For example, a sphere is closed since it has no boundary; but a disk is not closed, since it has a circle for a boundary curve.

The term is also used for higher and lower dimensional objects: a circle is a closed curve, since it has no boundary points, but a line segment is not closed since its endpoints are its boundaries.

* 11/1/94 dpvc@geom.umn.edu -- *

*The Geometry Center*