Re: ParametricPlot Problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg54235] Re: [mg54217] ParametricPlot Problem*From*: "David Park" <djmp at earthlink.net>*Date*: Mon, 14 Feb 2005 00:57:51 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

James, The basic problem is the NDSolve returns the solution inside two pairs of brackets {{ x -> ...}} and when you make the substitution it leaves one pair of brackets and that doesn't work for the parametrization. So select the inner solution as follows. (You didn't give us a y function so I made one up.) deqns = {x''[t] + 0.5x'[t] + x[t] + 0.1 x[t]^2 == Sin[0.1t], x[0] == 1, x'[0] == 0}; dsol = NDSolve[deqns, x, {t, 0, 400}][[1,1]] x -> InterpolatingFunction[{{0., 400.}}, <>] ParametricPlot[{Evaluate[x[t] /. dsol], Sin[2*Pi*(t/50)]}, {t, 0, 200}, PlotPoints -> 100, Frame -> True, ImageSize -> 400]; David Park djmp at earthlink.net http://home.earthlink.net/~djmp/ From: James [mailto:cannonjunk at hotmail.com] To: mathgroup at smc.vnet.net Hi, I'm trying to do a Parametric Plot of a 2nd order differential equation: 1. dee = x''[t] + 0.5 x'[t] + x[t] + 0.1 x[t]^2 == Sin[0.1t]; 2. constraints = { x[0] == 1, x'[0] == 0}; 3. sol = NDSolve[ {dee, constraints}, x[t], {t, 0, 400} ]; 4. ParametricPlot[ {Evaluate[ x[t] /. sol], y [t]}, {t, 0, 200}] but it keeps telling me "ParametricPlot::pptr: "\!\({Evaluate[x[t] /. \[InvisibleSpace]sol]}\) does \ not evaluate to a pair of real numbers at t = 8.333333333333334`*^-6." etc. This is despite the fact that it works if I do a straight "Plot" (ie, I just remove the "Parametric" part of 4.) If anyone can suggest how I might correct this problem I would appreciate it a lot since I'm quite out of ideas. Thanks, James -- Mathematica 5.0 UK