function [U,V]=sample_field(X,Y)
% sample call
% phase_portrait('sample_field',-2:0.1:2,-2:0.1:2,200,0.02,30)
  U=X.^3-2*X.^2-3*X+1-Y.^2;
  V=Y.^3-2*Y.^2-3*Y+1-X.^2;

