Clopen set


In topology, a clopen set (a portmanteau of closed-open set) in a topological space is a set which is both open and closed. That this is possible may seem counter-intuitive, as the common meanings of open and closed are antonyms, but their mathematical definitions are not mutually exclusive. A set is closed if its complement is open, which leaves the possibility of an open set whose complement is also open, making both sets both open and closed, and therefore clopen. As described by topologist James Munkres, unlike a door, "a set can be open, or closed, or both, or neither!"[1] emphasizing that the meaning of "open"/"closed" for doors is unrelated to their meaning for sets (and so the open/closed door dichotomy does not transfer to open/closed sets). This contrast to doors gave the class of topological spaces known as "door spaces" their name.

Examples edit

In any topological space   the empty set and the whole space   are both clopen.[2][3]

Now consider the space   which consists of the union of the two open intervals   and   of   The topology on   is inherited as the subspace topology from the ordinary topology on the real line   In   the set   is clopen, as is the set   This is a quite typical example: whenever a space is made up of a finite number of disjoint connected components in this way, the components will be clopen.

Now let   be an infinite set under the discrete metric – that is, two points   have distance 1 if they're not the same point, and 0 otherwise. Under the resulting metric space, any singleton set is open; hence any set, being the union of single points, is open. Since any set is open, the complement of any set is open too, and therefore any set is closed. So, all sets in this metric space are clopen.

As a less trivial example, consider the space   of all rational numbers with their ordinary topology, and the set   of all positive rational numbers whose square is bigger than 2. Using the fact that   is not in   one can show quite easily that   is a clopen subset of   (  is not a clopen subset of the real line  ; it is neither open nor closed in  )

Properties edit

  • A topological space   is connected if and only if the only clopen sets are the empty set and   itself.
  • A set is clopen if and only if its boundary is empty.[4]
  • Any clopen set is a union of (possibly infinitely many) connected components.
  • If all connected components of   are open (for instance, if   has only finitely many components, or if   is locally connected), then a set is clopen in   if and only if it is a union of connected components.
  • A topological space   is discrete if and only if all of its subsets are clopen.
  • Using the union and intersection as operations, the clopen subsets of a given topological space   form a Boolean algebra. Every Boolean algebra can be obtained in this way from a suitable topological space: see Stone's representation theorem for Boolean algebras.

See also edit

Notes edit

  1. ^ Munkres 2000, p. 91.
  2. ^ Bartle, Robert G.; Sherbert, Donald R. (1992) [1982]. Introduction to Real Analysis (2nd ed.). John Wiley & Sons, Inc. p. 348. (regarding the real numbers and the empty set in R)
  3. ^ Hocking, John G.; Young, Gail S. (1961). Topology. NY: Dover Publications, Inc. p. 56. (regarding topological spaces)
  4. ^ Mendelson, Bert (1990) [1975]. Introduction to Topology (Third ed.). Dover. p. 87. ISBN 0-486-66352-3. Let   be a subset of a topological space. Prove that   if and only if   is open and closed. (Given as Exercise 7)

References edit