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Re: FindRoot s all

  • To: mathgroup at
  • Subject: [mg49422] Re: FindRoot s all
  • From: Bill Rowe <readnewsciv at>
  • Date: Mon, 19 Jul 2004 07:46:12 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

On 7/18/04 at 8:09 AM, mathma18 at (Narasimham G.L.) wrote:

>y[u_,c_]=c^u-u^c-1 ; Plot[y[x,2],{x,-5,5}];
>FindRoot[y[x,2]==0,{x,-5,5}]; " Solution settles to a value outside
>the interval of roots of its derivative (humps and valleys),it may
>be the problem of Newton-Raphson diverging tangets.How to capture
>all roots [in this case {x -> 0, 1}] ? It should be valid for all

You didn't say what version of Mathematica you were using.  I get


"5.0 for Mac OS X (June 10, 2003)"

y[u_, c_] = c^u - u^c - 1; 
FindRoot[y[x, 2] == 0, {x, -5, 5}]

{x -> 4.257461914447932}

with no error messages

To find other roots with FindRoot, simply choose different starting points, i.e.

FindRoot[y[x, 2] == 0, {x, 0.5}]

{x -> 1.0000000000000004}

There is a package written by Ted Ersek availble on the Wolfram web site that is very useful for this type of problem. With it, you can do

<< "Enhancements`RootSearch`"; 
RootSearch[y[x, 2] == 0, {x, -5, 5}]

{{x -> 0.}, {x -> 1.}, {x -> 4.257461914447932}}

and get all the roots at once.
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