Proving Linear System Ax=b Consistent iff Rank A = m

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Discussion Overview

The discussion revolves around the conditions under which the linear system represented by the equation Ax=b is consistent for all column vectors b, specifically exploring the relationship between the rank of matrix A and the consistency of the system. The scope includes theoretical aspects of linear algebra and the implications of matrix rank on solution existence.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants express confusion about the meaning of "consistent for all column vectors b" in the context of the linear system Ax=b.
  • It is suggested that if the system is consistent for all b, then b must lie in the column space of A.
  • One participant notes that a system of linear equations is consistent if it has at least one solution, though this solution may not be unique.
  • Another participant questions what the implications are for the column space of A if b can be any vector.

Areas of Agreement / Disagreement

Participants generally agree on the definition of consistency in relation to solutions of the system, but there is uncertainty regarding the implications of consistency for all b and the nature of the column space of A.

Contextual Notes

There are limitations in understanding the implications of the rank of A and the conditions under which the system is consistent for all b, as well as the dependence on definitions of column space and matrix rank.

eyehategod
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let A be a mxn matrix.
prove that the system of linear equations Ax=b is consistnet for all column vectors b if and only if the rank of A is m.

I have no idea how to start, can anyone helo me out?
 
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what does it mean if the matrix equation is consistent for all vectors b?
 
i guess my problem is that i don't quite understand when it says "consistent for all column vectors b."
 
also it would mean that b is in the column space of A.
 
eyehategod said:
also it would mean that b is in the column space of A.

yes but any b?
 
Last edited:
eyehategod said:
i guess my problem is that i don't quite understand when it says "consistent for all column vectors b."

A system of linear equations is consistent if it has a solution. Of course, this solution need not be unique.
 
radou said:
A system of linear equations is consistent if it has a solution. Of course, this solution need not be unique.
A matrix equation, Ax= b, is "consistent" if it has at least one solution. "Ax= b is consistent for all b" means the equation Ax= b is consistent no matter what vector b is.

The OP said earlier, "also it would mean that b is in the column space of A." Okay. And if b is to be any member of A, what must the column space be?
 

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