Prove that a normed space is not Banach

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Discussion Overview

The discussion revolves around proving that a normed space with an uncountable Hamel basis is not a Banach space. Participants explore various approaches to the problem, including the use of the Baire theorem and the properties of Cauchy sequences.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests using the Baire theorem to demonstrate that the normed space is not Banach.
  • Another participant proposes finding a Cauchy sequence that does not converge as a method to show the space is not complete.
  • A participant points out that an infinite dimensional Banach space has an uncountable Hamel basis, questioning the generality of the claim.
  • One participant mentions constructing a finite dimensional closed subspace to reach a contradiction regarding the completeness of the space.
  • Another participant requests the exact wording of the exercise to clarify the problem.
  • A later reply emphasizes that a finite dimensional normed space is always Banach, suggesting a potential flaw in the proposed proof.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the proposed proof methods and the implications of having an uncountable Hamel basis. There is no consensus on how to approach the problem or whether the initial claim holds true.

Contextual Notes

Participants note that the exercise does not specify a particular normed space, which may affect the applicability of certain arguments. There is also uncertainty regarding the completeness of the space and the implications of finite dimensionality.

russel
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Hello everyone,
I have a problem and cannot solve it. Could you help? Here it is
We have a normed space and an uncountable Hamel basis of it. Prove that it is not a Banach space.
Should I use Baire theorem? Any suggestions?
 
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You're not giving us much information to work with. If you want to show something not Banach, then try to find a Cauchy sequence which does not converge.
There are other ways to prove this of course. A lot depends on the space in question.
 
You must have a particular space in mind that you didn't specify. An infinite dimensional Banach space necessarily has an uncountable Hamel basis so your result is not general. You need to show that your space is not complete. What is your space?
 
The exercise does not refer to a particular space. It is just a normed space X with an uncountable Hamel basis. A solution I came up with was to make a finite dimension closed subspace and show using baire that it is X, leading to a contadiction.
If the problem gave us a space for this example which one would that be?
 
Could you quote the exercise exactly as it was given??
 
russel said:
The exercise does not refer to a particular space. It is just a normed space X with an uncountable Hamel basis. A solution I came up with was to make a finite dimension closed subspace and show using baire that it is X, leading to a contadiction.
If the problem gave us a space for this example which one would that be?

But, how could you prove that a closed subspace of an undefined space equals the space itself?
 
Also notice that a finite dimensional normed space is always Banach, so your proof is likely incorrect.
 

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