What to do with a zero column in a system of linear equations?

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Discussion Overview

The discussion revolves around the implications of encountering a zero column in a system of linear equations, particularly regarding the treatment of corresponding variables. Participants explore whether such variables should be excluded from the solution set or considered free variables, with a focus on both homogeneous and inhomogeneous systems.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions the treatment of a zero column corresponding to a variable in a system of linear equations.
  • Another participant suggests that in a homogeneous system, the zero column indicates a free variable, while in an inhomogeneous system, it may lead to a contradiction if the corresponding constant is non-zero.
  • A participant describes their specific case of a 2 x 3 system and questions the necessity of testing for homogeneity given the presence of a zero column.
  • It is proposed that in the described case, the variable corresponding to the zero column is a free variable, and the system has infinitely many solutions, with the zero column variable not affecting the solution set.

Areas of Agreement / Disagreement

Participants express differing views on the implications of a zero column depending on the type of system (homogeneous vs. inhomogeneous), and the discussion remains unresolved regarding the necessity of testing for homogeneity in all cases.

Contextual Notes

Limitations include the lack of discussion on the definitions of homogeneous and inhomogeneous systems, and the specific conditions under which a zero column affects the solution set.

JinM
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Hello, just started a LA course, and I have a small question.

If I get a zero column while solving for a system of linear equations, and that column corresponds to, say, variable x_{5}, do we just not include it in the solution set since it appears in no equation of the system? Or is it a free variable (that has no purpose)?
 
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That depends on what kind of system you have. If it's a homogeneous system (Ax = 0) then you just have 0=0, and x_5 is indeed just a free variable. However, if you have an inhomogeneous (Ax= b) then you have a possibility that 0=b_5 and b_5 =\=0. In this case, you have an obviously false statement, and the system has no solutions.
 
Thanks. We still haven't discussed homogeneity. I don't see, though, how you get 0=0. I have a 2 x 3 system of linear equations and I eventually get this (through Gauss-Jordan),

[tex]x_1 + 3x_4 - x_5 -x_6 = 32[/tex]
[tex]x_2 +2x_4 -x_5 = 28[/tex]

x_3 is missing in both equations, hence the zero column that I mention before appearing in the matrix. Do I still have to test homogeneity to determine the solution set here?
 
In this case, x_3 is just a free variable. Since you only have 2 equations, neither of which gave 0=0 then your system does have a solution(infinitely many solutions in fact). x_3 would still be included in the solution set as a free variable just like x_4-x_6. It just turns out that nothing depends on the value of x_3.
 
Thanks, Vid. :smile:
 

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