lsode

[Sigfrid Lundberg]

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