Proving or Disproving X+Y as Open in Metric Spaces | Homework Help

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Homework Help Overview

The problem involves subsets X and Y of R², with the assertion that if X is open, then the sum X + Y is also open. Participants are tasked with either proving or disproving this statement.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster expresses skepticism about the statement, suggesting it may be false and considering the need for a counterexample. Another participant prompts consideration of the case where Y is a singleton, encouraging generalization from that scenario.

Discussion Status

Participants are actively engaging with the problem, exploring specific cases and relationships between the sets. There is a suggestion to express X + Y in terms of translations of X, indicating a productive line of reasoning is being developed.

Contextual Notes

Participants are navigating the implications of the openness of X and the nature of Y, which is described as non-empty but not further defined. The discussion hints at the need for clarity on the properties of Y in relation to the problem statement.

Mr_Physics
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Homework Statement



Let X and Y be subsets of R^2, both non-empty. If X is open, the the sum X+Y is open.

This is either supposed to be proved or disproved.


Homework Equations





The Attempt at a Solution



This strikes me as false since we are only given the X is open. However, I'm not sure how to disprove it other than creating a direct counter example. Any thoughts?
 
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What can you say about X + {y} if {y} is a singleton? Can you generalize from this?
 
No...I think I might need a hint.
 
X + {y} is just a translated copy of X. Specifically, if y = (a,b), then X + {y} is just X shifted right by a and up by b.

So if X is open, then X + {y} is open.

Now, think about X + Y, where Y is any nonempty set. Think of Y as a union of singletons {y}. Can you express X + Y in terms of the sets X + {y} where {y} are the singletons contained in Y?
 
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