Proving the Set of Solutions for AX=B is Not a Vector Space

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Homework Help Overview

The discussion revolves around proving that the set of solutions for the equation AX=B is not a vector space, where A is an mxn matrix and B is a non-zero mx1 matrix. Participants explore the implications of B being non-zero and the conditions required for a set to qualify as a vector space.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants question whether the solution set can be consistent if B is not in the column space of A. They discuss the necessity of the zero vector being part of the solution set and explore the implications of X=0 not solving the system. There are also inquiries about the definition of a vector space and its properties.

Discussion Status

The discussion is ongoing, with participants providing insights into the properties of vector spaces and the specific case of AX=B. Some participants have offered guidance on the conditions that must be met for a set to be a vector space, while others are still seeking clarification on these concepts.

Contextual Notes

There is a focus on the requirement that a vector space must contain a zero vector, and participants are examining the implications of B being non-zero on the closure properties of the solution set. The conversation reflects a mix of understanding and confusion regarding the definitions and properties of vector spaces.

Jaglowsd
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Let B be a non-zero mx1 matrix, and let A be an mxn matrix. Show that the set of solutions to the system AX=B is not a vector space.

I am thinking that I need to show that the solution is not consistent. In order to do so would I need to show that B is not in the column space of A?
 
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It's a lot simpler than that. A vector space is supposed to contain a zero vector.
 
So unless B is a zero vector than in any case AX=B can not be a vector space? It must be AX=0, or the null space?
 
Jaglowsd said:
So unless B is a zero vector than in any case AX=B can not be a vector space? It must be AX=0, or the null space?

That's kind of confusing, but if X=0 doesn't solve your system then the set of solutions isn't a vector space.
 
Dick said:
That's kind of confusing, but if X=0 doesn't solve your system then the set of solutions isn't a vector space.

Sorry, I am still trying to wrap my head around vector spaces. Could you elaborate why X=0 must solve the system to be a vector space.
 
Jaglowsd said:
Sorry, I am still trying to wrap my head around vector spaces. Could you elaborate why X=0 must solve the system to be a vector space.

When does a set of vectors constitute a vector space? You'll need to look up the definition if you can't recall. The answer is there.
 
If x and y both satisfy Ax= B, Ay= B, then A(x+ y)= Ax+ Ay= B+ B= 2B. Unless B= 0, x+ y does NOT satisfy the equation A(x+ y)= B so is NOT in this set. The set is not closed under addition, so is not a vector space.
 
Dick said:
When does a set of vectors constitute a vector space? You'll need to look up the definition if you can't recall. The answer is there.

1) Vector addition of vectors u,v
2) Scalar multiplication of a real number a, and u
3) A vector space has to have a zero vector
 
Since AX=B does not satisfy 3 because if X=0 then B must be a zero vector then it is not a vector space. Am I correct?
 
  • #10
Jaglowsd said:
Since AX=B does not satisfy 3 because if X=0 then B must be a zero vector then it is not a vector space. Am I correct?

Yes, you are correct. Some other properties don't work either as Halls pointed out. But that's the easiest one to check.
 
  • #11
Much appreciated for the help from the both of you.
 

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