Basis of Nullspace: Linear Algebra & Differential Equations

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    Basis Nullspace
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Discussion Overview

The discussion revolves around the concept of nullspace in linear algebra, particularly in relation to the properties of matrices, including invertibility and the implications of a zero determinant. Participants explore the definitions and relationships between nullspace, invertibility, and linear transformations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion regarding the relationship between the determinant of a matrix and the existence of a basis for its nullspace, initially stating that a zero determinant implies the matrix is invertible.
  • Another participant corrects this by stating that a zero determinant indicates the matrix is not invertible.
  • A later post clarifies that if a matrix is invertible, the only solution to the equation Ax = 0 is the trivial solution x = 0, suggesting that the nullspace is trivial in this case.
  • Participants discuss the implications of linear transformations and how solutions to Ax = r relate to the nullspace, questioning how to express the relationship between A, its inverse, and the nullspace.

Areas of Agreement / Disagreement

There is a disagreement regarding the initial claim about the determinant and invertibility. While one participant initially states that a zero determinant indicates invertibility, this is corrected by another participant. The discussion reflects varying levels of understanding about the implications of invertibility on the nullspace.

Contextual Notes

Participants have not fully resolved the implications of their statements regarding the nullspace and invertibility, and there are assumptions about the definitions of these terms that remain unexamined.

Who May Find This Useful

This discussion may be useful for students learning about linear algebra, particularly those grappling with the concepts of nullspace, matrix invertibility, and linear transformations.

dschmidt12
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I am in a linear algebra and differential equations course and have recently been learning how to find a basis for a nullspace, row space, or column space. However, I am EXTREMELY confused by a solution to a question in my textbook. The question asks to find the basis for the null space of a particular matrix. For the question at hand, the determinant is zero, which of course means that it is invertible. However, I'm confused as to why this means that there is no basis for this nullspace -- there is no relevant explanation in my text as far as I can tell. Thank you to anyone who can help solve this problem for me!
 
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If the determinant is zero then the matrix is not invertible
 
I'm sorry, I meant to write not zero...my mistake; that makes a big difference. :)
 
This all stems from the fact that matrices distribute over vectors and commute with scalar multiplication: i.e, the fact that matrices are linear transformations.

If Av=0 and Ax=r then A(v + x) = Av + Ax = 0 + r = r.

If I have one solution to Ax = r, then I also have a solution for every element of the nullspace. In this situation, how would I write A-1? Is it A-1r = x or A-1r = x + v? They both get sent to r by A.

This means that a matrix can only be invertible if the nullspace is trivial, in other words, if only zero gets sent to zero.
 
"Nullspace of A" is, by definition, the set of vectors, x, such that Ax= 0. If A has an inverse, take A^{-1} on both sides: A^{-1}Ax= x= A^{-1}0= 0.

That is, if A is invertible, Ax= 0 only for x= 0.

It is true, then that if A is invertible, it is one-to-one.
 

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