Is \(\mathbb{Q}(\sqrt{5})\) a Unique Factorization Domain?

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Discussion Overview

The discussion centers on whether the field \(\mathbb{Q}(\sqrt{5})\) is a Unique Factorization Domain (UFD). Participants explore the implications of unique factorization in this context, particularly in relation to the proof methods and comparisons with other known cases, such as \(\mathbb{Q}(\sqrt{-5})\).

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant notes that while \(\mathbb{Q}(\sqrt{-5})\) is not a UFD due to distinct prime factorizations, they are interested in establishing whether \(\mathbb{Q}(\sqrt{5})\) is a UFD.
  • Another participant suggests that the proof for \(\mathbb{Q}(\sqrt{5})\) might follow a similar structure to the proof of the fundamental theorem of arithmetic, focusing on the uniqueness aspect.
  • A participant questions whether it is sufficient to demonstrate that a general element \(a + b\sqrt{5}\) can have two distinct prime factorizations that differ by a unit.
  • One participant proposes that if unique factorization can be shown for integers, it should extend to rational coefficients by manipulating denominators, thus reducing the problem to integers.
  • Concerns are raised about applying the uniqueness proof to negative integers and whether the argument holds under such conditions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether \(\mathbb{Q}(\sqrt{5})\) is a UFD, and multiple approaches and uncertainties regarding the proof methods are present.

Contextual Notes

There are limitations regarding the assumptions made about coefficients and the treatment of negative integers, which remain unresolved in the discussion.

math_grl
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So you see it all over the place, \mathbb{Q}(\sqrt{-5}) is not a UFD by finding an element such that it has two distinct prime factorizations...but what about showing that \mathbb{Q}(\sqrt{5}) is a UFD?

I'm only concerned with this particular example, I might have questions later on regarding a general method.
 
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Isn't the proof goes like the proof of the fundamental theorem of arithmetic? the uniqueness part.
 
I guess it could be. So you are saying that you take some random element a + b\sqrt{5} \in \mathbb{Q}(\sqrt{5}) and claim there are two distinct prime factorizations and show they actually differ by a unit?
 
Well if you show it for the integers coeffiecients then it will obviously follow for rational coeffiecnts (I am referring to a,b), by taking common denominator in both sides and then multiply by the lcm of the two denominators of both sides we reducing the problem to integer coeffiecints.

I believe that because Q(\sqrt 5) is spanned by 1 and \sqrt 5
It's enough to show that (a+b\sqrt 5) c \sqrt 5=d+e\sqrt 5 and (a+b\sqrt 5) c =d+e\sqrt 5, where all the parameters are integers, is uniquely factorised (this obviously follows from the uniquness in simple integers).
Obviously in the proof we should impose the condition that gcd(a,b,c)=1 otherwise we can divide both sides by gcd(a,b,c).

I am just not sure how this will follow for negative integers, maybe this works upto a sign in the whole integers.
 

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