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Re: How to instruct Math to take a certain (e.g. real) type of results
*To*: mathgroup at smc.vnet.net
*Subject*: [mg105050] Re: [mg105018] How to instruct Math to take a certain (e.g. real) type of results
*From*: "David Park" <djmpark at comcast.net>
*Date*: Thu, 19 Nov 2009 05:23:08 -0500 (EST)
*References*: <19419730.1258547128749.JavaMail.root@n11>
If you are willing to permute the Mathematica multi-valued solutions and
match them up for the various terms, then you can verify the z == 0
solution.
w1^3 == -8 + z;
sols1 = Solve[%, w1]
w2^3 == 8 + z;
sols2 = RotateLeft[Solve[%, w2]]
w3^3 == z;
sols3 = Solve[%, w3]
sols1 /. z -> 0
sols2 /. z -> 0
sols3 /. z -> 0
(w1 /. sols1) + (w2 /. sols2) == (w3 /. sols3) /. z -> 0
True
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/
From: Alexei Boulbitch [mailto:Alexei.Boulbitch at iee.lu]
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
numerical answer
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
Phone: +352 2454 2566
Fax: +352 2454 3566
Website: www.iee.lu
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