# Artin–Schreier curve

In mathematics, an Artin–Schreier curve is a plane curve defined over an algebraically closed field of characteristic ${\displaystyle p}$ by an equation

${\displaystyle y^{p}-y=f(x)}$

for some rational function ${\displaystyle f}$ over that field.

One of the most important examples of such curves is hyperelliptic curves in characteristic 2, whose Jacobian varieties have been suggested for use in cryptography.[1] It is common to write these curves in the form

${\displaystyle y^{2}+h(x)y=f(x)}$

for some polynomials ${\displaystyle f}$ and ${\displaystyle h}$.

## Definition

More generally, an Artin-Schreier curve defined over an algebraically closed field of characteristic ${\displaystyle p}$ is a branched covering

${\displaystyle C\to \mathbb {P} ^{1}}$

of the projective line of degree ${\displaystyle p}$. Such a cover is necessarily cyclic, that is, the Galois group of the corresponding algebraic function field extension is the cyclic group ${\displaystyle \mathbb {Z} /p\mathbb {Z} }$. In other words, ${\displaystyle k(C)/k(x)}$ is an Artin–Schreier extension.

The fundamental theorem of Artin–Schreier theory implies that such a curve defined over a field ${\displaystyle k}$ has an affine model

${\displaystyle y^{p}-y=f(x),}$

for some rational function ${\displaystyle f\in k(x)}$ that is not equal for ${\displaystyle z^{p}-z}$ for any other rational function ${\displaystyle z}$. In other words, if we define polynomial ${\displaystyle g(z)=z^{p}-z}$, then we require that ${\displaystyle f\in k(x)\backslash g(k(x))}$.

## Ramification

Let ${\displaystyle C:y^{p}-y=f(x)}$ be an Artin–Schreier curve. Rational function ${\displaystyle f}$ over an algebraically closed field ${\displaystyle k}$ has partial fraction decomposition

${\displaystyle f(x)=f_{\infty }(x)+\sum _{\alpha \in B'}f_{\alpha }\left({\frac {1}{x-\alpha }}\right)}$

for some finite set ${\displaystyle B'}$ of elements of ${\displaystyle k}$ and corresponding non-constant polynomials ${\displaystyle f_{\alpha }}$ defined over ${\displaystyle k}$, and (possibly constant) polynomial ${\displaystyle f_{\infty }}$. After a change of coordinates, ${\displaystyle f}$ can be chosen so that the above polynomials have degrees coprime to ${\displaystyle p}$, and the same either holds for ${\displaystyle f_{\infty }}$ or it is zero. If that is the case, we define

${\displaystyle B={\begin{cases}B'&{\text{ if }}f_{\infty }=0,\\B'\cup \{\infty \}&{\text{ otherwise.}}\end{cases}}}$

Then the set ${\displaystyle B\subset \mathbb {P} ^{1}(k)}$ is precisely the set of branch points of the covering ${\displaystyle C\to \mathbb {P} ^{1}}$.

For example, Artin–Schreier curve ${\displaystyle y^{p}-y=f(x)}$, where ${\displaystyle f}$ is a polynomial, is ramified at a single point over the projective line.

Since the degree of the cover is a prime number, over each branching point ${\displaystyle \alpha \in B}$ lies a single ramification point ${\displaystyle P_{\alpha }}$ with corresponding ramification index equal to

${\displaystyle e(P_{\alpha })=(p-1){\big (}\deg(f_{\alpha })+1{\big )}+1.}$

## Genus

Since, ${\displaystyle p}$ does not divide ${\displaystyle \deg(f_{\alpha })}$, ramification indices ${\displaystyle e(P_{\alpha })}$ are not divisible by ${\displaystyle p}$ either. Therefore, Riemann-Roch theorem may be used to compute that genus of an Artin–Schreier curve is given by

${\displaystyle g={\frac {p-1}{2}}\left(\sum _{\alpha \in B}{\big (}\deg(f_{\alpha })+1{\big )}-2\right).}$

For example, for a hyperelliptic curve defined over a field of characteristic ${\displaystyle p=2}$ by equation ${\displaystyle y^{2}-y=f(x)}$ with ${\displaystyle f}$ decomposing as above, we have

${\displaystyle g=\sum _{\alpha \in B}{\frac {\deg(f_{\alpha })+1}{2}}-1.}$

## Generalizations

Artin–Schreier curves are a particular case of plane curves defined over an algebraically closed field ${\displaystyle k}$ of characteristic ${\displaystyle p}$ by an equation

${\displaystyle g(y^{p})=f(x)}$

for some separable polynomial ${\displaystyle g\in k[x]}$ and rational function ${\displaystyle f\in k(x)\backslash g(k(x))}$. Mapping ${\displaystyle (x,y)\mapsto x}$ yields a covering map from the curve ${\displaystyle C}$ to the projective line ${\displaystyle \mathbb {P} ^{1}}$. Separability of defining polynomial ${\displaystyle g}$ ensures separability of the corresponding function field extension ${\displaystyle k(C)/k(x)}$. If ${\displaystyle g(y^{p})=a_{m}y^{p^{m}}+a_{m-1}y^{p^{m-1}}+\cdots +a_{1}y^{p}+a_{0}}$, a change of variables can be found so that ${\displaystyle a_{m}=a_{1}=1}$ and ${\displaystyle a_{0}=0}$. It has been shown [2] that such curves can be built via a sequence of Artin-Schreier extension, that is, there exists a sequence of cyclic coverings of curves

${\displaystyle C\to C_{m-1}\to \cdots \to C_{0}=\mathbb {P} ^{1},}$

each of degree ${\displaystyle p}$, starting with the projective line.