Understanding Littlewood's Three Principles in Relation to the Lebesgue Integral

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Discussion Overview

The discussion revolves around Littlewood's three principles and their relation to the Lebesgue integral, focusing on the interpretation of the term "nearly" in various mathematical contexts. Participants seek to clarify these concepts and their significance in real analysis.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks for a simpler explanation of Littlewood's three principles and the meaning of "nearly" in contexts such as nearly a finite union of intervals, nearly continuous, and nearly uniformly convergent.
  • Another participant suggests that "nearly" is intentionally vague and that it is meant to be adapted to specific problems.
  • A participant proposes that "nearly" could be interpreted as "almost everywhere," prompting further discussion.
  • One participant disagrees with the interpretation of "nearly" as "almost everywhere," stating that the principles have specific meanings that do not equate to that phrase, referencing Lusin's Theorem and Egorov's Theorem for precise definitions.
  • A participant mentions having the same textbook, indicating a shared context among participants.

Areas of Agreement / Disagreement

There is disagreement regarding the interpretation of "nearly" and its equivalence to "almost everywhere." Some participants propose that "nearly" is vague and adaptable, while others assert that it has specific meanings tied to established theorems.

Contextual Notes

Participants reference specific theorems (Lusin's and Egorov's) for clarification of the principles, indicating that understanding these theorems is crucial for grasping the concepts discussed. The discussion does not resolve the ambiguity surrounding the term "nearly."

guildmage
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How do you explain Littlewood's three principles in simpler terms? What does "nearly" mean (as in nearly a finite union of intervals, nearly continuous, and nearly uniformly convergent)?

And why are these important if I'm going to study the Lebesgue integral?

I'm learning this on my own so I'm really having a hard time digesting what the book (Real Analysis by Royden) is saying.
 
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Those are the simpler terms. "Nearly" is purposely vague; these are approaches to problem solving that you're supposed to adapt to the problem of interest. Several commonly useful versions appear in the chapter; the introductory paragraphs to that section names them.
 
"nearly" is another way of saying "almost everywhere" isn't it?
 
@fourier jr: I would like to believe so.
 
fourier jr said:
"nearly" is another way of saying "almost everywhere" isn't it?

No.

One principle says: A set is nearly a finite union of intervals.

It does not mean "almost everywhere".

Another principle says: A function is nearly continuous. Precise meaning: see Lusin's Theorem. Again, it does not mean "almost everywhere".

The third is about uniform convergence. Precise meaning: Egorov's Theorem.
 
i've even got that book by royden. i guess i haven't looked at it in a while :(
 

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