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Re: Problems with NSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg88181] Re: Problems with NSolve
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 27 Apr 2008 04:59:46 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <fuumfl$8dr$1@smc.vnet.net>

kgwagh at gmail.com wrote:
> Hi all
> 
> I am trying to find the number of eigenvalue crossings for a matrix as
> a function of the parameter 'u', on which the elements of the
> (symmetric) matrix depend on linearly. The matrix elements also
> involve randomly chosen constants. The plan is to find the
> distribution of the crossings of these type of matrices as I scan over
> the random numbers.
> 
> So far I have been using the following :
> 
> NSolve[{chpoly[u, dim, \[Alpha], \[Gamma], \[Epsilon], x] == 0,
>   D[chpoly[u, dim, \[Alpha], \[Gamma], \[Epsilon], x], x] == 0}, {x,
>   u}, WorkingPrecision -> prec]
> 
> where chpoly is the characteristic polynomial of the matrix (with the
> eigenvalue variable being x) and \alpha, \gamma and \epsilon are
> constant parameter arrays of random numbers. For a double (or higher)
> degeneracy of the eigenvalues both the characteristic equation and its
> derivative should be zero. This approach has worked successfully only
> upto 12*12 matrices (where one such computation takes 40 secs on my
> laptop). For 13*13 my laptop takes 4000 sec. This seems to be somewhat
> surprising, because these polynomials are of the order 'n'  (where n
> is dimension of the matrix) in both u and x - and  n=13 does not sound
> very computationally unreasonable.  So I was wondering if there was a
> faster approach I could take.
> 
> Also, the problem essentially entails me to know the number (not the
> values) of the real solutions to this system of polynomials.
> CountRoots seemed ideal but it does not work for more than one
> equation. So is there any alternative along this route?
> 
> Any other alternatives are also welcome.
> Thanks,
> Kshitij.
> 

Could you post, please, a *fully working* example of what you are doing? 
It is really hard to optimize fuzzy functions (fuzzy not as in fuzzy 
logic but as in fuzzy definitions or concepts :-)

Regards,
-- Jean-Marc



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