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Re: Getting the value of the independent var from the dep.var using NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg63937] Re: Getting the value of the independent var from the dep.var using NDSolve
  • From: "Jens-Peer Kuska" <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 24 Jan 2006 01:30:23 -0500 (EST)
  • Organization: Uni Leipzig
  • References: <dptd4r$be$1@smc.vnet.net><dq2rgc$n0o$1@smc.vnet.net> <dqsohb$bui$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

a) the most initial value solvers have the desired accuracy only
   on the points that are calculated explicit
b) you must interpolate between two succesive 
values one for y[x]-1< 0 and
   y[x]-1>0  and try to locate the zero-crossing 
but typical the accuracy would be low

Regards
  Jens

"Borut Levart" <BoLe79 at gmail.com> schrieb im 
Newsbeitrag news:dqsohb$bui$1 at smc.vnet.net...
| Commenting on the last comment, I would like to 
say:
| - what a nice differential-equation platform 
NDSolve is,
| - and that the above example fails to find the 
peaks of Sin[x]:
|
| NDSolve[
|    {
|      y'[x] == Cos[x],
|      y[0] == 0}, y, {x, 0, 20},
|    Method -> {
|        "EventLocator",
|        "Event" -> y[x] - 1,
|        "EventAction" :> Sow[x]
|        }
|    ] // Reap
|
| Why is that?
| I guess the solver steps over the event, which 
in turn is not noticed.
| But could any algorithm not miss that?
| 



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