Having trouble understanding minimal polynomial problems

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Discussion Overview

The discussion revolves around the relationship between minimal polynomials and matrix representations. Participants explore whether a matrix can be uniquely determined from its minimal polynomial and the implications of eigenvalue multiplicities.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant expresses curiosity about deriving a matrix representation from a given minimal polynomial, questioning how this is possible given the potential loss of repeated eigenvalues.
  • Another participant asserts that multiple distinct matrices can share the same minimal polynomial, indicating that there is no unique matrix representation for a minimal polynomial.
  • It is noted that even with all eigenvalues having multiplicity 1, different eigenvectors could lead to different matrices.
  • A later reply acknowledges the initial participant's inquiry as valid and explains that similar matrices share the same minimal polynomial.
  • Participants discuss the construction of rational canonical forms from minimal polynomials, emphasizing that knowing only the minimal polynomial does not suffice to determine a unique form.
  • One participant suggests a specific example involving polynomials modulo a given polynomial and multiplication by x, indicating that the resulting matrix will have the desired minimal polynomial, though it is not unique.

Areas of Agreement / Disagreement

Participants generally agree that a minimal polynomial does not uniquely determine a matrix, with multiple competing views on how minimal polynomials relate to matrix representations and the implications of eigenvalue multiplicities.

Contextual Notes

Participants highlight limitations in deriving matrix representations solely from minimal polynomials, noting the need for additional information to identify specific matrix forms.

catsarebad
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i understand how to find minimal poly. if a matrix is given. i am curious if you can find the matrix representation if minimal polynomial is given.

i'm not exactly sure how you could since you can possibly lose repeated e-values when you write minimal polynomial. how can u create a n dimensional matrix from a polynomial with possibly lower power? idk just a dumb thought.
 
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Two distinct matrices (even of different sizes) can have the same minimal polynomial. So there is no "matrix representation" of a minimal polynomial.
 
Even if all eigenvalues were of "multiplicity" 1, they could correspond to different eigenvectors so to different matrices.
 
catsarebad said:
i understand how to find minimal poly. if a matrix is given. i am curious if you can find the matrix representation if minimal polynomial is given.

i'm not exactly sure how you could since you can possibly lose repeated e-values when you write minimal polynomial. how can u create a n dimensional matrix from a polynomial with possibly lower power? idk just a dumb thought.

I wouldn't call it a dumb thought, it is good that you are trying to develop a deeper understanding.

As was mentioned by other posters, distinct matrices can have the same minimal polynomial. In fact, similar matrices have the same minimal polynomial.

Given a minimal polynomial, you can construct particularly nice block matrices which are called rational canonical forms. Now for a given minimal polynomial for some nxn matrix, there is potentially more than one rational canonical forms you could construct. You would need to know more than just the minimal polynomial for your given transformation to determine which rat. canon. form is right for your transformation.
 
Let your vector space be polynomials modulo then given polynomial. Let the operator be multiplication by x. The matrix of the operator will have the desires minimal polynomial (of course not unique).
 

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