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

• To: mathgroup at smc.vnet.net
• Subject: [mg63918] Re: [mg63891] Re: Getting the value of the independent var from the dep.var using NDSolve
• From: Pratik Desai <pdesai1 at umbc.edu>
• Date: Sun, 22 Jan 2006 00:52:32 -0500 (EST)
• References: <dptd4r$be$1@smc.vnet.net><dq2rgc$n0o$1@smc.vnet.net> <200601210650.BAA11261@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Borut Levart wrote:

>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?
>
>  
>
If one is not afraid to use trignometry, one can try this
Clear[y,x]
sol1=NDSolve[{y'[x] == Cos[x],y[0] == 0}, y, {x, 0, 20},Method -> 
{EventLocator,"Event" ->Cos[x], "EventAction":> Sow[{x, y[x]}]}] // Reap
{{{y -> InterpolatingFunction[{{0., 20.}}, <>]}}, {{{1.5708,
           1.}, {4.71239, -1.}, {7.85398, 1.}, {10.9956, -1.}, {14.1372,
         1.}, {17.2788, -1.}}}}

Pratik

• References:
  • Re: Getting the value of the independent var from the dep.var using NDSolve
    • From: "Borut Levart" <BoLe79@gmail.com>