%
figure(1)
hold off
[x,y] = meshgrid(-2:.1:2,-2:.1:2);
% node
 vx=x; vy=2*y;
 quiver(x,y,vx,vy)
 grid minor
 title('unstable node')
 pause
 vx=-2*x; vy=x-4*y;
 quiver(x,y,vx,vy)
 grid minor
 title('stable node')
 pause
%saddle point 
vx=2*x; vy=-y;
 quiver(x,y,vx,vy)
 grid minor
 title('saddle point')
 pause
 % center
 vx=+2*y; vy=-2*x;
 quiver(x,y,vx,vy)
 grid minor
 title('center')
 pause
 % spirals
 vx=-2*x+y; vy=-x-y;
 quiver(x,y,vx,vy)
 grid minor
 title('stable spiral')
 
 
 pause
 vx=2*x-y; vy=x+y;
 quiver(x,y,vx,vy)
 grid minor
 title('unstable spiral')