Re: phase portrait

[Peter Gawthrop]

Hi Arnau,

 try adapting this script. I guess th is x and dth is y in your
 notation. You also need to write fun.m to give dx in terms of x=[th;dth] -
 equivalent to your f&g functions rolled into one.

 Peter.


  dt = 0.1;

  TH = [-1:0.1:1];
  DTH = 5*[-1:0.1:1];

  N = length(TH);
  M = length(DTH);

  i=0;
  THETA = zeros(N*M,1);
  DTHETA = zeros(N*M,1);
  D_THETA = zeros(N*M,1);
  D_DTHETA = zeros(N*M,1);

  for th=TH
    for dth=DTH
      i++;
      x = [th;dth];
      dx = fun(x);
      D_DTHETA(i) =  dx(1);
      D_THETA(i)  =  dx(2);
      THETA(i) = th;
      DTHETA(i) = dth;
    endfor
  endfor


  M = [THETA DTHETA D_THETA*dt D_DTHETA*dt];

  gset nokey
  axis([min(TH), max(TH), min(DTH), max(DTH)]);
  ylabel("$\\dot{\\theta}$")
  xlabel("$\\theta$")
  grid on;
  gplot M with vector


From: Arnau Mir Torres <address@hidden>
Subject: phase portrait
Date: Mon, 21 Feb 2005 15:36:45 +0100

> Hello.
> I want to plot a phase portrait for a system of differential equations 
> of the form:
> 
> xdot= f(t,x,y)
> ydot= g(t,x,y)
> 
> where t is the independent variable.
> 
> I want to plot in the x-y plane coordinates the phase portrait of the 
> previous differential equation.
> 
> Somebody can tells me if there exists a package or a script in octave 
> that does it?
> 
> Thanks in advance,
> Arnau.
> 
> 
> -- 
> Arnau Mir Torres
> Director Departament Matemàtiques i Informàtica.
> Universitat de les Illes Balearss.
> Edifici A. Turmeda, Campus de la UIB.
> Crta. de Valldemossa, km. 7,5.
> 07122 Palma de Mallorca.
> Baleares.
> Spain.
> Phone: +34 971 172987
> Fax: +34 971 173003
> e-mail: address@hidden
> web: http://dmi.uib.es/~arnau
> 
> 
> 
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Octave is freely available under the terms of the GNU GPL.

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