Subspace Questions: Checking 2 Sets in R^3

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In summary, A subspace is a set that is closed under addition and scalar multiplication, and contains the zero vector. To check whether a set is a subspace, you can use the definition or show that it is closed under addition and scalar multiplication. For the given sets, the first one is a subspace by definition, and for the second one, you can check that it is closed under addition and scalar multiplication by assuming two vectors satisfy the given equation and showing that their sum and scalar multiples also satisfy it. "All combinations" in this context means linear combinations, which is a set of vectors of the form a1 v1+...+an vn where the ak are scalars. To show that the column spaces of two matrices
  • #1
EvLer
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I am not sure about these 2 whether they are subspaces or not (i do know how to check whether it is a subspace or not)

subsets of R^3, subspace or not?
1.all combinations of (1,1,0) and (2,0,1)
2.plane of vectors (b1,b2,b3) that satisfy b3-b2+3b1 = 0

thanks for help.
 
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  • #2
What does a subspace mean to you?
 
  • #3
closed under addition and scalar multiplication... add two vectors and they should still be in the space, multiply by a const and the vector is still in space... also has to contain zero.
i am not sure which side to approach THESE two...

and what in the world is meant by "all combinations"?
 
  • #4
By combinations, I assume they mean linear combinations. A linear combination of a set of vectors v1,...,vn is any vector of the form a1 v1+...+an vn where the ak are scalars. The set of such vectors is clearly a subspace, and this is sometimes taken as the defintion. For the other one, assume two vectors satisfy the equation and show their sum and scalar multiples do as well.
 
  • #5
StatusX said:
For the other one, assume two vectors satisfy the equation and show their sum and scalar multiples do as well.

scalar multiple I get, but for sum not exactly sure:
(u3+v3) - (u2+v2) + 3(u1+v1) = 0
what does that give me?
 
  • #6
u3-u2+3u1=0, and similarly for v, so...
 
  • #7
oh ok, thanks.

one other quick question: if I factor a constant out of a row of a matrix B and get matrix A then can i say that B = 2A?
what if i factor a constant out of a column?
thanks again...

edit: also, how would i show that column spaces of 2 matrices are equal?
 
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1. What is a subspace?

A subspace is a subset of a vector space that satisfies the three properties of closure, addition, and scalar multiplication. In other words, it is a smaller space that exists within a larger vector space, and it contains all the necessary elements to perform vector operations such as addition and multiplication.

2. How do you check if two sets are subspaces in R^3?

To check if two sets are subspaces in R^3, you need to ensure that they satisfy the three properties of a subspace: closure, addition, and scalar multiplication. This means that if you add two vectors within the set, the resulting vector should also be within the set, and if you multiply a vector within the set by a scalar, the resulting vector should also be within the set.

3. What is the importance of checking if two sets are subspaces in R^3?

Checking if two sets are subspaces in R^3 is important because it ensures that the sets have the necessary properties to perform vector operations. This is crucial in many fields, such as physics, engineering, and computer graphics, where vector operations are commonly used to solve problems and model real-world phenomena.

4. Can a set be a subspace of another subspace in R^3?

Yes, a set can be a subspace of another subspace in R^3. This is because a subspace is a subset of a vector space, and therefore, it can also contain other subspaces within it. However, the nested subspace must still satisfy the three properties of closure, addition, and scalar multiplication to be considered a valid subspace.

5. What are some common mistakes when checking for subspaces in R^3?

One common mistake when checking for subspaces in R^3 is forgetting to check for closure. Another mistake is assuming that a set is a subspace without verifying all three properties. It is also important to note that the zero vector (0,0,0) must always be included in a subspace. Additionally, care should be taken to ensure that the sets are properly defined and that the operations are performed correctly.

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