A cycle is *trivial* if it is the boundary curve of some region
of the surface, and is *non-trivial* if
it does not bound a region. For example, on a sphere, every curve
bounds a region, so all cycles are trivial, but on a
torus, a curve that goes around the hole does
not bound a region, and so is non-trivial.

* 8/12/94 dpvc@geom.umn.edu -- *

*The Geometry Center*