function [U,V]=sample_field2(X,Y)
% sample call
% phase_portrait('sample_field2',-3:0.1:3,-3:0.1:3,200,0.03,30)
  U=sin(X.^3-2*X.^2-3*X+1-Y.^2);
  V=cos(Y.^3-2*Y.^2-3*Y+1-X.^2);

