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Eslam Badr

Publications and source records attributed to Eslam Badr.

18 recordsLinked to original sources
arXiv2026

Smooth plane curves with a unique outer Galois point and their automorphism groups

We consider smooth plane curves $\mathcal{X}$ of degree $d\geq4$, defined over an algebraically closed field of characteristic $0$, that possess a unique outer Galois point. This geometric condition forces the curve to be a cyclic covering of the projective line, and ensures that its automorphism group fits into a specific theoretical framework. For each possible non-cyclic reduced automorphism group $\operatorname{Aut}_{\operatorname{red}}(\mathcal{X})$, we fully characterize the defining equation of $\mathcal{X}$ and the precise structure of its full automorphism group $\operatorname{Aut}(\mathcal{X})$. This comprehensive analysis not only identifies the exact form of the equation for each automorphism type but also establishes the detailed criteria under which these scenarios can occur, thereby offering a complete classification of defining equations for smooth plane curves with a unique outer Galois point and a non-cyclic reduced automorphism group.

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arXiv2025

Rational Functions on the Projective Line from a Computational Viewpoint

An explicit invariant-theoretic description of the moduli space $\mathcal{M}_3^1$ of degree-three rational maps on $\mathbb{P}^1$ is developed. A cubic map $\phi$ is represented, up to conjugation, by the pair of binary forms $(f, g) \in V_4 \oplus V_2$ arising from its Clebsch--Gordan decomposition. From this representation one constructs weighted projective invariants $\xi_0, ..., \xi_5$ that embed $\mathcal{M}_3^1$ into $\mathbb{P}^5(2,2,3,3,4,6)$ onto the locus where the gcd of the weights of the non-zero coordinates equals $1$, together with absolute invariants defined as weight-zero rational functions of the $\xi_i$, normalized by an additional invariant $I_6$ of weight $6$. These absolute invariants determine the isomorphism class uniquely. The stratification of $\mathcal{M}_3^1$ is described explicitly by equations in the absolute invariants or polynomial relations among the $\xi_i$. Computational illustrations demonstrate that the resulting invariants provide an effective feature set for automated classification of automorphism groups. The methods suggest natural extensions to higher degrees.

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arXiv2023

On pseudo-real finite subgroups of $\operatorname{PGL}_3(\mathbb{C})$

Let $G$ be a finite subgroup of $\operatorname{PGL}_3(\mathbb{C})$, and let $σ$ be the generator of $\operatorname{Gal}(\mathbb{C}/\mathbb{R})$. We say that $G$ has a \emph{real field of moduli} if $^σG$ and $G$ are $\operatorname{PGL}_3(\mathbb{C})$-conjugates, that is, if $\exists\,ϕ\in\operatorname{PGL}_3(\mathbb{C})$ such that $ϕ^{-1}\,G\,ϕ=\,^σG$. Furthermore, we say that $\mathbb{R}$ is \emph{a field of definition for $G$} or that \emph{$G$ is definable over $\mathbb{R}$} if $G$ is $\operatorname{PGL}_3(\mathbb{C})$-conjugate to some $G'\subset\operatorname{PGL}_3(\mathbb{R})$. In this situation, we call $G'$ \emph{a model for $G$ over $\mathbb{R}$}. If $G$ has $\mathbb{R}$ as a field of definition but is not definable over $\mathbb{R}$, then we call $G$ \emph{pseudo-real}. In this paper, we first show that any finite cyclic subgroup $G=\mathbb{Z}/n\mathbb{Z}$ in $\operatorname{PGL}_3(\mathbb{C})$ has {a real field of moduli} and we provide a necessary and sufficient condition for $G=\mathbb{Z}/n\mathbb{Z}$ to be definable over $\mathbb{R}$; see Theorems 2.1, 2.2, and 2.3. We also prove that any dihedral group $\operatorname{D}_{2n}$ with $n\geq3$ in $\operatorname{PGL}_3(\mathbb{C})$ is definable over $\mathbb{R}$; see Theorem 2.4. Furthermore, we study all six classes of finite primitive subgroups of $\operatorname{PGL}_3(\mathbb{C})$, and show that all of them except the icosahedral group $\operatorname{A}_5$ are pseudo-real; see Theorem 2.5, whereas $\operatorname{A}_5$ is definable over $\mathbb{R}$. Finally, we explore the connection of these notions in group theory with their analogues in arithmetic geometry; see Theorem 2.6 and Example 2.7.

