Symbols:Abbreviations/L/LQR
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Abbreviation: LQR
Let $p$ and $q$ be distinct odd primes.
Then:
- $\paren {\dfrac p q} \paren {\dfrac q p} = \paren {-1}^{\dfrac {\paren {p - 1} \paren {q - 1} } 4}$
where $\paren {\dfrac p q}$ and $\paren {\dfrac q p}$ are defined as the Legendre symbol.
An alternative formulation is: $\paren {\dfrac p q} = \begin{cases} \quad \paren {\dfrac q p} & : p \equiv 1 \lor q \equiv 1 \pmod 4 \\ -\paren {\dfrac q p} & : p \equiv q \equiv 3 \pmod 4 \end{cases}$
The fact that these formulations are equivalent is immediate.
This fact is known as the Law of Quadratic Reciprocity, or LQR for short.