MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Help with Root function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg79439] Re: Help with Root function
  • From: jeremito <jeremit0 at gmail.com>
  • Date: Fri, 27 Jul 2007 05:41:13 -0400 (EDT)
  • References: <f89q55$5nu$1@smc.vnet.net>

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




  • Prev by Date: Re: Cyclic permutations
  • Next by Date: DynamicModule and InheritScope
  • Previous by thread: Re: Help with Root function
  • Next by thread: Re: Re: Help with Root function