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arXiv2022

The stratification by automorphism groups of smooth plane sextic curves

We obtain the list of automorphism groups for smooth plane sextic curves over an algebraically closed field K of characteristic p=0 or p>21. Moreover, we assign to each group a geometrically complete family over K describing its corresponding stratum, that is, a generic defining polynomial equation with parameters such that any curve in the stratum is K-isomorphic to a non-singular plane model obtained by specializing the values of those parameters over K.

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arXiv2019

On quadratic progression sequences on smooth plane curves

We study the arithmetic (geometric) progressions in the $x$-coordinates of quadratic points on smooth projective planar curves defined over a number field $k$. Unless the curve is hyperelliptic, we prove that these progressions must be finite. We, moreover, show that the arithmetic gonality of the curve determines the infinitude of these progressions in the set of $\overline{k}$-points with field of definition of degree at most $n$, $n\ge 3$.

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arXiv2018

Bielliptic smooth plane curves and quadratic points

Let $C_k$ be a smooth projective curve over a global field $k$, which is neither rational nor elliptic. Harris-Silverman, when $p=0$, and Schweizer, when $p>0$ together with an extra condition on the Jacobian variety $\operatorname{Jac}(C_k)$ arising from Mordell's conjecture, showed that $C$ has infinitely many quadratic points over some finite field extension $L/k$ inside $\overline{k}$ (a fixed algebraic closure of $k$) if and only if $C$ is hyperelliptic or bielliptic. Now, let $C_k$ be a smooth plane curve of a fixed degree $d\geq4$ with $p=0$ or $p>(d-1)(d-2)+1$ (up to an extra condition on $\operatorname{Jac}(C_k)$ in positive characteristic). Then, we prove that $C_k$ admits always finitely many quadratic points unless $d=4$. A so-called \emph{geometrically complete families} for the different strata of smooth bielliptic plane quartic curves by their automorphism groups, are given. Interestingly, we show (in a very simple way) that there are only finitely many quadratic extensions $k(\sqrt{D})$ of a fixed number field $k$, in which we may have more solutions to the Fermat's and the Klein's equations of degree $d\geq5$; $X^d+Y^d-Z^d=0$ and $X^{d-1}Y+Y^{d-1}Z+Z^{d-1}X=0$ respectively, than these over $k$ (the same holds for any non-singular projective plane equation of degree $d\geq 5$ over $k$, and also in general when $k$ is a global field after imposing an extra condition on $\operatorname{Jac}(C_k)$).

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arXiv2018

Hypersurface model-fields of definition for smooth hypersurfaces and their twists

Given a smooth projective variety of dimension $n-1\geq 1$ defined over a perfect field $k$ that admits a non-singular hypersurface modelin $\mathbb{P}^n_{\overline{k}}$ over $\overline{k}$, a fixed algebraic closure of $k$, it does not necessarily have a non-singular hypersurface model defined over the base field $k$. We first show an example of such phenomenon: a variety defined over $k$ admitting non-singular hypersurface models but none defined over $k$. We also determine under which conditions a non-singular hypersurface model over $k$ may exist. Now, even assuming that such a smooth hypersurface model exists, we wonder about the existence of non-singular hypersurface models over $k$ for its twists. We introduce a criterion to characterize twists possessing such models and we also show an example of a twist not admitting any non-singular hypersurface model over $k$, i.e for any $n\geq 2$, there is a smooth projective variety of dimension $n-1$ over $k$ which is a twist of a smooth hypersurface variety over $k$, but itself does not admit any non-singular hypersurface model over $k$. Finally, we obtain a theoretical result to describe all the twists of smooth hypersurfaces with cyclic automorphism group having a model defined over $k$ whose automorphism group is generated by a diagonal matrix.

