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

• To: mathgroup at smc.vnet.net
• Subject: [mg105018] How to instruct Math to take a certain (e.g. real) type of results
• From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
• Date: Wed, 18 Nov 2009 06:57:41 -0500 (EST)

```Dear Community,

I came to a problem, that I cannot check the solution y=0 of equation

In:= eq = (-8 + y)^(1/3) + (8 + y)^(1/3) == y^(1/3);
eq /. y -> 0

Out= False

just because the expression

In:= eq[] /. y -> 0

Out= 2 + 2 (-1)^(1/3)

Mathematica does not interpret as zero. And if I ask it to give a
In:= 2 + 2 (-1.)^(1/3)

Out= 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.
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```

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