Re: Help with find root needed

*To*: mathgroup at smc.vnet.net*Subject*: [mg88980] Re: Help with find root needed*From*: dh <dh at metrohm.ch>*Date*: Thu, 22 May 2008 06:17:21 -0400 (EDT)*References*: <g11r1s$a95$1@smc.vnet.net>

Hi Aaron, the slope at strating point 10 is too large and the algorithm overshoots. Choose a better starting point, e.g. Max[1,gamma] Further, if the root is near 1 you must allow Mathematica to search around 1. Simply change {z,10,1,10} to {z,10,0.5,10}. Here is the correction: K[h_,\[Gamma]_]:=z/.FindRoot[f[z,h,\[Gamma]]==0,{z,Max[1,\[Gamma]],0.5,10}]; hope this helps, Daniel Aaron Fude wrote: > Hi, > > In what follows, I'm looking for the largest root of a function, that > is also a function of two parameters. The Manipulate command shows me > that the root is consistently there (albeit near a singularity which > might be what's causing problems). However, FindRoot doesn't find it > as the plot shows. I get one error, but my guess is that it's > referring to one particular point on the plot. > > f[z_, h_, \[Gamma]_] := > Tanh[h*z] + (\[Gamma]^2 + 1 - z^2)/(\[Gamma]^2 - 1 - z^2) * > z/\[Gamma]; > Manipulate[ > Plot[f[z, h, \[Gamma]], {z, 0, 10}], {h, .1, 10}, {\[Gamma], .1, > 10}] > K[h_, \[Gamma]_] := > z /. FindRoot[f[z, h, \[Gamma]] == 0, {z, 10, 1, 10}]; > Plot3D[K[h, \[Gamma]], {h, .1, 10}, {\[Gamma], .1, 10}] > > Many thanks in advance! > > Aaron > -- Daniel Huber Metrohm Ltd. Oberdorfstr. 68 CH-9100 Herisau Tel. +41 71 353 8585, Fax +41 71 353 8907 E-Mail:<mailto:dh at metrohm.com> Internet:<http://www.metrohm.com>