What does the term complementary interval mean?

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Discussion Overview

The discussion revolves around the term "complementary interval" in the context of the Cantor set and its properties, particularly regarding continuity and convergence. Participants explore its meaning and implications within mathematical analysis.

Discussion Character

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

Main Points Raised

  • One participant seeks a definition of "complementary interval" as it relates to the Cantor set.
  • Another participant suggests that "complementary interval" may not be a technical term but rather a combination of the terms "complement" and "interval."
  • A participant provides context by mentioning continuity of the Cantor function at points of the Cantor set that have complementary intervals converging to them.
  • One participant speculates that complementary intervals could refer to non-overlapping intervals.
  • Another participant notes that the complement of the Cantor set in [0,1] consists of a countable union of disjoint open intervals, suggesting this might relate to the term in question.
  • A follow-up question arises about the meaning of intervals not in the Cantor set converging to points that are in the Cantor set.
  • A participant proposes a definition of convergence for sequences of sets, indicating a mathematical approach to understanding the concept.

Areas of Agreement / Disagreement

Participants express differing interpretations of the term "complementary interval," with no consensus reached on its precise meaning or application within the context of the Cantor set.

Contextual Notes

The discussion highlights the ambiguity of the term "complementary interval" and its dependence on specific mathematical contexts, particularly regarding the Cantor set and continuity. The concept of convergence for sequences of intervals is also under examination, with no resolution on its implications.

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What does the term "complementary interval" mean?

I'm studying the Cantor set and related notions right now, and I keep coming across the term "complementary interval." What does that mean? I can't find a definition spelled out anywhere!
 
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Where are you seeing this? How is it being used?

Without knowing any context, I would guess that "complementary interval" is not a technical term -- it's simply an English phrase built from the two technical terms "complement" and "interval", or maybe even from the English word "complementary" and the technical term "interval".
 


Ok. Allow me to provide some context: "The difficult part is verifying continuity of the Cantor function at points of the Cantor set that have a sequence of complementary intervals converging to them."
 


I know nothing about what you are studying, but a wild guess: could it mean that the intervals don't overlap? If I and I' are any two intervals, then they are complimentary intervals if there exists no points in I which are in I' ?
 


The complement of the Cantor set in [0,1] is a countable union of disjoint open intervals. Maybe they're talking about those?
 


Hurkyl said:
The complement of the Cantor set in [0,1] is a countable union of disjoint open intervals. Maybe they're talking about those?
Yes, that's a reasonable idea. But how do you make sense of intervals NOT in the Cantor set converging to points that ARE in the Cantor set? What does it even MEAN for a sequence of open intervals to converge to a point?
 


Well, the most straightforward idea I can think of is:

A sequence of sets Sn converges to a point P iff, for every open set U containing P, there exists an N such that n>N implies Sn is a subset of U.

Or equivalently, every sequence sn of points chosen so that sn is an element of Sn converges to P.
 

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