Micromass' big counterexample challenge

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SUMMARY

The forum discussion centers on a challenge posed by Micromass, inviting participants to provide counterexamples for ten false statements in basic analysis. Participants, including Samy_A, ResrupRL, and andrewkirk, successfully solved various statements by constructing detailed counterexamples, demonstrating their understanding of mathematical concepts such as open sets, diagonalizability of matrices, and the behavior of functions. The discussion emphasizes the importance of rigorous argumentation in mathematics and encourages creative problem-solving without reliance on external sources.

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  • Familiarity with properties of symmetric matrices and diagonalizability.
  • Knowledge of square-integrable functions and their limits.
  • Experience with constructing counterexamples in mathematical proofs.
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  • #61
mfb said:
That is basically what @Samy_A constructed, just with a much shorter argument.
Didn't he remove whole intervals? I am saying:
Take x irrational, then ##\mathbb R ## -{##x##} is your desired open set .

EDIT: It is open as the complement of the closed set {x}, and clearly ##\mathbb Q \subset G \subset \mathbb R ##.
 
Last edited:
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  • #62
WWGD said:
Didn't he remove whole intervals?
If you take the union of all his sets (to get the final set) you should get the same with both approaches.
 
  • #63
micromass said:
I hope some of these statements were surprising to some of you
o yes, especially for those who did not read Bernard R. Gelbaum, John M. H. Olmsted: Counterexamples in Analysis. :)
 

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