Compactness of [0, 1] in a specific topology

  • Thread starter Thread starter radou
  • Start date Start date
  • Tags Tags
    Specific Topology
Click For Summary
SUMMARY

The discussion centers on the compactness of the interval [0, 1] within the topology on R where the complement of open sets is either countable or all of R. It is established that [0, 1] is compact when covered by sets whose complements are finite, as a finite subcover can be constructed. However, the case where the complement is countably infinite presents a challenge, illustrated by a cover consisting of sets like U1 = R\{1/n} for positive integers n, which does not yield a finite subcover despite covering [0, 1]. The definitions and lemmas from Munkres are referenced to clarify the conditions for compactness.

PREREQUISITES
  • Understanding of topological spaces and their properties.
  • Familiarity with the concept of compactness in topology.
  • Knowledge of open sets and their complements in a given topology.
  • Ability to apply definitions and lemmas from topology textbooks, specifically Munkres.
NEXT STEPS
  • Study the concept of compactness in different topological spaces.
  • Explore the implications of countable versus finite complements in topology.
  • Learn about the various types of covers and subcovers in topology.
  • Review Munkres' definitions and lemmas related to compactness for deeper understanding.
USEFUL FOR

Mathematics students, particularly those studying topology, as well as educators and researchers interested in the properties of compact spaces and their applications in analysis.

radou
Homework Helper
Messages
3,149
Reaction score
8

Homework Statement



Let R have the topology consisting of all the sets A such that R\A is either countable or all of R. Is [0, 1] a compact subspace in this topology?

The Attempt at a Solution



If U covers R, if it consists of sets of type A such that R\A is finite, then [0, 1] is compact, since, if we take any set A from U, and if A doesn't cover [0, 1], then there's a finite number of points missing to cover [0, 1], let's say x1, ..., xn, and for each xi we can take a set Ai from U containing xi until we cover [0, 1], so we have a finite subcover (consisting of n+1 sets of U).

The interesting case is where R\A is countably infinite.

Let U be a cover for [0, 1] consisting of the following sets:

U1 = R\{1/n : n is a positive integer}
U2 = R\{1/(n+1) : n is a positive integer}
U3 = R\{1/(n+2) : n is a positive integer}, etc.

The family U covers [0, 1], right? But it doesn't have a finite subcover. Does this work?

Edit: by the way, Munkres says:

Definition. If Y is a subspace of X, a collection A of subsets of X is said to cover Y if the union of its elements contains Y.

Lemma 26.1. Let Y be a subspace of X, Then Y is compact if and only if every covering of Y by sets open in X contains a finite subcollection covering Y.

So I considered sets open in R, in the given topology. I hope there's no "subspace issue" I ignored here unwillingly.
 

Similar threads

  • · Replies 12 ·
Replies
12
Views
3K
Replies
12
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 2 ·
Replies
2
Views
557
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
Replies
5
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
Replies
3
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K