Why category in Baire's Category Theorem?

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Discussion Overview

The discussion revolves around the terminology used in Baire's Category Theorem, specifically the use of the word "category" in relation to the density of intersections of open sets in complete metric spaces. Participants explore the historical context and definitions associated with the theorem.

Discussion Character

  • Conceptual clarification, Meta-discussion

Main Points Raised

  • One participant questions the significance of the term "category" in Baire's Category Theorem and expresses a desire to understand its meaning.
  • Another participant notes that the theorem originally distinguished between "sets of the first category" and "sets of the second category," which are now referred to as meagre and nonmeagre sets.
  • A participant reflects on the complexity of the term "meagre" and suggests that the details may not be adequately covered in their reading material.
  • Another participant encourages a focus on the basic definition of a meagre set as a starting point for understanding.
  • A participant humorously comments on the categorization of sets, implying that meagre sets are considered small or negligible, while suggesting an informal hierarchy of set sizes.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the term "category" and its implications, indicating that there is no consensus on a clear explanation or understanding of the concept.

Contextual Notes

Some participants indicate that the definitions and implications of "meagre" sets may not be fully explained in their current resources, suggesting a potential limitation in understanding the broader context of the theorem.

goedelite
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Why "category" in Baire's Category Theorem?

I have recently begun reading a monograph on topology, "Introduction to ..." by Gamelin and Greene. Ultimately, I would learn the reason, but patience is not my strong suit. So, why the word "category" in the name of the theorem, which has to do with the density of an intersection of open sets in a complete metric space?
 
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The Baire theorem originally dealt with "sets of the first category" and "sets of the second category". Now we call them meagre sets and nonmeagre sets (or fat sets in dutch).

See http://en.wikipedia.org/wiki/Meagre_set
 


micromass: Thanks for your reply with the citation in wikipedia. I tried reading it and concluded that the import of "category" or "meagre" is a rather detailed matter. That is why, I suppose, it is not explained in my monograph.

I shall have to remain ignorant, allay my impatience by turning my attention elsewhere at least for a while.
 


goedelite, I think you should be able to understand the first line of the "Definition" section on the wikipedia page linked to above. That's all you need to know to "know" what a meagre subset is.
 


Mr Vodka: Alas, there is much that I suppose I should be able to do but cannot. That a meagre set is in some sense small or negligible is reassuring in that I could assume there is a categorization of sets: meagre, teeny-weeny bikini, small, medium, large, economy and over-sized.
 

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