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MathGroup Archive 2002

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Re: To plot solutions, FindRoot as a function

  • To: mathgroup at
  • Subject: [mg37396] Re: To plot solutions, FindRoot as a function
  • From: bghiggins at (Brian Higgins)
  • Date: Sat, 26 Oct 2002 02:03:51 -0400 (EDT)
  • References: <ap861r$53e$>
  • Sender: owner-wri-mathgroup at


There are a few problems with your code that you need to address
before it will work:
(i) The output from FindRoot is in the form of a replacement rule:
x->something. And this will not work as input for the plotting
(ii) You should use Plot3D not Plot;
(iii) Your initial guess is not in the neighborhood of the root so you
always get x=0 (or its approximation)

The following changes have been made and now your code woirks

sol[a_, b_] := x /. FindRoot[a*Tanh[b*x] == x, {x, 2}]
Plot3D[sol[a, b], {a, 1, 2}, {b, 1, 2}, PlotPoints -> 40]



Veniamin Abalmassov <V.Abalmassov at> wrote in message news:<ap861r$53e$1 at>...
> Hello,
> I'd like to plot the solution of an equation which depends on two 
> parameters, e.g.
> a*Tanh[b*x] == x
> So, I'd like to see it in 3D, one axis is "a", other is "b", and the last 
> is "solution".
> I've tried naively to do it as follows:
> sol[a_,b_]:=FindRoot[a*Tanh[b*x] == x, {x, {0.01, 0.1}}]
> Plot[sol[a,b], {a, 1, 2}, {b, 1, 2}]
> And it doesn't work. Help me, please. What can I do?
> Thanks a lot,
> Veniamin

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