Is There a Proof for p=1 mod 4 if p|x^2+1?

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SUMMARY

The discussion centers on proving that if an odd prime p divides the expression n = x² + 1, then p must satisfy the condition p ≡ 1 mod 4. The participants explore various modular arithmetic approaches, particularly focusing on the implications of p being congruent to 3 mod 4. A hint regarding the order of x in the multiplicative group of integers modulo p is suggested as a potential pathway to the proof.

PREREQUISITES
  • Understanding of modular arithmetic, specifically congruences.
  • Familiarity with prime numbers and their properties.
  • Knowledge of the structure of the multiplicative group of integers modulo p.
  • Basic concepts of quadratic residues and non-residues.
NEXT STEPS
  • Study the properties of quadratic residues in modular arithmetic.
  • Learn about the structure and order of elements in the group ##\mathbb{Z}/p\mathbb{Z}^\times##.
  • Research proofs related to quadratic forms and their implications in number theory.
  • Explore the implications of Fermat's theorem on sums of two squares.
USEFUL FOR

Mathematicians, number theorists, and students studying modular arithmetic and prime number properties will benefit from this discussion.

TDA120
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Homework Statement



Let n be a whole number of the form ##n=x^2+1## with ##x \in Z##, and p an odd prime that divides n.
Proof: ##p \equiv 1 \mod 4##.

Homework Equations



The Attempt at a Solution



The only relevant case is if p=3 mod 4.

If I try to calculate mod 3, or mod 4, or mod p, I'm not getting anywhere.
 
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Welcome to PF, TDA120! :smile:

Someone gave me a hint: what is the order of x in ##\mathbb{Z}/p\mathbb{Z}^\times##? :wink:

Happy biking!
 

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