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MathGroup Archive 2005

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Re: problem with FindRoot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54088] Re: problem with FindRoot
  • From: "qfwfq" <qfwfq_0 at yahoo.com>
  • Date: Thu, 10 Feb 2005 02:46:27 -0500 (EST)
  • References: <cud7jj$2vm$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Thanks Mr. Wolf,

Your solution works very well. The only problem was Mathematica 5.0
does not like your assignment. It said:

function::lvset: Local variable specification spec contains expr which
is an assignment to var; only assignments to symbols are allowed.

But it has been easy to solve it.

BTW, and about your last comment
>but this won't garantee anything for your solutions.
you're right. Sometimes, the solutions Mathematica provides to the
simultaneous solution of the set of equations are not correct. This is
the reason because my initial guess to FindRoot is {24}. Do you know a
better way to solve a set of equations with lots of exponentials. I
only know an estimation of where the solutions are.

Thnaks again,

Jose



Wolf, Hartmut wrote:
> >-----Original Message-----
> >From: qfwfq [mailto:qfwfq_0 at yahoo.com]
To: mathgroup at smc.vnet.net
> >Sent: Tuesday, February 08, 2005 11:31 AM
> >To: mathgroup at smc.vnet.net
> >Subject: [mg54088]  problem with FindRoot
> >
> >Hi all!
> >
> >I have four equations (eq1=0, ..., eq4=0) with the the four
> >unknowns (y1, y2, y3, y4) in exponentials. I know the interval
> >where the solutions are, and I solve this equation set by
> >means of FindRoot by obtaining set of solutions. Then, I make
> >a selection between the obtained results.
> >
> >However, I have a dependency between variables: y1<y2<y3<y4
> >
> >
> >mR = Table[FindRoot[{eqn1 == 0, eqn2 == 0, eqn3 == 0, eqn4 ==
> >0}, {y1, Random[Real, {a, b}]}, {y2, Random[Real, {a, b}]},
> >{y3, Random[Real, {a, b}]}, {y4, Random[Real, {a, b}]}], {24}];
> >
> >My question is
> >
> >Is there any way of include the relation between y1, ..., y4
> >in the selection of them by Random. Something like {y2,
> >Random[Real, {y1, b}]}, {y3, Random[Real, {y2, b}]}, ...
> >
> >I hope I have clearly explained my problem.
> >
> >Thanks
> >
> >
>
> Would this be what you intended, ordered starting values?
>
> mR = Table[
>       With[{{s1, s2, s3, s4} = Sort[Table[Random[Real, {a, b}],
{4}]]},
>         FindRoot[{eqn1 == 0, eqn2 == 0, eqn3 == 0, eqn4 == 0}, {y1,
s1},
> {y2,
>             s2}, {y3, s3}, {y4, s4}]], {24}];
>
> but this won't garantee anything for your solutions.
> 
> --
> Hartmut Wolf


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