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The law of quadratic reciprocity is a theorem in number theory that describes the conditions under which a quadratic congruence has solutions. Specifically, it relates the solvability of the congruence (x^2 \equiv p \pmod{q}) to the solvability of (x^2 \equiv q \pmod{p}), where (p) and (q) are distinct prime numbers.1

This law was first observed by Adrien-Marie Legendre and later rigorously proven by Carl Friedrich Gauss. It is a fundamental result in the study of quadratic residues and has connections to other areas of number theory, including the theory of quadratic forms and algebraic integers. The law of quadratic reciprocity is considered a cornerstone of elementary number theory.1

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