cubic reciprocity law

In a ring $\mathbb{Z}/n\mathbb{Z}$, a cubic residue is just a value of the function $x^3$ for some invertible element $x$ of the ring. Cubic residues display a reciprocity phenomenon similar to that seen with quadratic residues. But we need some preparation in order to state the cubic reciprocity law.

$\omega$ will denote $\frac{-1+i\sqrt{3}}{2}$, which is one of the complex cube roots of $1$. $K$ will denote the ring $K=\mathbb{Z}[\omega ]$. The elements of $K$ are the complex numbers $a+b\omega$ where $a$ and $b$ are integers. We define the norm $N:K\to \mathbb{Z}$ by

$$N(a+b\omega )=a^2-ab+b^2$$

or equivalently

$$N(z)=z\overline{z}.$$

Whereas $\mathbb{Z}$ has only two units (meaning invertible elements), namely $\pm 1$, $K$ has six, namely all the sixth roots of 1:

$$\pm 1\mathit{\hspace{1em}\hspace{1em}}\pm \omega \mathit{\hspace{1em}\hspace{1em}}\pm {\omega }^2$$

and we know ${\omega }^2=-1-\omega$. Two nonzero elements $\alpha$ and $\beta$ of $K$ are said to be associates if $\alpha =\beta \mu$ for some unit $\mu$. This is an equivalence relation, and any nonzero element has six associates.

$K$ is a principal ring, hence has unique factorization. Let us call $\rho \in K$ “irreducible” if the condition $\rho =\alpha \beta$ implies that $\alpha$ or $\beta$, but not both, is a unit. It turns out that the irreducible elements of $K$ are (up to multiplication by units):

– the number $1-\omega$, which has norm 3. We will denote it by $\pi$.

– positive real integers $q\equiv 2(mod3)$ which are prime in $\mathbb{Z}$. Such integers are called rational primes in $K$.

– complex numbers $q=a+b\omega$ where $N(q)$ is a prime in $Z$ and $N(q)\equiv 1(mod3)$.

For example, $3+2\omega$ is a prime in $K$ because its norm, 7, is prime in $\mathbb{Z}$ and is 1 mod 3; but 7 is not a prime in $K$.

Now we need some convention whereby at most one of any six associates is called a prime. By convention, the following numbers are nominated:

– the number $\pi$.

– rational primes (rather than their negative or complex associates).

– complex numbers $q=a+b\omega$ where $N(q)\equiv 1(mod3)$ is prime in $\mathbb{Z}$ and

$a$ $\equiv$ $2(mod3)$

$b$ $\equiv$ $0(mod3).$

One can verify that this selection exists and is unambigous.

Next, we seek a three-valued function analogous to the two-valued quadratic residue character $x\mapsto \left(\frac{x}{p}\right)$. Let $\rho$ be a prime in $K$, with $\rho \ne \pi$. If $\alpha$ is any element of $K$ such that $\rho \nmid \alpha$, then

$${\alpha }^{N(\rho )-1}\equiv 1(mod\rho ).$$

Since $N(\rho )-1$ is a multiple of 3, we can define a function

$${\chi }_{\rho }:K\to \{1,\omega ,{\omega }^2\}$$

by

${\chi }_{\rho }(\alpha )$ $\equiv$ ${\alpha }^{(N(\rho )-1)/3}\text{ if }\rho \nmid \alpha$

${\chi }_{\rho }(\alpha )$ $=$ $0\text{ if }\rho \mid \alpha .$

${\chi }_{\rho }$ is a character, called the cubic residue character mod $\rho$. We have ${\chi }_{\rho }(\alpha )=1$ if and only if $\alpha$ is a nonzero cube mod $\rho$. (Compare Euler’s criterion.)

At last we can state this famous result of Eisenstein and Jacobi:

Theorem (Cubic Reciprocity Law): If $\rho$ and $\sigma$ are any two distinct primes in $K$, neither of them $\pi$, then

$${\chi }_{\rho }(\sigma )={\chi }_{\sigma }(\rho ).$$

The quadratic reciprocity law has two “supplements” which describe $\left(\frac{-1}{p}\right)$ and $\left(\frac{2}{p}\right)$. Likewise the cubic law has this supplement, due to Eisenstein:

Theorem: For any prime $\rho$ in $K$, other than $\pi$,

$${\chi }_{\rho }(\pi )={\omega }^{2m}$$

where

$m$ $=$ $(\rho +1)/3\mathit{\hspace{1em}\hspace{1em}}\text{ if }\rho \text{ is a rational prime}$

$m$ $=$ $(a+1)/3\mathit{\hspace{1em}\hspace{1em}}\text{ if }\rho =a+b\omega \text{ is a complex prime.}$

Remarks: Some writers refer to our “irreducible” elements as “primes” in $K$; what we have called primes, they call “primary primes”.

The quadratic reciprocity law would take a simpler form if we were to make a different convention on what is a prime in $\mathbb{Z}$, a convention similar to the one in $K$: a prime in $\mathbb{Z}$ is either 2 or an irreducible element $x$ of $\mathbb{Z}$ such that $x\equiv 1(mod4)$. The primes would then be 2, -3, 5, -7, -11, 13, …and the QRL would say simply

$$\left(\frac{p}{q}\right)\left(\frac{q}{p}\right)=1$$

for any two distinct odd primes $p$ and $q$.

Title	cubic reciprocity law
Canonical name	CubicReciprocityLaw
Date of creation	2013-03-22 13:41:26
Last modified on	2013-03-22 13:41:26
Owner	mathcam (2727)
Last modified by	mathcam (2727)
Numerical id	7
Author	mathcam (2727)
Entry type	Topic
Classification	msc 11A15
Related topic	QuadraticReciprocityRule
Defines	cubic residue
Defines	cubic residue character