Intersection of Connected Sets: True or False?

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Discussion Overview

The discussion centers around the question of whether the intersection of two connected sets is itself connected. Participants explore definitions and counterexamples related to connectedness in topology, with a focus on the implications of intersections being empty or non-empty.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant questions the truth of the statement, suggesting that if the intersection is empty, the sets would not be connected.
  • Another participant asserts that there are simple counterexamples to the claim.
  • A different participant argues that the empty set is connected by definition, as it has no non-empty subsets to partition it.
  • One participant provides an example with singleton sets, questioning if their empty intersection could serve as a counterexample given the empty set's connectedness.
  • Another participant emphasizes the need for a non-empty set to demonstrate disconnection.
  • A later reply suggests visualizing two connected sets that intersect in a way that creates disconnected regions, using the example of connected subsets of a circle.

Areas of Agreement / Disagreement

Participants express differing views on the nature of connectedness and the implications of empty intersections, indicating that multiple competing views remain without a consensus.

Contextual Notes

Participants reference definitions of connectedness and the properties of the empty set, but there are unresolved assumptions regarding the nature of intersections and the conditions under which sets are considered connected.

beetle2
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I have been asked if the following is true or false

the intersection of two connected sets is connected ?

I would have thought that if their intersection was empty they wouldn't be connected.

If they were disjoint or course it would be empty.


any ideas?
 
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there are simple counter examples
 
The empty set definitely is connected. Take a look at the definition of not connected again... the partioning sets are required to be non-empty themselves (otherwise every set is disconnected). The empty set has no non-empty subsets so must be connected by default
 
So are you saying that if I take two non empty connected sets ie.

A= {1} and B={2} as singletons are connected then the intersection of (A n B ) = the empty set which is itself connected?
 
Last edited:
So I need to find at least an non empty set to be dissconnected.
 
beetle2 said:
So are you saying that if I take two non empty connected sets ie.

A= {1} and B={2} as singletons are connected then the intersection of (A n B ) = the empty set which is itself connected?

he is saying that the empty set is connected so that it will not serve for a counter example
 
Thanks guys
 
Just try to imagine two connected sets that cross over each other in two disconnected regions; it's not too difficult if you think about it e.g. take two connected subsets of a circle which cover the circle but are not the whole circle.
 

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