Re: NDSolve problem

• To: mathgroup at smc.vnet.net
• Subject: [mg6531] Re: [mg6511] NDSolve problem
• From: Richard Finley <trfin at fiona.umsmed.edu>
• Date: Mon, 31 Mar 1997 23:01:32 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

It would seem that there is no real reason to use the original FFunc1[y]
since you can easily just use the root: theta = n Pi/y for the n of your
choice.  If you use this instead of FFunc1[y] in NDSolve there shouldn't be
a problem.

At 02:43 AM 3/27/97 -0500, you wrote:
>Hi,
>
>I am trying to solve a scalar ODE numerically but since evaluating the
>derivative involves a FindRoot operation, I get error messages.
>
>Example:
>
>If the function that gives the derivative is
>
>FFunc1[y_]:= theta /. FindRoot[Cos[y*theta]==0.0, {theta,2,5}];
>
>( which is pretty much equivalent to FFunc2[y_]:= N[ArcCos[0]/y]; )
>
>Trying NDSolve as follows just spouts out errors
>
>NDSolve[{y'[x]==FFunc1[y[x]], y[0]==1.}, {y}, {x,0,1}]
>
>FindRoot::precw: Warning: The precision of the argument function
>        (Cos[y[x] theta] - 0.) is less than WorkingPrecision (16).
>FindRoot::frnum: Function {Cos[2. y[x]]} is not a length 1 list
>        of numbers at theta = 2..
>ReplaceAll::reps: {FindRoot[Cos[y[x] theta] == 0., {theta, 2, 5}]}
>        is neither a list of replacement rules nor a valid dispatch
>        table, and so cannot be used for replacing.
>
>Does anyone know how I could get around this problem?
>
>Cheers,
>
>Zahir
>
>

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