arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 2202.05765math.AG
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特征$p$中具有大线性自同构群的平面曲线

Plane curves with a large linear automorphism group in characteristic $p$

H. Borges, G. Korchmáros, P. Speziali

摘要:

设$G$是定义在$q$阶的有限域$\mathbb{F}_q$上的三维射影群$\mathrm{PGL}(3,q)$的一个子群,看作是$\mathrm{PGL}(3,K)$的一个子群,其中$K$是$\mathbb{F}_q$的一个代数闭包.对于七个非偶发的,最大子群$G$的$\mathrm{PGL}(3,q)$,我们调查(投影,不可约)平面曲线定义的$K$是左不变的$G$。对于每一个,我们计算$G$不变曲线的最小度$d(G)$,提供度$d(G)$的所有$G$不变曲线的分类,并确定所有$G$不变曲线的度谱中的第一个间隙$\varepsilon(G)$。证明了除非$G$唯一确定,否则次数$d(G)$的曲线属于依赖于$G$的束.我们还指出,$G$ -不变曲线的程度$d(G)$有特殊的几何特征,如Frobenius非经典性和不寻常的变化的数量$\mathbb{F}_{q^i}$ -合理的点。对于由$\mathrm{PGL}(3,q)$的大子群保持不变的平面曲线的大多数例子,曲线的整个自同构群是线性的,即,$\mathrm{PGL}(3,K)$的一个亚组。虽然这似乎是一个普遍的行为,我们表明,相反的情况下,也可以发生一些不可约的平面曲线,即曲线有一个大组的线性自同构,但它的全自同构群是非线性的。

英文摘要:

Let $G$ be a subgroup of the three dimensional projective group $\mathrm{PGL}(3,q)$ defined over a finite field $\mathbb{F}_q$ of order $q$, viewed as a subgroup of $\mathrm{PGL}(3,K)$ where $K$ is an algebraic closure of $\mathbb{F}_q$. For the seven nonsporadic, maximal subgroups $G$ of $\mathrm{PGL}(3,q)$, we investigate the (projective, irreducible) plane curves defined over $K$ that are left invariant by $G$. For each, we compute the minimum degree $d(G)$ of $G$-invariant curves, provide a classification of all $G$-invariant curves of degree $d(G)$, and determine the first gap $\varepsilon(G)$ in the spectrum of the degrees of all $G$-invariant curves. We show that the curves of degree $d(G)$ belong to a pencil depending on $G$, unless they are uniquely determined by $G$. We also point out that $G$-invariant curves of degree $d(G)$ have particular geometric features such as Frobenius nonclassicality and an unusual variation of the number of $\mathbb{F}_{q^i}$-rational points. For most examples of plane curves left invariant by a large subgroup of $\mathrm{PGL}(3,q)$, the whole automorphism group of the curve is linear, i.e., a subgroup of $\mathrm{PGL}(3,K)$. Although this appears to be a general behavior, we show that the opposite case can also occur for some irreducible plane curves, that is, the curve has a large group of linear automorphisms, but its full automorphism group is nonlinear.

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