How Do You Prove That sup(S ∪ T) Equals max{supS, supT}?

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    Analysis Proof
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Discussion Overview

The discussion revolves around proving the statement that the supremum of the union of two sets, sup(S ∪ T), equals the maximum of their individual suprema, max{supS, supT}. The focus is on the theoretical aspects of this proof without assuming any subset relationships between the sets S and T.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant requests assistance in completing or starting the proof, expressing frustration with the task.
  • Another participant suggests beginning with the definition of the supremum of the union of two sets.
  • A participant asserts that the problem is trivial, arguing that the largest number in the union must be the largest from either set, but acknowledges the complexity introduced by the concept of suprema.
  • One participant expresses understanding of the answer but struggles with demonstrating that the maximum exists in the union and grapples with the idea that supT may not be contained within sup(S ∪ T).
  • Another participant clarifies the definition of supremum and explains how it relates to the elements of the sets, suggesting that if L is the larger of the two suprema, it must also be the supremum of the union.
  • There is a request for the original poster to attempt the reverse direction of the proof.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement on the definitions and intuitive understanding of the problem, but there is no consensus on how to effectively demonstrate the proof. Disagreement exists regarding the perceived difficulty of the proof and the clarity of the definitions involved.

Contextual Notes

Participants highlight the importance of definitions and the existence of elements in the context of suprema, but there are unresolved aspects regarding the completeness of the proof and the assumptions made about the sets.

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Please help with analysis proof!

hey all,

i was wondering if anyone could offer some advice on how to complete or even begin this proof. here it is:

prove that sup(S union T)= max{supS,supT}. Do not assume S is a subset of T.


AHHHHHH!

thanks.
 
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step by step: what is the definition of sup{SuT}?
 
this is trivial. you have not even tried it have you?

i.e. intuitively, the biggest number in the union of two sets is either the biggest number in one or the other.

this problem is the same plus the clumsiness of sups.
 
mathwonk said:
this is trivial. you have not even tried it have you?

i.e. intuitively, the biggest number in the union of two sets is either the biggest number in one or the other.

this problem is the same plus the clumsiness of sups.

no, no I've tried. I understand the answer, the problem for me is showing it. Mainly that the max of the set exists in the union.

i guess another problem is i can't get around SupT doesn't have to exist in Sup SUT, but it could be the max of the set.

Oh well.

p.s. what's with the rage? I am not claiming to be an expert.


thanks to you two for replying.
 
It'ds not rage its frustration. sorry. start as suggested with the definition of sup. L is the sup of a set S if no element of the set S is larger than L, but elements of S do get as close to L as desired. Hence if L is the larger of the sups of S and T then L is at least as large as all elements of both S and T, and if L is the sup of S say, then surely there are elements of S hence also of SuT which get as close to L as you want. hence L is alkso the sup of SuT.

Where did you get stuck here? Thjs follows immediately from the definitions of the words.


now you try the other direction.
 
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