>> [x,y]=meshgrid([-5:0.5:5],[-5:0.5:5]);
>> z1=sqrt(x.^2+y.^2-4);
>> z2=z1.*((x.^2+y.^2-4)>=0);
>> surf(x,y,z2)
>> z3=-z2;
>> hold on
>> surf(x,y,z3)