From help-octave-request  Thu Jun 24 15:20:36 1993
Subject: lsode
From: siglun at volterra dot teorekol dot lu dot se (Sigfrid Lundberg)
To: help-octave
Date: Thu, 24 Jun 93 22:18 MET DST


I'm a newbie octave user. Everything seem to work as expected, but there
is one I don't understand. I have written a tiny function:

	function xdot = volterra ( x , t) 

	#
	#  Integrate lotka-volterra
	#
	  global x xdot;

	  r = 0.25;
	  k = 1.4;
	  a=1.5;
	  b=0.16;
	  c=0.9;
	  d=0.8;


	  xdot (1) = r*x (1) * (1 - x (1)/k) - a*x (1)*x (2)/(1+b*x (1));
	  xdot (2) = c*a*x (1)*x (2)/(1+b*x (1))-d*x (2);

	endfunction

Then I start octave, and load this function:

	Octave, version 0.72.  Copyright (C) 1992, 1993, John W. Eaton.
	This is free software with ABSOLUTELY NO WARRANTY.
	For details, type `warranty'.

	octave:1> eval("volterra");
	error: `x' undefined

	octave:2> x=[1;6]
	x =

	  12
	   6

I can evaluate this function

	octave:4> volterra(x,5)
	ans =

	  -59.701
	   28.488

But, if I try to use lsode for solving this predator-prey model


	octave:18> x=[1;6]
	x =

	  1
	  6

	octave:19> lsode ('volterra',x,30,0.01)
	ans =

	  1  6

nothing interesting happens. What am I doing wrong?


Regards,

Sigfrid