Are Disjoint Sets in a Topological Space Always Open When Their Union is Open?

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Homework Help Overview

The discussion revolves around the properties of disjoint sets in a topological space, specifically whether a collection of disjoint sets whose union is open must each be open individually.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to clarify their question regarding disjoint sets and their openness in the context of topology.
  • Some participants question the assumptions behind the original statement and explore counterexamples, such as the trivial topology.
  • Others suggest considering specific types of topological spaces, like locally path connected spaces, to further investigate the properties of the sets in question.

Discussion Status

Participants are actively engaging with the question, providing counterexamples and seeking clarification on the definitions involved. There is a recognition of the complexity of the topic, with no explicit consensus reached yet.

Contextual Notes

There are indications of confusion regarding the original question, as well as a need to clarify the definitions of open sets and the nature of the topology being discussed.

ehrenfest
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Homework Statement


If you have a collection of disjoint open sets in a general topological space whose union is open, is it true that each of them individually must be open? Why?

EDIT: this makes absolutely no sense. here is what I meant to ask:
EDIT:If you have a collection of disjoint sets in a general topological space whose union is open, is it true that
EDIT:each of them individually must be open? Why?

Homework Equations


The Attempt at a Solution

 
Last edited:
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As stated the answer is trivially yes because they are disjoint and OPEN. Did you really mean to ask if you had a collection of disjoint set's whose union in a topological space is open is each necessarily open. In that case the answer is false consider the trivial topology on {a,b,c}. Then the collection {{a}, {b}, {c}} is a collection of disjoint sets whose union is open but none of the ndividual sets is open.
 
d_leet said:
As stated the answer is trivially yes because they are disjoint and OPEN. Did you really mean to ask if you had a collection of disjoint set's whose union in a topological space is open is each necessarily open. In that case the answer is false consider the trivial topology on {a,b,c}. Then the collection {{a}, {b}, {c}} is a collection of disjoint sets whose union is open but none of the ndividual sets is open.

What if we are in a locally path connected and path connected space?
 
What are the original sets that comprise the union? They can't be open individually if you are being asked that very question.
 
Mathdope said:
What are the original sets that comprise the union? They can't be open individually if you are being asked that very question.

Sorry. Reread the EDIT. I am foolish.
 
ehrenfest said:
Sorry. Reread the EDIT. I am foolish.

Consider any non-open set X and it's complement \bar{X}.
Their union is clearly open since it's the entire space, they're disjoint by construction, and X is non-open by hypothesis.
 

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