Re: How to instruct Math to take a certain (e.g. real) type of

• To: mathgroup at smc.vnet.net
• Subject: [mg105047] Re: How to instruct Math to take a certain (e.g. real) type of
• From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
• Date: Thu, 19 Nov 2009 05:22:34 -0500 (EST)
• References: <he0oa3\$4or\$1@smc.vnet.net>

```Hi Alexei,

You may try the RealOnly package available at
http://library.wolfram.com/infocenter/MathSource/6771/ . There may be
side effects, be careful.

Cheers -- Sjoerd

On Nov 18, 2:11 pm, Alexei Boulbitch <Alexei.Boulbi... at iee.lu> wrote:
> Dear Community,
>
> I came to a problem, that I cannot check the solution y=0 of equation
>
> In[345]:= eq = (-8 + y)^(1/3) + (8 + y)^(1/3) == y^(1/3);
> eq /. y -> 0
>
> Out[346]= False
>
> just because the expression
>
> In[347]:= eq[[1]] /. y -> 0
>
> Out[347]= 2 + 2 (-1)^(1/3)
>
> Mathematica does not interpret as zero. And if I ask it to give a
> In[348]:= 2 + 2 (-1.)^(1/3)
>
> Out[348]= 3.+ 1.73205 \[ImaginaryI]
> It returns the complex root out of the three possible.
>
> My question is the following:
>
> 1) How should I instruct Mathematica to take a certain  root that I want
> of say, (-1)^(1/3)?
>
> 2) I think there is a general possibility instruct Mathematica that all
> calculations should be done on reals only. Is it right?
>
> Thank you, Alexei
>
> --
> Alexei Boulbitch, Dr., habil.
> Senior Scientist
>
> IEE S.A.
> ZAE Weiergewan
> 11, rue Edmond Reuter
> L-5326 Contern
> Luxembourg
>
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>
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>
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```

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