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MathGroup Archive 2005

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Re: Re: Hold problems with FindRoot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg57126] Re: [mg57073] Re: Hold problems with FindRoot
  • From: Chris Chiasson <chris.chiasson at gmail.com>
  • Date: Mon, 16 May 2005 21:35:24 -0400 (EDT)
  • References: <428457A1.3020501@bjornerstedt.org> <200505140858.EAA09467@smc.vnet.net> <acbec1a405051410265918dd00@mail.gmail.com> <4288E5A7.90105@bjornerstedt.org>
  • Reply-to: Chris Chiasson <chris.chiasson at gmail.com>
  • Sender: owner-wri-mathgroup at wolfram.com

I don't know the answer. I think FindRoot does attempt to symbolically
find the derivative of the system with respect to the independant
variables. Without knowing exactly what goes on inside of FindRoot, I
guess that whatever error it ran into -- when you didn't have the
PatternTest -- prevented FindRoot from realizing that it needed to
switch to the "secant method" for estimating the derivative.

On 5/16/05, Jonas Björnerstedt <lists at bjornerstedt.org> wrote:
> Thanks! This solved my problem. Is this an undocumented restriction in
> how FindRoot can be used, or where can I find more documentation?
> 
> Jonas
> 
> Chris Chiasson wrote:
> 
> >Try changing the definitions of f and g, via a PatternTest, so that
> >they are only defined for numeric arguments.
> >
> >f[x_?NumericQ,y_?NumericQ]=blahblah1
> >g[x_?NumericQ,y_?NumericQ]=blahblah2
> >
> >On 5/14/05, Jonas Björnerstedt <lists at bjornerstedt.org> wrote:
> >
> >
> >>Hello again,
> >>
> >>There was a small mistake in my previous posting. It should have said:
> >>
> >>FindRoot[Evaluate[{f[x,y]==0, g[x,y]==0}],{{x,1},{y,1}}]
> >>
> >>where f and g are functions that work correctly. Trying to solve one
> >>equation for two unknowns would cause other problems than I am currently
> >>having.
> >>
> >>Jonas.
> >>
> >>Jonas Björnerstedt wrote:
> >>
> >>
> >>
> >>>Hello all,
> >>>
> >>>I have problems getting FindRoot to evaluate the equations it
> >>>evaluates, despite using Evaluate on the equations. I have a rather
> >>>complex function f[x,y] that works correctly with individual values x
> >>>and y. Evaluating the following expression does not work however.
> >>>
> >>>FindRoot[Evaluate[f[x,y]==0],{{x,1},{y,1}}]
> >>>
> >>>When evaluating, I get similar errors as when I evaluate f without
> >>>inserting values for x and y. I can also do a Plot3D on f without
> >>>using Evaluate.
> >>>
> >>>Does anybody have an idea what the problem is?
> >>>
> >>>Jonas
> >>>
> >>>
> >>>
> >>
> >>
> >
> >
> >
> >
> 
> 


-- 
Chris Chiasson
http://chrischiasson.com/
1 (810) 265-3161


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