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Re: To plot solutions, FindRoot as a function

  • To: mathgroup at
  • Subject: [mg37353] Re: To plot solutions, FindRoot as a function
  • From: "Borut L" <gollum at>
  • Date: Fri, 25 Oct 2002 02:46:33 -0400 (EDT)
  • References: <ap861r$53e$>
  • Sender: owner-wri-mathgroup at

Hi Veniamin,

First, there is always a trivial solution, i.e. x = 0.

You are probably interested in the additional one, yes?. Depending on the
values of a and b, it exists in Reals or it doesn't.

I would carefully avoid continuous analysis in such problems and looked for
a discrete solution. You can always increase resolution.

Here we go.

getSol[a_,b_,x_]:=FindRoot[Evaluate[a Tanh[b x] == x],
    {x,Sqrt[3 (a b-1)/(a b^3)]}]



(* continuous :) *)

A note : You should check for the critical a and b, i.e. where the
non-trivial solution vanishes. (e.g. a*b == 1). If you are thinking where
I've got the initial value for FindRoot, think twice

Series[a Tanh[b x],{x,0,3}]//Normal

Hope that helps,


"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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