Solving Square Matrix Similarity to Diagonal Matrix

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Homework Help Overview

The problem involves a square matrix A that satisfies the equation A^2 - 8A + 15I = 0. The tasks include demonstrating that A is similar to a diagonal matrix and showing the existence of matrices with specific trace values for positive integers k ≥ 8.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the implications of rewriting the original equation and question the validity of the approach for different matrix sizes.
  • Some suggest using the Cayley-Hamilton theorem and minimal polynomials to analyze the characteristic equation of A.
  • There are discussions about eigenvalues and eigenvectors, with some participants expressing uncertainty about how to demonstrate the existence of a full set of eigenvectors.
  • Questions arise regarding the relationship between the minimal polynomial and the eigenvalues of A, as well as the implications for diagonalizability.

Discussion Status

The discussion is ongoing, with various participants contributing ideas and questioning assumptions. Some guidance has been offered regarding the use of minimal polynomials and the Cayley-Hamilton theorem, but there is no explicit consensus on the methods or interpretations being explored.

Contextual Notes

Participants note that the original equation may only apply to 2x2 matrices, leading to confusion about its validity for n x n matrices. There is also mention of the complexity of higher-order cases and the implications of eigenvalue multiplicity.

  • #31
Well, I believe we have the answer for (a), don't you?

As for (b), I believe we can use the fact that any diagonal matrix with only 3's and 5's on the diagonal should be a solution for the equation...
 
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  • #32
I like Serena said:
Well, I believe we have the answer for (a), don't you?

As for (b), I believe we can use the fact that any diagonal matrix with only 3's and 5's on the diagonal should be a solution for the equation...

Aah, thus (b) turns out to the following problem: for each k>8, prove that k can be written as the sum of 3's and 5's.

This should be fun :biggrin:
 
  • #33
micromass said:
Aah, thus (b) turns out to the following problem: for each k>8, prove that k can be written as the sum of 3's and 5's.

This should be fun :biggrin:

Hey! What about k=8? :wink:
 
  • #34
I like Serena said:
Hey! What about k=8? :wink:

Aaaaah, of course, k=8 is the most important one!

Hint for the OP: first try to write k=8,9,...,15 as the sum of 3's and 5's. The rest of the numbers are easy (so I claim)
 
  • #35
I'm confused as to what k actually is
 
  • #36
Maybe_Memorie said:
I'm confused as to what k actually is

Your problem states:

"(b) Show that for every positive integer k >= 8 there exists a matrix A
satisfying the above condition with tr(A) = k."


So let's start with k=8.
Can you find an nxn matrix A with tr(A)=8 that satisfies: A^2 - 8A + 15I = 0?

If you can, then the next question is:
Suppose k=9.
Can you find an nxn matrix A with tr(A)=9 that satisfies: A^2 - 8A + 15I = 0?

...
 
  • #37
8 = 3 + 5
9 = 3 + 3 + 3
10 = 5 + 5
11 = 3 + 3 + 5
12 = 3 + 3 + 3
13 = 5 + 5 + 3
14 = 5 + 3 + 3 + 3
15 = 5 + 5 + 5

Every k>15 will be a sum of previous numbers >8, so therefore it is clear to see that there is a matrix with the required condition with tr(A)=k and k>8
 
  • #38
Maybe_Memorie said:
12 = 3 + 3 + 3

Forgot a +3 here. But the rest is totally correct!
 

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