Gluing points of [0, 1] to get [0, 1]^2

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Discussion Overview

The discussion revolves around the concept of gluing points of the interval [0, 1] to obtain the square [0, 1]^2 through the framework of Peano's space-filling curve. Participants explore the implications of this mapping, the nature of the points being glued, and the properties of the resulting quotient space.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the continuous map f: I -> I^2, as described by Peano's space-filling curve, suggests that all points of I may be glued together, though justifications for this are sought.
  • One participant suggests gluing points x and y whenever f(x) = f(y), leading to a bijection and a continuous induced map, which could imply a homeomorphism.
  • Another viewpoint emphasizes that the square can be seen as the image of the line, where overlapping points on the line are identified as the same.
  • A later reply introduces the idea that space-filling curves might be uniform limits of continuous functions on closed intervals, hinting at the continuity of the limit and its implications for the mapping.
  • Participants mention the existence of other sequences of uniformly continuous functions that can produce similar limits, such as the Devil's staircase.

Areas of Agreement / Disagreement

Participants express various interpretations of how points are glued together and the implications of this process, indicating that multiple competing views remain without a consensus on the specifics of the gluing process.

Contextual Notes

Some claims depend on assumptions about the properties of continuous functions and the nature of quotient spaces, which are not fully resolved in the discussion.

GridironCPJ
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By Peano's space-filling curve, there exists a continuous map f: I -> I^2 whos image fills up the entire square I^2 (where I=[0, 1]). This can also be represented by gluing points of I together. Which points of I get glued together? I was looking at the proof of Peano's space-filling curve and I kind of get the idea it's all of them, although I'm having trouble justifying that. Would anyone like to shed some light on this?
 
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GridironCPJ said:
This can also be represented by gluing points of I together. Which points of I get glued together? I was looking at the proof of Peano's space-filling curve and I kind of get the idea it's all of them, although I'm having trouble justifying that.

Can you explain what you mean by this?
 
Hmm, how about you glue together any two points x,y whenever f(x)=f(y)? The map f applied to this quotient space will then be a bijection.

The universal property of quotients will mean that the induced map is continuous and of course a continuous bijection from a compact space to a Hausdorff one is a homeomorphism, so we are done :)
 
Last edited:
Of course, another way you can easily see this is to just think that the square is the image of the line, but where overlapping points on the line are the same.
 
Jamma said:
Hmm, how about you glue together any two points x,y whenever f(x)=f(y)? The map f applied to this quotient space will then be a bijection.

The universal property of quotients will mean that the induced map is continuous and of course a continuous bijection from a compact space to a Hausdorff one is a homeomorphism, so we are done :)

cool.

So the square is a quotient space of the interval.
 
GridironCPJ said:
By Peano's space-filling curve, there exists a continuous map f: I -> I^2 whos image fills up the entire square I^2 (where I=[0, 1]). This can also be represented by gluing points of I together. Which points of I get glued together? I was looking at the proof of Peano's space-filling curve and I kind of get the idea it's all of them, although I'm having trouble justifying that. Would anyone like to shed some light on this?

I think but am not sure that space filling curves are the uniform limits of continuous functions on closed intervals. A theorem states that this limit is itself continuous.

These functions are those maze like curves. They are designed - I think - to approach arbitrarily closely to every point in the square.

This trick applies to other sequences of uniformly continuous functions to produce other weird limits such as the Devil's staircase,
 

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