Solving U, V, W Subspaces Problem

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Discussion Overview

The discussion revolves around a problem concerning the properties of subspaces U, V, and W in the context of direct sums in vector spaces. Participants are exploring whether the condition U (direct sum) W = V (direct sum) W implies that U equals V, engaging in theoretical reasoning and counter-example exploration.

Discussion Character

  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents the problem of proving or disproving the statement regarding the equality of subspaces U and V under the condition of their direct sums with W.
  • Another participant questions the formulation of the problem, suggesting that the implications of the direct sum condition should be carefully considered, particularly regarding the representation of vectors in V.
  • A participant expresses a belief that the statement may not be true but admits to being unable to prove it.
  • Another participant suggests looking for a counter-example to demonstrate that the statement is not necessarily true, indicating that this approach could yield a better evaluation.
  • A later reply acknowledges an error in understanding and prompts a reevaluation of the implications of the direct sum definitions on the subspaces involved.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the validity of the statement. There are competing views regarding the implications of the direct sum conditions and whether a counter-example can be found to disprove the statement.

Contextual Notes

Participants express uncertainty about the implications of the direct sum definitions and the conditions under which the equality holds. There are unresolved mathematical steps regarding the proof or disproof of the statement.

mivanova
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Hi,
I have thius problem to solve. Please, help me!

1. Prove or disprove if U, V, W are subspaces of V for which
U (dir sum) W = V (dir sum) W then U=V

Thank you in advance!
 
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Did you mistype the question? Does it really read V (direct sum) W ?

If it's indeed correct then think carefully about what this implies for any vector v in V and how it may be expressed as a unique sum of vectors v,w from V and W. What does it say about w?

And with this in mind, look at the left-hand side. Is this sufficient alone to conclude U=V?
 
Hi,
It's indeed (direct sum) and I think that the statement is it's not true. I can't prove it though.
Thanks!
 
If you can't prove it, try looking for a counter-example. Providing a single counter-example without showing why the statement isn't necessarily true would give you full marks, whereas doing the latter only gets you about half marks.
 
Oh man, I can't believe I made such a stupid error. Ok, forget what I said earlier and look at V (dir sum) W. What is the subspace spanned by that, taking into account the the definition of direct sum?

What does that say about W? After you're done with this, think about the subspace spanned by U (dir sum) W, and what does it mean for U when the equality stated in the proposition holds.
 

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