Quadratic Integers: Understanding the Theorem and Proving 32 = ab in Q[sqrt -1]

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Discussion Overview

The discussion revolves around the properties of quadratic integers in the quadratic field Q[sqrt -1], specifically focusing on a theorem related to their forms based on congruences mod 4. Participants explore the implications of this theorem, the proof of the equation 32 = ab for relatively prime quadratic integers, and the concept of relative primality among certain quadratic expressions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire whether the theorem about quadratic integers implies that if a and b are rational integers and the quadratic number conforms to its congruency mod 4, then the quadratic number is an integer.
  • There is a question about how to prove that 32 = ab where a and b are relatively prime quadratic integers in Q[sqrt -1], with a being expressed as e(g^2) where e is a unit and g is a quadratic integer.
  • One participant suggests looking for two quadratic integers of the form A +/- B(sqrt -1) to find products that equal an integer, indicating a method to cancel the quadratic part.
  • Participants discuss the relative primality of expressions a + b sqrt d and a - b sqrt d, questioning whether they are relatively prime when a, b, and d are rational integers and d is not a perfect square.
  • Another participant expresses uncertainty about the conditions under which two quadratic integers are relatively prime, suggesting that if certain factors are relatively prime to a, then the two expressions might be relatively prime, but acknowledges the need for further exploration.

Areas of Agreement / Disagreement

The discussion contains multiple competing views and uncertainties regarding the implications of the theorem, the proof of the equation, and the concept of relative primality among quadratic integers. No consensus is reached on these points.

Contextual Notes

Participants express various assumptions about the forms of quadratic integers and their properties, but these assumptions remain unresolved. The discussion also highlights the dependence on specific definitions and the need for further clarification on the conditions for relative primality.

squelchy451
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For the theorem that states that in quadratic field Q[sqrt d], if d is congruent to 1 mod 4, then it is in the form (a + b sqrt d)/2 and if it's not, it's in the form a + b sqrt d where a and b are rational integers, is it saying that if a and b are rational integers and the quadratic number are in the form according to its congruency mod 4, then the quadratic number is an integer?

Also, how would you prove that if 32 = ab where a and b are relatively prime quadratic integers in Q[sqrt -1], a = e(g^2) where e is a unit and g is a quadratic integer in Q [sqrt -1].
 
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squelchy451 said:
For the theorem that states that in quadratic field Q[sqrt d], if d is congruent to 1 mod 4, then it is in the form (a + b sqrt d)/2 and if it's not, it's in the form a + b sqrt d where a and b are rational integers, is it saying that if a and b are rational integers and the quadratic number are in the form according to its congruency mod 4, then the quadratic number is an integer?

Also, how would you prove that if 32 = ab where a and b are relatively prime quadratic integers in Q[sqrt -1], a = e(g^2) where e is a unit and g is a quadratic integer in Q [sqrt -1].

When you have the product of two quadratic integers equal an integer, look first for two of the form A +/- B(sqrt -1). That is one use negative B and the other uses positive B so that the quadratic part cancels out.
 
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On a side note, are a + b sqrt d and a - b sqrt d where a, b, and d are rational integers (and d is not a perfect square) relatively prime?
 
squelchy451 said:
On a side note, are a + b sqrt d and a - b sqrt d where a, b, and d are rational integers (and d is not a perfect square) relatively prime?

Well if 2, b and sqrt d each are relatively prime to a, then I would say that the two factors are relatively prime but I am not sure. However, I think you have to look to cancel the quadratic part in another way since 4 + 4i and 4 - 4i are not relatively prime. Maybe try playing around with numbers 1,16 and i in lieu of 4 and i.
 

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