function dx = predprey(t,x)
%
%  The predator-prey problem. 
%

a = 1; b = 0.002; c = 0.0006; d = 0.5;

dx = zeros(size(x));
dx(1) = a*x(1)-b*x(1)*x(2);
dx(2) = c*x(1)*x(2) - d*x(2);


