Solving Isomorphic Rings: \mathbb{F}_5[x]/(x^2+2) & \mathbb{F}_5[x]/(x^2+3)

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Discussion Overview

The discussion revolves around the isomorphism between the rings \(\mathbb{F}_5[x]/(x^2+2)\) and \(\mathbb{F}_5[x]/(x^2+3)\). Participants explore the requirements for establishing a ring homomorphism and the necessary properties of the mapping.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests finding a homomorphism \(\phi:R\to S\) that is linear and satisfies \(\phi(1)=1\).
  • Another participant questions what property the image of \(x\) must satisfy for the isomorphism.
  • A different participant proposes that mapping \(x\) to \(2x\) might work, providing a series of equivalences to support this idea.
  • One participant mentions that if an explicit isomorphism is not required, the isomorphism can be inferred from the fact that both fields are generated by adjoining a root of an irreducible quadratic to a field of order 5, leading to both having 25 elements.
  • Another participant confirms that an explicit isomorphism must be shown, indicating a requirement to demonstrate the isomorphism 'the long way.'

Areas of Agreement / Disagreement

Participants express differing views on whether an explicit isomorphism is necessary, with some suggesting that the isomorphism can be inferred from properties of finite fields, while others insist on the need for a detailed construction.

Contextual Notes

There is an assumption that the properties of the mappings and the structure of the rings are understood, but the discussion does not resolve the specific steps needed to construct the isomorphism explicitly.

Who May Find This Useful

Readers interested in ring theory, particularly those studying isomorphisms in finite fields and polynomial rings.

JackTheLad
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Hi guys,

I'm trying to show that \mathbb{F}_5[x]/(x^2+2) and \mathbb{F}_5[x]/(x^2+3) are isomorphic as rings.

As I understand it, I have to find the homomorphism \phi:R\to S which is linear and that \phi(1)=1.

I'm just struggling to find what I need to send x to in order to get this work.
 
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Well, what property must the image of x satisfy?


If all else fails, there aren't many possibilities, you could just try them all.
 
Actually, I think x --> 2x might do it, because

x^2 + 2 \equiv 0
(2x)^2 + 2 \equiv 0
4x^2 + 2 \equiv 0
4(x^2 + 3) \equiv 0
x^2 + 3 \equiv 0

Is that all that's required?
 
Do you have to provide an explicit isomorphism? If not you can just use the fact that finite fields with the same cardinality are isomorphic...both of these fields are generated by adjoining a root of an irreducible quadratic to a field of order 5, and hence both have 25 elements.
 
Yeah, unfortunately I do have to show the explicit isomorphism (we're supposed to do it 'the long way')
 

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