Rank(A) + nullity(A) = no. of cols of A (WHY?)

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Discussion Overview

The discussion revolves around the rank-nullity theorem in linear algebra, specifically addressing why the sum of the rank and nullity of a matrix equals the number of its columns. Participants express confusion about this relationship and seek clarification without relying on linear transformations.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant expresses confusion about the rank-nullity theorem and its relation to the number of columns in a matrix.
  • Another suggests that a proof can be found online and mentions that understanding will improve with familiarity with linear algebra concepts like nullspaces.
  • A participant requests an explanation that does not involve linear transformations, indicating their current module has not covered that topic yet.
  • One participant emphasizes that the rank-nullity theorem should be an intuitive part of linear algebra and suggests reviewing previous exercises to understand it better.
  • Another participant mentions that understanding will become clearer once linear operators and their matrix representations are learned, and suggests looking into Gaussian elimination as a way to investigate the theorem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to explain the theorem without using linear transformations. There are multiple viewpoints on the best approach to understanding the relationship between rank, nullity, and the number of columns.

Contextual Notes

Some participants indicate limitations in their current understanding due to the progression of their coursework, which has not yet covered certain foundational concepts like linear transformations.

nyxynyx
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Hello! I am confused over why rank(A) + nullity(A) = n = no. of columns of A, not no. of rows or something else.

My lecturer showed me something like a mxn matrix postmultiplied with a x-vector to get R^n, thus n = no. of cols. Makes sense when he was explaining but when i stepped out i realized that i didnt get it. Any help pls? Thanks!
 
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You can probably find a proof somewhere online.

It's pretty quite intuitive after you get further into Linear Algebra and have become more comfortable with Nullspaces and such. I'll find a link with a proof.
 
I can't one what doesn't use linear transformations!
 
My module hasn't reached transformations yet :(. Is there a explanation why its equal to no. of columns without talking about transformation?
 
In my humble opinion, the rank-nullity theorem is not something you really want to "explain" -- it should be part of the foundation of your intuition for linear algebra. If you're looking for understanding, your best bet is probably to review previous exercises where you actually solved systems of equations and apply the rank-nullity theorem to describe the system.

e.g. you may have done an exercise solving Ax=b, where A is 3x3, and got a two-dimensional space of solutions. The rank-nullity theorem says that the rank of A is one -- so confirm that by computing the rank of A!
 
Hurkyl said:
In my humble opinion, the rank-nullity theorem is not something you really want to "explain" -- it should be part of the foundation of your intuition for linear algebra. If you're looking for understanding, your best bet is probably to review previous exercises where you actually solved systems of equations and apply the rank-nullity theorem to describe the system.

e.g. you may have done an exercise solving Ax=b, where A is 3x3, and got a two-dimensional space of solutions. The rank-nullity theorem says that the rank of A is one -- so confirm that by computing the rank of A!

Exactly. It comes around. If it hasn't yet, continue solving systems. :biggrin:
 
Once you learn something about linear operators and their matrix representation, it should become formally clear.

Edit: actually, you can investigate this fact by going into Gaussian elimination.
 
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