Proving Limit Points in Point Sets for Scientists

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Discussion Overview

The discussion revolves around proving a property of limit points in the context of point sets, specifically addressing whether a point that is a limit point of the union of two sets must also be a limit point of at least one of the individual sets. The scope includes theoretical reasoning and mathematical proofs related to topology and limit points.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant presents the problem of proving that if point p is a limit point of the union of sets H and K, then p must be a limit point of either H or K.
  • Another participant suggests that a sequence in the union H U K tending to p must have a subsequence lying in either H or K.
  • A different participant clarifies that the problem is not merely about sets but involves topology, defining regions as open sets in that topology.
  • This participant outlines a proof strategy by assuming p is not a limit point of H and demonstrating that this leads to the conclusion that p must be a limit point of K.
  • A follow-up post expresses gratitude for the help but raises a new question about proving that the intersection of two regions is also a region.

Areas of Agreement / Disagreement

Participants appear to engage in a constructive exploration of the problem, with some agreement on the definitions and strategies for proof. However, the discussion does not reach a consensus on the overall proof or the specific question about the intersection of regions.

Contextual Notes

There is an assumption that the definition of limit points and the concept of regions are understood within the context of topology, but the proof regarding the intersection of regions remains unaddressed.

Who May Find This Useful

This discussion may be useful for those interested in topology, limit points, and mathematical proofs, particularly in the context of point set theory.

Timberwoods
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i just ran into a hard problem, may be any of you guy can help...
prove that if the point p is a limit point of H U K where H and K are point sets, then p is a limit point of H or p is a limit point of K.
Given definition of a limit point is: a point p is said to be a limit point of a point set M if every region containing p contains a point of M distinct from p.
Thanks for your time.
 
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So there is a sequence in HuK tending to p. It must have a subsequnce lying in either H or K, musn't it?
 
Not really a "set" problem is it? I assume you have a topology on a set having H and K as subsets and a "region" is an open set in that topology.

Suppose p is a limit point of HUK. Then, by definition, each region of p (every open set containing p) contains a point of HUK different from p.

If p is a limit point of H, we are done so we can assume that is not true.
(This is a standard technique: we are asked to prove "a OR b" so we assume a is NOT true and prove b must be true.)

If p is NOT a limit point of H, there exist a region V containing p which contains no member of H (other than, possibly, p itself). Of course, since p is a limit point of HUK, V must contain a member of HUK which means it must contain a member of K. Let U be any region containing p and look at U intersect V (which is non-empty).
 
thanks, it helps a lot, but in doing so, i still need to prove that the intersection of 2 regions is a region, which i haven't proved it yet, would you give me a hand on that?
 

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