The saddle surface must be three-dimensional. Where did you get the two-dimensional saddle surface? Two-dimensional or saddle surface?

A sphere is also three-dimensional. I wonder what the landlord means.

If the spherical surface x 2+y 2+z 2 =1intersects with the saddle surface z=xy, a curve is obtained.

The expression of curve can only be expressed by two equations simultaneously, and it is hard to say what curve it is.

Just like the four-leaf lotus of Guanyin. Put two opposite leaves in Bai Wan, and then the edge of the lotus is this curve.

But the projection of this curve on the xoy plane is expressed as x 2+y 2+(xy) 2 = 1,

The shape is as follows: