On the Maximality, Weierstrass Semigroups, and Automorphism Group of the Curve $Y^{q+1}=X^n(X^n+1)$

Abstract

We study the algebraic curve over ${\mathbb{F}}_{q^2}$ defined by $y^{q+1}=x^n(x^n+1)$, where $n$ is a positive integer coprime to the characteristic. We first prove (when $q$ is odd) that the nonsingular model of this curve is ${\mathbb{F}}_{q^2}$-maximal if and only if $n\mid (q+1)$. Writing $n=\frac{q+1}{m}$, we obtain a family of maximal curves parameterized by the divisors $m$ of $q+1$, which extends the previously studied case $m=3$ corresponding to maximal curves with the third largest possible genus. For this family, we determine the Weierstrass semigroups at several classes of rational points, including those lying above the branch points of the natural projection. These semigroups are described explicitly in terms of $q$ and $m$, and exhibit different behaviors depending on the arithmetic properties of $m$. Moreover, we determine the full automorphism group of the curve under a mild condition on the characteristic. Our results extend an earlier work on the case $m=3$ and provide new insight into the structure of this family of maximal curves.