Re: To plot solutions, FindRoot as a function

• To: mathgroup at smc.vnet.net
• Subject: [mg37396] Re: To plot solutions, FindRoot as a function
• From: bghiggins at ucdavis.edu (Brian Higgins)
• Date: Sat, 26 Oct 2002 02:03:51 -0400 (EDT)
• References: <ap861r\$53e\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Veniamin,

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
routine;
(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)

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

Cheers,

Brian

Veniamin Abalmassov <V.Abalmassov at unibas.ch> wrote in message news:<ap861r\$53e\$1 at smc.vnet.net>...
> 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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