Proving Equivalence of Standard and Basis-Generated Topologies on RxR

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Discussion Overview

The discussion revolves around proving the equivalence of the standard topology on RxR and the topology generated by a basis consisting of open disks. The context is primarily academic, focusing on a theoretical understanding relevant to an exam preparation.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant expresses difficulty in proving the equivalence of the two topologies and seeks assistance.
  • Another participant questions the understanding of the natural metric topology of R and suggests that it coincides with open disks in RxR.
  • A participant asks for clarification on the topology generated by the basis of open disks.
  • One participant emphasizes the importance of understanding the definitions of a basis and the topology generated by a basis, recommending a personal review of these definitions.
  • A participant explains that for any open set U in R2, one can find open disks and rectangles that fit within U, suggesting that this leads to understanding how open rectangles are open sets in the product topology of RxR.
  • There is a suggestion that the union of rectangles can be used to demonstrate the equivalence of the two topologies.

Areas of Agreement / Disagreement

The discussion does not appear to reach a consensus, as participants express varying levels of understanding and different approaches to the problem. Some participants provide guidance while others seek clarification.

Contextual Notes

There may be limitations in understanding the definitions and theorems related to topologies and bases, as well as the application of these concepts to the problem at hand.

Who May Find This Useful

This discussion may be useful for students studying topology, particularly those preparing for exams or seeking clarification on the concepts of topological equivalence and basis-generated topologies.

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I am having trouble proving this statement. Please help as I am trying to study for my exam, which is tomorrow

Prove that the standard topology on RxR is equivalent to the one generated by the basis consisting of open disks.

Thanks :)
 
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What are you having trouble understanding? What is the natural metric topology of R? This is essentially the same that is put on RxR, pointwise. Can you see how this coincides with open disks? It is not a hard problem.
 
What is the topology generated by the basis consisting of open disks?
 
What is the definition of a basis? What is the definition of the topology generated by a basis? Read carefully the definition and think if you completely understand every word and every statement there. Then close your book, write the definitions yourself on a blank piece of paper, as precisely as you can. Then open the book and compare - did you skip or added something? Once this is done, it will be easier to discuss the details and the difficulties you still may have.
 
Suppose a U is an open set in R2 (consider the usual euclidean topology - we are drawing a distinction between R2 and RxR).

Then for each point x in U, we can find an open disc Dx about x small enough so that it fits in U. If we like, we can also find a "rectangle" Rx small enough to fit inside Dx (top figure):

attachment.php?attachmentid=28528&stc=1&d=1285474877.png


Do you understand how the product topology in RxR naturally makes "open rectangles" open sets? It should follow very quickly from the definitions. What happens, now, if we take the union over all x in U of the rectangles Rx?

Similarly, for any rectangle, we can find an open disc inside it (bottom figure). Is this clear?
 

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