odd behavior of NDSolve
- To: mathgroup@smc.vnet.net
- Subject: [mg11580] odd behavior of NDSolve
- From: Selwyn Hollis <shollis@peachnet.campus.mci.net>
- Date: Tue, 17 Mar 1998 10:43:01 -0500
- Organization: CampusMCI
I've discovered a very disturbing problem with NDSolve. I'd like to know
if others can reproduce this.
Here's an example:
f[x_, y_] := 1 - Exp[-(x^2 + y^2)]
fx[x_, y_] = D[f[x, y], x]; fy[x_, y_] = D[f[x, y], y];
Clear[x0, y0, u0, v0];
diffeqs =
{x''[t] == -fx[x[t], y[t]],
y''[t] == -fy[x[t], y[t]],
x[0] == x0, x'[0] == u0, y[0] == y0, y'[0] == v0}
endTime = 12.03;
{x0, y0, u0, v0} = {4, 3, -0.7, -0.17};
soln = Flatten[NDSolve[diffeqs, {x, y}, {t, 0, endTime}]];
r[t_] = {x[t], y[t]} /. soln;
path = ParametricPlot[r[t], {t, 0, endTime},
PlotRange -> {{-4, 4}, {-4, 4}}];
You should see a curve with a slight bend to it. Now change endTime to
12.04. The bend is gone. Shouldn't we get the same curve for 0 < t <
12.03? Either I'm missing something or NDSolve has a serious problem.
(BTW, I'm running Mathematica 3.0 and Mac OS 8.1)
--
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Dr. Selwyn Hollis
Associate Professor of Mathematics
Armstrong Atlantic State University
Savannah, GA 31419 USA
<http://www.math.armstrong.edu/faculty/hollis/>
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