I plotted $z = x^2+y^2$ and got this shape. Is there a name for this shape?
How about $z \ge x^2 + y^2$ ?
Best Answer
The surface is a "circular paraboloid" (a special case of the "elliptic paraboloid"). Slices by planes perpendicular to the $z$ axis are (concentric) circles, while slices by planes parallel to the $z$ axis are (identical) parabolas.
It's an example of a surface of revolution, in this case obtained by revolving the basic parabola shape (think $y = x^2$) about its axis.
The filled counterpart would simply be a "solid circular paraboloid".
I don't believe such a shape has a single-word name like "sphere" or "cube" associated to it. However, in mathematics we can characterize such a shape as "the wedge of two spheres" and write it symbolically as
$$S^2\vee S^2$$
$S^2$ denotes the "$2$-sphere" (Wikipedia link). Note that in mathematics this refers specifically to the "hollow" sphere; if you meant in your question to refer to a "filled-in" sphere, then the correct mathematical word is "$3$-ball" (Wikipedia link) and you would write $B^3\vee B^3$ instead.
The $\vee$ in the middle is the "wedge sum" operation (Wikipedia link). It takes two "shapes" (i.e., topological spaces) and glues them together at a single point. But of course, taking two spheres and attaching them at a single point produces the same shape as starting with one sphere and pinching it in the middle.
Best Answer
The surface is a "circular paraboloid" (a special case of the "elliptic paraboloid"). Slices by planes perpendicular to the $z$ axis are (concentric) circles, while slices by planes parallel to the $z$ axis are (identical) parabolas.
It's an example of a surface of revolution, in this case obtained by revolving the basic parabola shape (think $y = x^2$) about its axis.
The filled counterpart would simply be a "solid circular paraboloid".