Quadratic reciprocity

Definition

Quadratic reciprocity is a theorem in number theory that gives a criterion for determining the solvability of quadratic equations modulo prime numbers. It establishes a fascinating relationship between the solvability of two different quadratic equations, revealing how the properties of prime numbers interact in modular arithmetic. This theorem is one of the cornerstones of number theory and connects deeply with concepts like modular arithmetic, prime factorization, and the distribution of prime numbers.

5 Must Know Facts For Your Next Test

1. The law of quadratic reciprocity states that for distinct odd primes p and q, if p is a quadratic residue modulo q, then q is a quadratic residue modulo p, and vice versa.
2. The theorem can be expressed using the Legendre symbol: (p/q)(q/p) = (-1)^{((p-1)(q-1)/4)}.
3. Quadratic reciprocity was first conjectured by Leonhard Euler and proved by Carl Friedrich Gauss in his work 'Disquisitiones Arithmeticae'.
4. There are several generalizations of quadratic reciprocity, including cubic and higher reciprocity laws, which extend its principles to other polynomial forms.
5. Understanding quadratic reciprocity requires knowledge of modular arithmetic, particularly how to compute quadratic residues and non-residues for various primes.

Review Questions

• How does the law of quadratic reciprocity connect the properties of two distinct odd primes?
  • The law of quadratic reciprocity establishes a direct link between two distinct odd primes p and q regarding their quadratic residues. It states that if p is a quadratic residue modulo q, then q is also a quadratic residue modulo p, creating an elegant symmetry between these primes. This relationship highlights how the behavior of primes under modular arithmetic can reflect each other's properties.
• Describe the significance of the Legendre symbol in relation to quadratic reciprocity and provide an example of its use.
  • The Legendre symbol serves as a crucial tool in number theory for encapsulating whether a number is a quadratic residue modulo a prime. In terms of quadratic reciprocity, it allows us to formulate the relationship between two primes succinctly: (p/q)(q/p) = (-1)^{((p-1)(q-1)/4)}. For example, if we evaluate (5/7), we determine whether 5 is a residue modulo 7, helping us understand their interrelationships under the rules established by quadratic reciprocity.
• Critically evaluate how Gauss's proof of quadratic reciprocity influenced later developments in number theory.
  • Gauss's proof of quadratic reciprocity laid foundational groundwork for modern number theory by introducing rigorous techniques that are still relevant today. His approach not only provided insight into the behavior of primes but also sparked interest in further exploration of reciprocity laws, leading to advancements in algebraic number theory. The methods developed by Gauss inspired later mathematicians to generalize these concepts and apply them in various fields, showcasing the theorem's lasting impact on mathematical research and discovery.

Related terms

Legendre symbol: A notation that encodes whether a number is a quadratic residue modulo a prime, defined as (a/p) where 'a' is the integer and 'p' is the prime.

Quadratic residue: An integer 'a' is a quadratic residue modulo 'p' if there exists an integer 'x' such that x² ≡ a (mod p).

Gauss's Lemma: A criterion for determining the value of the Legendre symbol based on the number of quadratic residues among a set of integers.