Linear Algebra: Direct sum proof

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Homework Help Overview

The discussion revolves around proving a property related to direct sums in linear algebra, specifically showing that the intersection of two subspaces U and V of a vector space W is trivial (i.e., U ∩ V = {0}) when W is expressed as a direct sum of U and V.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the definition of a direct sum and its implications, questioning whether the initial understanding aligns with the formal definition. There are attempts to clarify the problem by suggesting alternative formulations and discussing the uniqueness of vector representations in the context of direct sums.

Discussion Status

The discussion is ongoing, with participants seeking clarification on definitions and expressing uncertainty about how to approach the proof. Some guidance has been offered regarding rewriting the problem and considering the implications of having a non-zero vector in both subspaces.

Contextual Notes

There appears to be some confusion regarding the definitions and necessary steps to prove the statement, indicating a need for further exploration of the concepts involved in direct sums and subspace intersections.

loesch.19
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Let U and V be subspaces of a vector space W. If W=U [tex]\oplus[/tex] V, show U [tex]\bigcap[/tex] V={0}.

I'm a bit lost on this one... as I thought this was essentially the definition of direct sum. I'm unsure where to start. Any help would be great!
 
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What's the definition of a direct sum you're using?
 
vela said:
What's the definition of a direct sum you're using?

I wasn't sure if that was necessary info or not... looks like I was wrong :)

If U and V are subspaces of vector space W, and each w in W can be written uniquely as a sum u+v where u is in U and v is in V then W is a direct sum of U and V.
 
So were you able to do the problem?
 
Hurkyl said:
So were you able to do the problem?

No... I still need help.
 
Well, surely you can do something on it -- even if it's just rewriting the problem in a less opaque form.

e.g. do you know anything about proving two subspaces equal? (Or two sets?)
 
loesch.19 said:
Let U and V be subspaces of a vector space W. If W=U [tex]\oplus[/tex] V, show U [tex]\bigcap[/tex] V={0}.

I'm a bit lost on this one... as I thought this was essentially the definition of direct sum. I'm unsure where to start. Any help would be great!

I think you should add the definition as follows:

If U and V are subspaces of a Vector space W and each [tex]w \in W[/tex] can be written as the unique sum as u+v where [tex]u \in U[/tex] and [tex]v \in V[/tex] then

W is the direct sum of U and V and can be written [tex]W = U \oplus V[/tex]
 
Suppose there were a non-zero vector, w, in both U and V and let u be any vector in U. Now, write u as two different sums of vectors in U and V.
 

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