Re: NDSolve problem
- To: mathgroup at smc.vnet.net
- Subject: [mg6528] Re: [mg6511] NDSolve problem
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Mon, 31 Mar 1997 23:01:29 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Zahir Bishnani <z.bishnani at damtp.cam.ac.uk>
[mg6511] NDSolve problem
>>>>>>>>>>>
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}];
Trying NDSolve as follows just spouts out errors
NDSolve[{y'[x]==FFunc1[y[x]], y[0]==1.}, {y}, {x,0,1}]
<<<<<<<<<<<<
Zahir
The problem arises from the way that you are using FindRoot.
Trace the evaluation of FFunc1 by using a variant that prints out
the values of theta as it is evaluated.
FFunc1P[y_]:=
theta /. FindRoot[Print[theta];Cos[y*theta]==0.0, {theta,2,5}];
For example (leaving out the message)
FFunc1P[0.005]
2.
5.
2.
5.
60.6231
631.9
339.622
306.864
314.173
314.159
314.159
Try some more values for y.
The roots that you find are all over!
The graph
Plot[Cos[3 x], {x,-1,6}]
explains this.
Allan Hayes
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk