Trying to understand terms in a problem

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Discussion Overview

The discussion revolves around understanding specific terms in a mathematical problem related to topology, particularly focusing on the concepts of "non-crossing" and "non-connected" in the context of convex discs in the plane. Participants explore definitions and seek guidance on how to approach the problem.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant asks for clarification on the terms "non-crossing" and "non-connected," suggesting they may originate from topology.
  • Another participant provides a definition of connectedness from topology, explaining that a topological space is connected if it cannot be represented as a disjoint union of two non-trivial open sets.
  • Several participants express their limited understanding of topology and request hints or initial steps to tackle the problem.
  • A participant defines a convex disc as any compact, convex set with non-empty interior.
  • One participant challenges the assumption that the problem is easy, indicating difficulty in finding a quick proof and suggesting consulting the person who assigned the problem.

Areas of Agreement / Disagreement

Participants generally express uncertainty about the problem and its terminology, with no consensus on the ease of the problem or the best approach to take.

Contextual Notes

There are limitations in the participants' understanding of topology, which may affect their ability to engage with the problem fully. The definitions of "non-crossing" and "non-connected" are not universally agreed upon, and the problem's complexity is acknowledged.

Mr Davis 97
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Here is the problem: Let ##C## be a convex disc in the plane, and ##C_1## and ##C_2## be two translates of ##C##. Prove that ##C_1## and ##C_2## are non-crossing, that is, it isn't possible that both ##C_1 - C_2## and ##C_2 - C_1## are non-connected.

Here is my question: What exactly do the terms "non-crossing" and "non-connected" mean? Are these terms coming from topology?
 
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Crossing is defined in your problem.

Connectedness is indeed a term from topology. A topological space ##(X, \mathcal{T})## is connected if the only clopen (= both closed and open) sets are ##\emptyset## and ##X##. Or equivalently, ##X## is not a disjoint union of two (non-trivial) open sets.
 
So as someone who has a limited understanding of topology, what would be a hint or a first step for this?
 
Mr Davis 97 said:
So as someone who has a limited understanding of topology, what would be a hint or a first step for this?

How does your book define disk?
 
A convex disc is any compact, convex set with non-empty interior
 
Math_QED said:
How does your book define disk?
Any hints? I feel for someones who knows topology this would be an easy problem
 
Mr Davis 97 said:
Any hints? I feel for someones who knows topology this would be an easy problem

It isn't an easy problem. I can't find a quick proof for your statement. Maybe ask to the person who assigned you this problem. Good luck.
 

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