Jacobi Symbol

Definition (Jacobi symbol)

Let bZb\in\mathbb{Z} and nNn\in\mathbb{N} odd. We factor nn as a unique product of prime powers: n=p1α1prαrn=p_{1}^{\alpha_{1}}\dots p_{r}^{\alpha_{r}}We define the Jacobi Symbol as a product of Legendre symbols via (bn):=(bp1)α1(bpr)αr\left( \frac{b}{n} \right):=\left( \frac{b}{p_{1}} \right)^{\alpha_{1}}\dots\left( \frac{b}{p_{r}} \right)^{\alpha_{r}}

Proposition

If nn is odd then the satisfies (1n)=(1)n12\left( \frac{-1}{n} \right)=(-1)^{\frac{n-1}{2}}

Proposition

If nn is odd and positive, then (2n)=(1)n218\left( \frac{2}{n} \right)=(-1)^{\frac{n^{2}-1}{8}}

Theorem (Quadratic Reciprocity of Jacobi)

If m,nm,n are coprime odd integers, then (mn)=(1)(m1)(n1)4(nm) \left( \frac{m}{n} \right)=(-1)^{\frac{(m-1)(n-1)}{4}}\left( \frac{n}{m} \right)