[x,y] = meshgrid(-10:0.2:10);
z1 = (x.^2-2*y.^2) + eps;
a=input('a=?');
z2=a*ones(size(x));
subplot(1,2,1);
mesh(x,y,z1);hold on;mesh(x,y,z2);
v=[-10,10,-10,10,-100,100];axis(v);grid;
hold off
r0=abs(z1-z2)<=1;
xx=r0.*x;yy=r0;zz=r0.*z2;
subplot(1,2,2);
plot3(xx(r0~=0),yy(r0~=0),zz(r0~=0),'*');
axis(v);grid;