Equivalence of Irreducibility Tests for Polynomials Under Variable Substitution

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Discussion Overview

The discussion revolves around the equivalence of irreducibility tests for polynomials when subjected to variable substitutions, specifically examining whether such transformations affect the fundamental nature of the polynomial and the validity of various irreducibility tests. The scope includes theoretical aspects of polynomial algebra and the implications of variable substitutions on polynomial properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that substituting x with y + a in a polynomial does not fundamentally change the polynomial, as it is merely a translation.
  • Others question whether the same holds true for more complex substitutions, such as x = a^n y^n + ... + a_0, and seek theorems that support these claims.
  • A participant introduces the idea that if a polynomial f(x) is irreducible, it may imply that f(x+a) is also irreducible, and discusses the implications of this in terms of polynomial factorization.
  • Another participant mentions an isomorphism between polynomial rings that arises from the substitution, suggesting that irreducibility can be preserved under certain conditions related to field coefficients.

Areas of Agreement / Disagreement

Participants express differing views on the impact of variable substitutions on irreducibility, with some asserting that translations do not alter the polynomial's fundamental properties while others explore the nuances of more complex substitutions. The discussion remains unresolved regarding the general applicability of irreducibility tests under various transformations.

Contextual Notes

Limitations include the lack of consensus on theorems that definitively prove the relationships between irreducibility and variable substitutions, as well as the dependence on the nature of the coefficients involved.

Simfish
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So if we have, say, a polynomial f(x) = a_n x^n + ... + a_0 and, say, use the substitution x = y + a, then do ALL irreducibility tests work the same? And do all OTHER tests also work the same? Is the polynomial FUNDAMENTALLY the same? And what theorem is there to prove this?
 
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Simfish said:
So if we have, say, a polynomial f(x) = a_n x^n + ... + a_0 and, say, use the substitution x = y + a, then do ALL irreducibility tests work the same? And do all OTHER tests also work the same? Is the polynomial FUNDAMENTALLY the same? And what theorem is there to prove this?

It is the same in terms of x. Of course the polynomial in terms of y is a translation of the polynomial which was in terms of x.
 
Okay. It's a translation. Would the translation work even if you try x = a^n y^n + ... + a_0? And are there theorems to prove this?
 
Are you asking if the irreducibility of f(x) implies that of f(x+a)? Well suppose f(x+a) factors nontrivially into g(x)h(x), then what can we say about f(x)=f(x+a-a)?

If you want a more 'high level' explanation of this, and if your coefficients come from a field F, then consider the isomorphism F[x] -> F[x] given by x->x+a. This induces an isomorphism F[x]/<f(x)> =~ F[x]/<f(x+a)>. f(x) is irreducible iff F[x]/<f(x)> is a field iff f(x+a) is irreducible.
 
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