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double d science forum Guru
Joined: 03 May 2005
Posts: 640

Posted: Sat Jul 15, 2006 11:49 am Post subject:
Eliminating multiple roots in mathematica's "Solve" function



I am using mathematica to solve a set of coupled equations numerically.
But every time mathematica solves the equations, it gives me all the
roots (which are many). I only want the
real, positive roots. How do I tell mathematica to automatically
suppressed those other roots? 

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T Wake science forum Guru
Joined: 08 May 2005
Posts: 1978

Posted: Sat Jul 15, 2006 3:56 pm Post subject:
Re: Eliminating multiple roots in mathematica's "Solve" function



In <1152964191.809085.312450@m73g2000cwd.googlegroups.com>, sent to
sci.physics on Saturday 15 July 2006 12:49, markdemers15@hotmail.com
(markdemers15@hotmail.com) had a brainstorm and wrote:
Quote:  I am using mathematica to solve a set of coupled equations numerically.
But every time mathematica solves the equations, it gives me all the
roots (which are many). I only want the
real, positive roots. How do I tell mathematica to automatically
suppressed those other roots?

Dont you want to include one of your normal racist or antimathematician
diatribes here? Or are you saving it for your sock puppets?

T Wake
Usenet.es7 at gishpuppy.com 

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Math Freak science forum beginner
Joined: 11 Jul 2005
Posts: 43

Posted: Sat Jul 15, 2006 11:56 pm Post subject:
Re: Eliminating multiple roots in mathematica's "Solve" function



On 15 Jul 2006 04:49:51 0700, markdemers15@hotmail.com
wrote:
Quote:  I am using mathematica to solve a set of coupled equations numerically.
But every time mathematica solves the equations, it gives me all the
roots (which are many). I only want the
real, positive roots. How do I tell mathematica to automatically
suppressed those other roots?

I'm not posting this to help you on your question,
sorry.
Your reference to Mathematica reminded me of a rather
powerful software (well, at least in the setting of the
years it first came out) I used a long time back, but
not accessible to me anymore. Its name is Math CAD v. 2
and I only have it on those large 5"1/4 inch floppies.
It was very useful and I paid two hundred and some
dollars for it in the 80s dollars. I've wished to use
it again for some time but cannot find a free download
of it or something. Too old I guess, but quite powerful
and versatile enough for me and my purposes. Do you, or
anybody else here, happen to have an electronic copy of
it on your computer? If so, email me a copy and I'll
send you my two original 5"1/4 inch floppies so
copyright matters etc are observed. I have to keep the
user's manual though (a nice exquisitely made binder in
the style of those days... ).

"marde kheradmande honarpishe rA
omr dobAyast darin ruzgAr
tA be yeki tajrobe Amukhtan
bA degari tajrobe bordan bekAr"
 Sa'di 

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Paul Abbott science forum addict
Joined: 19 May 2005
Posts: 99

Posted: Mon Jul 17, 2006 4:01 pm Post subject:
Re: Eliminating multiple roots in mathematica's "Solve" function



In article <1152964191.809085.312450@m73g2000cwd.googlegroups.com>,
markdemers15@hotmail.com wrote:
Quote:  I am using mathematica to solve a set of coupled equations numerically.
But every time mathematica solves the equations, it gives me all the
roots (which are many). I only want the
real, positive roots. How do I tell mathematica to automatically
suppressed those other roots?

One solution is to use Reduce instead of Solve. A simple example:
entering
Reduce[{x^13 + x  1 == 0, x > 0}, x]
only the first root is real and positive:
x == Root[1 + #1 + #1^13 & , 1, 0]
Numerically, this is
N[%]
x == 0.8597566716878293
Cheers,
Paul
_______________________________________________________________________
Paul Abbott Phone: 61 8 6488 2734
School of Physics, M013 Fax: +61 8 6488 1014
The University of Western Australia (CRICOS Provider No 00126G)
AUSTRALIA http://physics.uwa.edu.au/~paul 

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double d science forum Guru
Joined: 03 May 2005
Posts: 640

Posted: Mon Jul 17, 2006 8:13 pm Post subject:
Re: Eliminating multiple roots in mathematica's "Solve" function



Thank you! Are you related to Costello?
Paul Abbott wrote:
Quote:  In article <1152964191.809085.312450@m73g2000cwd.googlegroups.com>,
markdemers15@hotmail.com wrote:
I am using mathematica to solve a set of coupled equations numerically.
But every time mathematica solves the equations, it gives me all the
roots (which are many). I only want the
real, positive roots. How do I tell mathematica to automatically
suppressed those other roots?
One solution is to use Reduce instead of Solve. A simple example:
entering
Reduce[{x^13 + x  1 == 0, x > 0}, x]
only the first root is real and positive:
x == Root[1 + #1 + #1^13 & , 1, 0]
Numerically, this is
N[%]
x == 0.8597566716878293
Cheers,
Paul
_______________________________________________________________________
Paul Abbott Phone: 61 8 6488 2734
School of Physics, M013 Fax: +61 8 6488 1014
The University of Western Australia (CRICOS Provider No 00126G)
AUSTRALIA http://physics.uwa.edu.au/~paul 


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