When Do Span Intersections Equal Span of Intersections in Vector Spaces?

In summary, the question asks for a condition and proof for when the span of the intersection of two subsets of a vector space is equal to the intersection of the spans of those subsets. However, the conjecture that this is true if and only if the two subsets are vector spaces is incorrect, as shown by a counterexample. Further information and clarification is needed in order to provide a correct answer.
  • #1
batballbat
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Homework Statement


[tex] S_1 [/tex] and [tex] S_2 [/tex] are subsets of a vector space. When is this:[tex] span(S_1 \cap S_2) = span(S_1) \cap span(S_2) [/tex] true? Prove it.

Homework Equations


The Attempt at a Solution


conjecture: iff the two subsets are vector spaces.
 
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  • #2
batballbat said:

Homework Statement


[itex]S_1[/itex] and [itex]S_2[/itex] are subsets of a vector space. When is this:[itex]span(S_1 \cap S_2) = span(S_1) \cap span(S_2)[/itex] true? Prove it.

Homework Equations


The Attempt at a Solution


conjecture: iff the two subsets are vector spaces.

It's true when both sides are subsets of each other.

Choose an arbitrary element in each set and show it belongs to the other set both ways.
 
  • #3
sorry, but that is of no help. I am asking for a condition and a proof for "iff".
 
  • #4
Well, what do you know and what have you tried? Do you know what "span" means?

Or do you just want someone to do the problem for you?
 
  • #5
ok. please delete this post.
 
  • #6
batballbat said:

Homework Statement


[tex] S_1 [/tex] and [tex] S_2 [/tex] are subsets of a vector space. When is this:[tex] span(S_1 \cap S_2) = span(S_1) \cap span(S_2) [/tex] true? Prove it.


Homework Equations





The Attempt at a Solution


conjecture: iff the two subsets are vector spaces.
It's easy to see that your guess is wrong. Let ##\{e_1,e_2\}## be the standard basis of ##\mathbb R^2##. Let ##S_1=\{e_1\}## and ##S_2=\{e_1,e_2\}##. We have $$\operatorname{span}S_1\cap\operatorname{span} S_2 =\operatorname{span}(S_1\cap S_2)$$ but neither ##S_1## nor ##S_2## is a subspace.

You will have to put in some effort of your own if you want help with the problem. In particular, you should include the definition of "span". Is ##\operatorname{span}\emptyset## defined?
 

Related to When Do Span Intersections Equal Span of Intersections in Vector Spaces?

What is a linear algebra span proof?

A linear algebra span proof is a mathematical method used to prove that a vector is a linear combination of other vectors in a given vector space.

Why is a linear algebra span proof important?

A linear algebra span proof is important because it helps to understand the relationship between different vectors and their span in a vector space. It also allows us to determine whether a vector can be expressed as a linear combination of other vectors in a given space.

How is a linear algebra span proof performed?

A linear algebra span proof is performed by finding the coefficients that satisfy a linear combination of vectors that result in the given vector. This is typically done by setting up a system of equations and using techniques such as Gaussian elimination to solve for the coefficients.

What is the difference between a linear algebra span proof and a linear independence proof?

A linear algebra span proof shows that a vector can be expressed as a linear combination of other vectors, while a linear independence proof shows that a set of vectors are not linearly dependent on each other. In other words, a linear algebra span proof focuses on the span of a given vector, while a linear independence proof focuses on the relationships between a set of vectors.

Why is it important to understand linear algebra span proofs?

Understanding linear algebra span proofs is important in many areas of mathematics, physics, and engineering. It allows us to solve systems of linear equations, determine the dimension of a vector space, and perform other important calculations. It also provides a foundation for more advanced topics such as linear transformations and eigenvectors.

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