Linear transformation and find basis

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Homework Help Overview

The discussion revolves around finding a basis for R² such that the linear transformation defined by the matrix A results in a diagonal representation. The matrix A is given as a 2x2 matrix with specific entries, and participants are exploring the eigenvalues and corresponding eigenvectors necessary for diagonalization.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the identification of eigenvalues, with some initially misinterpreting the entries of the matrix as eigenvalues. There is an exploration of the implications of not obtaining a linearly dependent system for certain eigenvalues. Questions arise regarding the necessity of solving for eigenvalues rather than relying on diagonal entries.

Discussion Status

The conversation is active, with participants providing feedback on each other's reasoning. Some guidance has been offered regarding the correct approach to finding eigenvalues, and there is an acknowledgment of misunderstandings related to the properties of diagonal matrices. Multiple interpretations of the problem are being explored, particularly concerning the definitions and requirements for a basis in R².

Contextual Notes

There is a noted confusion regarding the nature of the matrix A, as it is not diagonal, which is central to the problem. Participants are also grappling with the definitions of eigenvalues and the requirements for constructing a basis from eigenvectors.

fk378
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Homework Statement


Define T: R2-->R2 by T(x)=Ax
Find a basis B for R2 with the property that [T]_B is diagonal.

A=
0 1
-3 4


The Attempt at a Solution


The eigenvalues of a diagonal matrix are its diagonal entries, so here the eigenvalues are 1, and -3. For eigenvalue=1 I get the basis [1,1] and for eigenvalue -3 I do not get a linearly dependent system. However, accidentally solved the system for eigenvalue=3 and got the basis [1,3] which is supposed to be correct. Can anyone see what I'm doing wrong?
 
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The eigenvalues are 3 and 1. Not -3 and 1. If for -3 you don't get a linearly dependent system doesn't that make you wonder what's going wrong? Start by fixing that. I really don't think you needed to post this question.
 
Last edited:
Oh so I would need to solve for the eigenvalues here, not just look at the diagonals? I think I confused this matrix with a triangular matrix.
 
Of course you need to solve for the eigenvalues. I don't know what sort of rule you made up to read them off of the opposite diagonal, but it's clearly not right. Like I said, the fact you didn't get a linearly dependent system should immediately say, that's not an eigenvalue!
 
fk378 said:

Homework Statement


Define T: R2-->R2 by T(x)=Ax
Find a basis B for R2 with the property that [T]_B is diagonal.

A=
0 1
-3 4


The Attempt at a Solution


The eigenvalues of a diagonal matrix are its diagonal entries, so here the eigenvalues are 1, and -3.
Yes, the eigenvalues of a diagonal matrix the entries on the main diagonal. But this is NOT a diagonal matrix- you are asked to change it into one. Furthermore, -3 and 1 are not on the main diagonal!

For eigenvalue=1 I get the basis [1,1] and for eigenvalue -3 I do not get a linearly dependent system. However, accidentally solved the system for eigenvalue=3 and got the basis [1,3] which is supposed to be correct. Can anyone see what I'm doing wrong?
What do MEAN by "the basis [1,3]"? That's a single vector. A basis for a two dimensional vector space consists of two vectors.
 

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