Maximal Subspaces in Vector Spaces: Using Zorn's Lemma to Prove Existence

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Discussion Overview

The discussion revolves around proving that every vector space V has a maximal subspace using Zorn's Lemma. Participants explore the implications of proper subspaces, the nature of unions of subspaces, and the existence of bases in vector spaces, addressing both finite and infinite-dimensional cases.

Discussion Character

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

Main Points Raised

  • One participant expresses difficulty in proving that every totally ordered subcollection of proper subspaces has an upper bound in the context of Zorn's Lemma.
  • Another participant asserts that the union of a chain of subspaces is indeed a subspace, but emphasizes the necessity for it to be a proper subspace of V.
  • Some participants suggest using the idea of taking a basis of V and deleting one element to form a maximal subspace.
  • There is a discussion about whether the problem implies the existence of a basis for V, with some participants questioning if V is finite-dimensional.
  • One participant clarifies that every vector space has a basis, regardless of dimensionality, and that Zorn's Lemma is applicable for proving this in infinite-dimensional cases.
  • Another participant proposes a method involving the collection of all subspaces not containing a specific vector v in V as a way to apply Zorn's Lemma.

Areas of Agreement / Disagreement

Participants generally agree on the applicability of Zorn's Lemma and the existence of bases in vector spaces, but there is disagreement regarding the implications of proper subspaces and the nature of unions of subspaces. The discussion remains unresolved on the best approach to prove the existence of maximal subspaces.

Contextual Notes

Some limitations include the dependence on the definitions of proper subspaces and bases, as well as the unresolved nature of whether V is finite-dimensional or infinite-dimensional.

mathboy
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Maximal subspace

Problem: Prove that every vector space V has maximal subspace, i.e. a proper subspace that is not properly contained in a proper subspace of V.

I let A be the collection of all proper subspaces of V, but I can't prove that every totally ordered subcollection of A has an upper bound in A. The problem that the union of proper subspaces is not necessarily a proper subspace of V. What do I do now?
 
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But the union of a chain of subspaces is a subspace.
 
mathboy said:
Problem: Prove that every vector space V has maximal subspace, i.e. a proper subspace that is not properly contained in a proper subspace of V.

I let A be the collection of all proper subspaces of V, but I can't prove that every totally ordered subcollection of A has an upper bound in A. The problem that the union of proper subspaces is not necessarily a proper subspace of V. What do I do now?

Think basis elements.
 
morphism said:
But the union of a chain of subspaces is a subspace.

But it has to be a proper subspace of V.

For example { span{1}, span{1,x}, span{1,x,x^2}, span{1,x,x^2,x^3}, ... } is a chain of proper subspaces of R[x], but its union is all of R[x], which is not a proper subspace of R[x].
 
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JasonRox's idea is good, take a basis of V and delete one element. The span of that would have to be a maximal subspace.

But I'm assuming that mathboy wants to use Zorn's lemma. In that case choose any v in V, and let A be the collection of all subspaces not containing v. This time the upper bound of any chain will be a proper subspace. The maximal element of A would be a maximal subspace of V.
 
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andytoh said:
But it has to be a proper subspace of V.
Oops! I should learn to read! Thanks for pointing that out. :smile:
 
JasonRox's idea is good, take a basis of V and delete one element. The span of that would have to be a maximal subspace.

I don't think the problem implies there IS a basis of V (unless it turns out all vector spaces have a basis, and I just don't know that yet)
 
Office_Shredder said:
I don't think the problem implies there IS a basis of V (unless it turns out all vector spaces have a basis, and I just don't know that yet)

Is V finite-dimensional? Is the book assuming that?

Do you know what finite-dimensional is?
 
Every vector space V has a basis, whether it is finite-dimensional or not. In mathboy's problem V can be infinite-dimensional and the result is still true.

If you want to prove that V has a basis if V is infinite-dimensional, you would have to use Zorn's lemma as well. Ultimately, mathboy's problem rests on Zorn's Lemma.


My approach to mathboy's problem is: Choose any v in V, and let A be the collection of all subspaces not containing v and then use Zorn's lemma. But I'm trying to figure out if there is a better partially ordered set to use, because my A seems a little clumsy (though I believe it would still get the job done).
 
  • #10
andytoh said:
Every vector space V has a basis, whether it is finite-dimensional or not. In mathboy's problem V can be infinite-dimensional and the result is still true.

Of course I know this!

Ok, a vector space has a basis {v_1,...}, now delete one vector from there and span that that set. What do you get?

Voila!
 
  • #11
Thanks guys. I forgot to say that I have to use Zorn's Lemma. But I know how to proceed now. I will use the collection of all proper subspaces that does not contain some fixed v in V.
 

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