closefigure(2)syms x yf=2.*x.^4+y.^4-2.*x.^2-2.*y.^2+3;fx=diff(f,x);fy=diff(f,y);fxx=diff(f,x,2);fyy=diff(fy,y);fxy=diff(fx,y);fxfyfxxfyyfxy[a,b]=solve(fx,fy)t=fxx*fyy-fxy^2;ttmp=2^(1/2)/2;k1= subs(t,[x,y],[-tmp,0])k2=subs(t,[x,y],[tmp,0])k3=subs(t,[x,y],[-tmp,-1])k4=subs(t,[x,y],[tmp,-1])k5=subs(t,[x,y],[-tmp,1])k6=subs(t,[x,y],[tmp,1])k7=subs(t,[x,y],[0,0])k8=subs(t,[x,y],[0,-1])k9=subs(t,[x,y],[0,1]) z1=subs(f,[x,y],[-tmp,0]) z2=subs(f,[x,y],[tmp,0]) z3=subs(f,[x,y],[-tmp,-1]) z4=subs(f,[x,y],[tmp,-1])z5=subs(f,[x,y],[-tmp,1])z6=subs(f,[x,y],[tmp,1])z7=subs(f,[x,y],[0,0])z8=subs(f,[x,y],[0,-1])z9=subs(f,[x,y],[0,1])  u=-3/2:0.1:3/2;v=-3/2:0.1:3/2;[x,y]=meshgrid(u,v);z=2.*x.^4+y.^4-2.*x.^2-2.*y.^2+3;surf(x,y,z); hold on contourf(x,y,z)  plot(-tmp,0,‘*r‘) plot(tmp,0,‘*r‘) plot(-tmp,-1,‘*r‘)   plot3(-tmp,0,z1,‘*g‘)  plot3(tmp,0,z2,‘*g‘)  plot3(-tmp,-1,z3,‘*g‘)  plot3(tmp,-1,z4,‘*g‘)  plot3(-tmp,1,z5,‘*g‘)  plot3(tmp,1,z6,‘*g‘)  plot3(0,0,z7,‘*g‘)  plot3(0,-1,z8,‘*g‘)  plot3(0,1,z9,‘*g‘) % axis equal