Subspace Question: Why Not Closed Under Addition?

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In summary, the conversation discusses the concept of a set being closed under addition and how it applies to a specific example. The participants also clarify the meaning of x^2=y^2 and explore alternative methods of proving a set is not closed under addition.
  • #1
Nope
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Homework Statement



[PLAIN]http://img683.imageshack.us/img683/4530/unledkw.jpg
can someone please explain why it is not closed under addition?
My textbook did not explain very well, but I understand this can be zero vector and it is closed under scalar multiplication.
thanks!

Homework Equations


The Attempt at a Solution

 
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  • #2
[x,y]=[1,-1] satisfies x^2=y^2. So does [1,1]. Does their sum?
 
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  • #3
oh ,
(1+1, 1-1)
(2,0) not in (x,y)
am i correct?
or can you explain to me in word?
 
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  • #4
Nope said:
can you show me?, because I don't get the closed under addition part at all..

Do you agree [1,-1] and [1,1] are in your subspace? What's the sum [1,-1]+[1,1]? It's vector addition, right?
 
  • #5
yes, (2,0)
 
  • #6
Nope said:
yes, (2,0)

Is [2,0] in your set where x^2+y^2=1?
 
  • #7
Nope said:
oh ,
(1+1, 1-1)
(2,0) not in (x,y)
am i correct?
or can you explain to me in word?

Right! (2,0) is not in (x,y) such that x^2=y^2. So your set is NOT closed under addition.
 
  • #8
not in the set, but why is x^2+y^2 equal to one?
 
  • #9
Nope said:
not in the set, but why is x^2+y^2 equal to one?

It isn't. I meant to write x^2=y^2 or x^2-y^2=0. My mistake.
 
  • #10
is there another way to prove it? like using x1 or y2
 
  • #11
Nope said:
is there another way to prove it? like using x1 or y2

The most direct way to prove a set is NOT closed under addition is to find two elements in the set whose sum is NOT in the set. I'm not sure why you would want another way.
 
  • #12
I got it, thanks!
 

1. What is subspace?

A subspace is a subset of a vector space that satisfies the properties of a vector space. This means that it is closed under addition and scalar multiplication, and contains the zero vector.

2. Why is it important for a subspace to be closed under addition?

Closed under addition means that when two vectors in a subspace are added together, the result must also be in the subspace. This is important because it ensures that the subspace remains a valid vector space.

3. Why is subspace not always closed under addition?

Subspaces are not always closed under addition because the addition of two vectors may result in a vector that is not within the subspace. For example, if a subspace is limited to a specific dimension, adding two vectors from different dimensions may result in a vector that is not in the subspace.

4. How can we prove that a subspace is not closed under addition?

To prove that a subspace is not closed under addition, we can provide a counterexample. This means finding two vectors in the subspace that, when added together, result in a vector that is not in the subspace.

5. Can a subspace be closed under addition but not scalar multiplication?

No, a subspace must be closed under both addition and scalar multiplication in order to be a valid vector space. If a subspace is not closed under scalar multiplication, it may not satisfy the properties of a vector space, such as closure under addition.

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