Maximal Elements in a Bounded Set

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Discussion Overview

The discussion revolves around the concept of maximal elements in a bounded set, particularly in the context of specific examples and questions posed by a participant. The scope includes theoretical exploration and mathematical reasoning related to set theory.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant requests verification and corrections on a set of questions related to maximal elements.
  • Another participant agrees with most of the questions but challenges the reasoning in question 5b, suggesting consideration of the set $\displaystyle\left\{1-\frac{1}{n+1}\:\bigg|\: n\in\mathbb{N}\right\}$.
  • A different participant expresses agreement with most points but raises a concern regarding question 5.2, specifically mentioning the context of an open interval.
  • A participant elaborates on the set $S=\left\{0,\frac12,\frac23,\frac34,\ldots\right\}\subset\mathbb{Q}$, noting that while it is bounded above by $1$, no element of $S$ can be maximal due to the existence of greater elements within the set.

Areas of Agreement / Disagreement

Participants generally agree on the correctness of most questions, but there are multiple competing views regarding the specific reasoning in questions 5b and 5.2, indicating that the discussion remains unresolved.

Contextual Notes

The discussion highlights the dependence on definitions of maximal elements and the implications of boundedness in set theory, with some assumptions about the nature of the sets being discussed remaining unexamined.

ertagon2
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Could someone please check these questions? Please correct them if necessary, with an explanation if you could.
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Hi ertagon2,

Everything looks OK, except 5b. Think about the set $\displaystyle\left\{1-\frac{1}{n+1}\:\bigg|\: n\in\mathbb{N}\right\}$.
 
I agree with everything except 5.2. Consider an open interval.
 
castor28 said:
Hi ertagon2,

Everything looks OK, except 5b. Think about the set $\displaystyle\left\{1-\frac{1}{n+1}\:\bigg|\: n\in\mathbb{N}\right\}$.

I don't think I understand. Can you elaborate?
 
ertagon2 said:
I don't think I understand. Can you elaborate?
Hi ertagon2,

This is the set $\displaystyle S=\left\{0,\frac12,\frac23,\frac34,\ldots\right\}\subset\mathbb{Q}$. This set is bounded above (by $1$). In fact, $1$ is the least upper bound of $S$, but it is not an element of $S$.

No element of $S$ can be maximal, because, for each element $\left(1 - \dfrac{1}{n+1}\right)\in S$, $\left(1 - \dfrac{1}{n+2}\right)$ is greater and also an element of $S$.
 

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