%
% phase portrait of predator prey system
% as contourplots of phi(x,y)
%
x=linspace(0,3);
y=linspace(0,3);
[X,Y]=meshgrid(x,y);
%
a=1;
b=2;
c=1;
d=1;
phi=-c*log(X)-a*log(Y)+d*X+b*Y;
contour(X,Y,phi)
