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Re: Re: Help with Root function

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  • Subject: [mg79532] Re: [mg79439] Re: Help with Root function
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sat, 28 Jul 2007 05:43:26 -0400 (EDT)
  • Reply-to: hanlonr at cox.net

The answer you got contained Root. 

Look up Root in the help browser and refer to SEE ALSO in that entry. It refers you to several entries including ToRadicals.


Bob Hanlon

---- jeremito <jeremit0 at gmail.com> wrote: 
> Thank you all who offered the solution to this problem.  The answer
> is:
> 
> Eigenvalues[B]//ToRadicals
> 
> How simple, if you know how to do it.  My follow-up question is:
> 
> How could I (or anyone) have found that on their own?  I searched in
> the documentation, but couldn't find it until I knew what to search
> for.
> 
> Thanks again,
> Jeremy
> 
> On Jul 26, 5:39 am, jeremito <jerem... at gmail.com> wrote:
> > I am trying to find the eigenvalues of a 3x3 matrix with non-numeric
> > elements.  This requires finding the roots of cubic polynomials.
> > Mathematica can do this, but I know how to interpret its output.  For
> > example
> >
> > In[1]:= B = {{a, 1, 1}, {1, b, 1}, {1, 1, c}}
> >
> > Out[1]= {{a, 1, 1}, {1, b, 1}, {1, 1, c}}
> >
> > In[2]:= Eigenvalues[B]
> >
> > Out[2]= {Root[-2 + a + b + c -
> >     a b c + (-3 + a b + a c + b c) #1 + (-a - b - c) #1^2 + #1^3 &,
> >   1], Root[-2 + a + b + c -
> >     a b c + (-3 + a b + a c + b c) #1 + (-a - b - c) #1^2 + #1^3 &,
> >   2], Root[-2 + a + b + c -
> >     a b c + (-3 + a b + a c + b c) #1 + (-a - b - c) #1^2 + #1^3 &,
> >   3]}
> >
> > How can I get Mathematica to give me the full answer?  I know it is
> > long and ugly, but at least I can do something with it.  I can't do
> > anything with what it gives me now.  Does that make sense?
> > Thanks,
> > Jeremy
> 
> 
> 



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