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Re: FindRoot with algebraic expressions

  • To: mathgroup at
  • Subject: [mg122847] Re: FindRoot with algebraic expressions
  • From: Bill Rowe <readnews at>
  • Date: Sat, 12 Nov 2011 07:34:34 -0500 (EST)
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On 11/11/11 at 4:54 AM, dashiell at (Mirko) wrote:

>I was wondering how FindRoot works when trying to use a algebraic
>initial point. I.e.:

>FindRoot[h[theta]==0,{theta[m], ???}] where h is function of theta,

Are you aware FindRoot is designed to return numeric answers not
algebraic answers? Also, in Mathematica, theta[m] is interpreted
as the *function* theta evaluated at m, i.e., not a variable.
That is

FindRoot[h[theta]==0 {theta[m]

is asking Mathematica to find the root of the function h with
respect to the function theta, probably not what you mean.

If theta were defined, then it should be possible to do

FindRoot[h[theta] == 0, {m, m0}]

assuming theta was defined using Set (not SetDelayed) and m0 is
given a numeric value. The above would return a value for m that
makes h[theta] 0.

>for example:
>h[theta_]=1/(1-m)*theta[m]  (notice that this is for a given m).

What are you expecting here? Until theta is defined, the only
possible solution is the trivial solution, i.e., theta[m] is 0

>What should I use as a starting point?

Someone here likely can help you if you post details of the
problem you are actually trying to solve. If you are trying to
find a symbolic root to an expression, you need something other
than FindRoot which is a numeric function.

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