Topology

• When Is a Covering Map a Homeomorphism?
  A covering map is a homeomorphism exactly when every fiber has one point; the same local-sheet argument compares two covering spaces.
• What Is the Difference Between an Arbitrary and a Finite Collection?
  An arbitrary collection may have any number of members, while a finite collection has finitely many; see how this affects unions and intersections.
• What Is the Difference Between Bounded, Unbounded, Open, and Closed Sets?
  Open, closed, bounded, and unbounded describe different properties. Learn their definitions, classify common subsets of the real line, and avoid common mix-ups.