Open Subsets in Metric Space A with Discrete Metric d

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Homework Help Overview

The discussion revolves around the properties of open subsets in a metric space defined by the discrete metric. Participants are exploring the implications of this metric on the nature of open and closed sets within the context of a non-empty set A.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • Participants are questioning whether singletons are considered open sets under the discrete metric and discussing the nature of unions of open sets. There is an exploration of the relationship between open and closed sets, particularly focusing on the complements of closed sets.

Discussion Status

Some participants have provided hints and guidance regarding the properties of open balls in this metric space. There is an ongoing exploration of the implications of these properties, particularly concerning singletons and their unions, but no consensus has been reached yet.

Contextual Notes

Participants are working under the assumption that every subset of A is closed, which is a key point in their exploration of open sets. The discussion is framed within the constraints of the discrete metric, which influences the definitions being considered.

Mr_Physics
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Homework Statement



Let A be a non-empty set and let d be the discrete metric on X. Describe what the open subsets of X, wrt distance look like.


Homework Equations





The Attempt at a Solution



I think that the closed sets are the subsets of A that are the complement of a union of singletons, or in other words, every subset is closed.

But what does that say about the open subsets?
 
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Are the singletons open sets under this metric?

What can you say about a union of open sets?
 
I think a union of open sets is open.

Not sure if singletons are open or not.
 
Every subset is closed, you say, that is correct. What does this imply for the open sets? Remember that the open sets are exactly the complements of the closed sets!
 
Mr_Physics said:
Not sure if singletons are open or not.

Hint: If you take a point x in X, then what points does the open ball of radius 1/2, centered at x, consist of?
 
An open ball of radius 1/2 I guess would just be a singleton, right?
 
Mr_Physics said:
An open ball of radius 1/2 I guess would just be a singleton, right?

That's right. Therefore every singleton is an open set - in fact, an open ball.

Now what if you have a union of singletons? Is that an open set?
 

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