bit...
Meilleures salutations
Florian Jaccard
EICN-HES
e-mail : jaccardf at eicn.ch
-----Message d'origine-----
De : Jack [mailto:sosolala at hotmail.com]
Envoyé : jeu., 2. mai 2002 08:49
À : mathgroup at smc.vnet.net
Objet : [mg34082] Real valued parameters/variables
Hello
I would like to know if there is a possibility to tell Mathematica,
that I am only interested in real valued solutions, respectively that
some parameters and variables can only real valued.
It would be comfortable if one could give this information at the
beginning of a Mathematica notebook and at each command.
Thanks for your help
Jack