# Power residue symbol

In algebraic number theory the n-th power residue symbol (for an integer n > 2) is a generalization of the (quadratic) Legendre symbol to n-th powers. These symbols are used in the statement and proof of cubic, quartic, Eisenstein, and related higher reciprocity laws.

## Background and notation

Let k be an algebraic number field with ring of integers ${\mathcal {O}}_{k}$ that contains a primitive n-th root of unity $\zeta _{n}.$ Let ${\mathfrak {p}}\subset {\mathcal {O}}_{k}$ be a prime ideal and assume that n and ${\mathfrak {p}}$ are coprime (i.e. $n\not \in {\mathfrak {p}}$ .)

The norm of ${\mathfrak {p}}$ is defined as the cardinality of the residue class ring (note that since ${\mathfrak {p}}$ is prime the residue class ring is a finite field):

$\mathrm {N} {\mathfrak {p}}:=|{\mathcal {O}}_{k}/{\mathfrak {p}}|.$ An analogue of Fermat's theorem holds in ${\mathcal {O}}_{k}.$ If $\alpha \in {\mathcal {O}}_{k}-{\mathfrak {p}},$ then

$\alpha ^{\mathrm {N} {\mathfrak {p}}-1}\equiv 1{\bmod {\mathfrak {p}}}.$ And finally, suppose $\mathrm {N} {\mathfrak {p}}\equiv 1{\bmod {n}}.$ These facts imply that

$\alpha ^{\frac {\mathrm {N} {\mathfrak {p}}-1}{n}}\equiv \zeta _{n}^{s}{\bmod {\mathfrak {p}}}$ is well-defined and congruent to a unique $n$ -th root of unity $\zeta _{n}^{s}.$ ## Definition

This root of unity is called the n-th power residue symbol for ${\mathcal {O}}_{k},$ and is denoted by

$\left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}=\zeta _{n}^{s}\equiv \alpha ^{\frac {\mathrm {N} {\mathfrak {p}}-1}{n}}{\bmod {\mathfrak {p}}}.$ ## Properties

The n-th power symbol has properties completely analogous to those of the classical (quadratic) Legendre symbol ($\zeta$ is a fixed primitive $n$ -th root of unity):

$\left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}={\begin{cases}0&\alpha \in {\mathfrak {p}}\\1&\alpha \not \in {\mathfrak {p}}{\text{ and }}\exists \eta \in {\mathcal {O}}_{k}:\alpha \equiv \eta ^{n}{\bmod {\mathfrak {p}}}\\\zeta &\alpha \not \in {\mathfrak {p}}{\text{ and there is no such }}\eta \end{cases}}$ In all cases (zero and nonzero)

$\left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}\equiv \alpha ^{\frac {\mathrm {N} {\mathfrak {p}}-1}{n}}{\bmod {\mathfrak {p}}}.$ $\left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}\left({\frac {\beta }{\mathfrak {p}}}\right)_{n}=\left({\frac {\alpha \beta }{\mathfrak {p}}}\right)_{n}$ $\alpha \equiv \beta {\bmod {\mathfrak {p}}}\quad \Rightarrow \quad \left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}=\left({\frac {\beta }{\mathfrak {p}}}\right)_{n}$ ## Relation to the Hilbert symbol

The n-th power residue symbol is related to the Hilbert symbol $(\cdot ,\cdot )_{\mathfrak {p}}$ for the prime ${\mathfrak {p}}$ by

$\left({\frac {\alpha }{\mathfrak {p}}}\right)_{n}=(\pi ,\alpha )_{\mathfrak {p}}$ in the case ${\mathfrak {p}}$ coprime to n, where $\pi$ is any uniformising element for the local field $K_{\mathfrak {p}}$ .

## Generalizations

The $n$ -th power symbol may be extended to take non-prime ideals or non-zero elements as its "denominator", in the same way that the Jacobi symbol extends the Legendre symbol.

Any ideal ${\mathfrak {a}}\subset {\mathcal {O}}_{k}$ is the product of prime ideals, and in one way only:

${\mathfrak {a}}={\mathfrak {p}}_{1}\cdots {\mathfrak {p}}_{g}.$ The $n$ -th power symbol is extended multiplicatively:

$\left({\frac {\alpha }{\mathfrak {a}}}\right)_{n}=\left({\frac {\alpha }{{\mathfrak {p}}_{1}}}\right)_{n}\cdots \left({\frac {\alpha }{{\mathfrak {p}}_{g}}}\right)_{n}.$ For $0\neq \beta \in {\mathcal {O}}_{k}$ then we define

$\left({\frac {\alpha }{\beta }}\right)_{n}:=\left({\frac {\alpha }{(\beta )}}\right)_{n},$ where $(\beta )$ is the principal ideal generated by $\beta .$ Analogous to the quadratic Jacobi symbol, this symbol is multiplicative in the top and bottom parameters.

• If $\alpha \equiv \beta {\bmod {\mathfrak {a}}}$ then $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}=\left({\tfrac {\beta }{\mathfrak {a}}}\right)_{n}.$ • $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}\left({\tfrac {\beta }{\mathfrak {a}}}\right)_{n}=\left({\tfrac {\alpha \beta }{\mathfrak {a}}}\right)_{n}.$ • $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}\left({\tfrac {\alpha }{\mathfrak {b}}}\right)_{n}=\left({\tfrac {\alpha }{\mathfrak {ab}}}\right)_{n}.$ Since the symbol is always an $n$ -th root of unity, because of its multiplicativity it is equal to 1 whenever one parameter is an $n$ -th power; the converse is not true.

• If $\alpha \equiv \eta ^{n}{\bmod {\mathfrak {a}}}$ then $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}=1.$ • If $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}\neq 1$ then $\alpha$ is not an $n$ -th power modulo ${\mathfrak {a}}.$ • If $\left({\tfrac {\alpha }{\mathfrak {a}}}\right)_{n}=1$ then $\alpha$ may or may not be an $n$ -th power modulo ${\mathfrak {a}}.$ ## Power reciprocity law

The power reciprocity law, the analogue of the law of quadratic reciprocity, may be formulated in terms of the Hilbert symbols as

$\left({\frac {\alpha }{\beta }}\right)_{n}\left({\frac {\beta }{\alpha }}\right)_{n}^{-1}=\prod _{{\mathfrak {p}}|n\infty }(\alpha ,\beta )_{\mathfrak {p}},$ whenever $\alpha$ and $\beta$ are coprime.