Proving Polynomial Properties: Existence of nxn Matrix in Field k

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Discussion Overview

The discussion revolves around proving the existence of an nxn matrix with entries in a field k that has a given degree n polynomial p(x) as its characteristic polynomial. The scope includes theoretical aspects of linear algebra and polynomial properties.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions how to prove that for any degree n polynomial p(x) with coefficients in a field k, there exists an nxn matrix with characteristic polynomial p.
  • Another participant discusses the characteristic polynomial of a diagonal matrix and notes the complexity of finding matrices for polynomials with roots outside the field, referencing the polynomial x^2 + 1 and its associated matrix with real entries.
  • This participant also mentions the concept of algebraically closed fields and suggests that the proof may be simpler in those cases.
  • A different participant expresses interest in the general case, particularly when the field k is the reals.
  • Another participant suggests looking into companion matrices as a potential approach to the problem.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of the problem, particularly regarding the existence of matrices for polynomials with roots outside the field. There is no consensus on a definitive approach or solution.

Contextual Notes

Participants note limitations regarding the generalizability of results, particularly in relation to the field being algebraically closed and the implications for matrices with real entries.

DeadWolfe
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How might one prove that for any degree n polynomial p(x) with coefficients lying in a field k, there exists any nxn matrix with entries in k with characteristic polynomial p?
 
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If you have a diagonal matrix with entries [itex]\lambda_1, \lambda_2, \cdots, \lambda_n[/itex] then what is its characteristic polynomial?

[addition]It's not as easy as I thought. For example, the polynomial [itex]x^2 + 1[/itex] has real coefficients, two complex roots, and there is a matrix with real entries - namely [[0 1] [-1 0]] - of which it is the characteristic polynomial. So it's not as easy as just taking diag(first root, second root, ...) in the case where the roots can be outside the field (this has a name, I think it is "algebraically closed"?). Of course changing the basis does not change the characteristic polynomial and in my example that is the trick to get a real matrix but you'd have to prove that it is generally possible). But the proof is easy for algebraically closed fields (if that is indeed the name).
 
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Yes, but of course I was interested in the general case (and particularly when k=reals)
 
Look into companion matrices.
 

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