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arXiv2018

A class of pseudoreal Riemann surfaces with diagonal automorphism group

A Riemann surface $\mathcal{S}$ having field of moduli $\mathbb{R}$, but not a field of definition, is called \emph{pseudoreal}. This means that $\mathcal{S}$ has anticonformal automorphisms, but non of them is an involution. We call a Riemann surface $\mathcal{S}$ \emph{plane} if it can be described by a smooth plane model of some degree $d\geq4$ in $\mathbb{P}^2_{\mathbb{C}}$. We characterize pseudoreal-plane Riemann surfaces $\mathcal{S}$, whose conformal automorphism group $\operatorname{Aut}_+(\mathcal{S})$ is $\operatorname{PGL}_3(\mathbb{C})$-conjugate to a finite non-trivial group $G$ that leaves invariant infinitely many points of $\mathbb{P}^2_{\mathbb{C}}$. In particular, we show that such pseudoreal-plane Riemann surfaces exist only if $\operatorname{Aut}_+(\mathcal{S})$ is cyclic of even order $n$ dividing the degree $d$. Explicit examples are given, for any degree $d=2pm$ with $m>1$ odd, $p$ is prime and $n=d/p$.

math.AG↗
arXiv2017

The Picard Group of Brauer-Severi Varieties

In this note we provide explicit generators of the Picard groups of cyclic Brauer-Severi varieties defined over the base field. In particular, for all Brauer-Severi surfaces. To produce these generators we use the Twisting Theory for smooth plane curves.

math.NT↗
arXiv2017

Riemann surfaces defined over the reals

The known (explicit) examples of Riemann surfaces not definable over their field of moduli are not real whose field of moduli is a subfield of the reals. In this paper we provide explicit examples of real Riemann surfaces which cannot be defined over the field of moduli.

math.AG↗
arXiv2017

Plane model-fields of definition, fields of definition, the field of moduli of smooth plane curves

Given a smooth plane curve $\overline{C}$ of genus $g\geq 3$ over an algebraically closed field $\overline{k}$, a field $L\subseteq\overline{k}$ is said to be a \emph{plane model-field of definition for $\overline{C}$} if $L$ is a field of definition for $\overline{C}$, i.e. $\exists$ a smooth curve $C'$ defined over $L$ where $C'\times_L\overline{k}\cong \overline{C}$, and such that $C'$ is $L$-isomorphic to a non-singular plane model $F(X,Y,Z)=0$ in $\mathbb{P}^2_{L}$. {In this short note, we construct a smooth plane curve $\overline{C}$ over $\overline{\mathbb{Q}}$, such that the field of moduli of $\overline{C}$ is not a field of definition for $\overline{C}$, and also fields of definition do not coincide with plane model-fields of definition for $\overline{C}$.} As far as we know, this is the first example in the literature with the above property, since this phenomenon does not occur for hyperelliptic curves, replacing plane model-fields of definition with the so-called hyperelliptic model-fields of definition.

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arXiv2017

A note on the stratification by automorphisms of smooth plane curves of genus 6

In this note, we give a so-called representative classification for the strata by automorphism group of smooth $\bar{k}$-plane curves of genus $6$, where $\bar{k}$ is a fixed separable closure of a field $k$ of characteristic $p = 0$ or $p > 13$. We start with a classification already obtained by the first author and we use standard techniques. Interestingly, in the way to get these families for the different strata, we find two remarkable phenomenons that did not appear before. One is the existence of a non $0$-dimensional final stratum of plane curves. At a first sight it may sound odd, but we will see that this is a normal situation for higher degrees and we will give a explanation for it. We explicitly describe representative families for all strata, except for the stratum with automorphism group $\mathbb{Z}/5\mathbb{Z}$. Here we find the second difference with the lower genus cases where the previous techniques do not fully work. Fortunately, we are still able to prove the existence of such family by applying a version of Luroth's theorem in dimension $2$.

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arXiv2016

On twists of smooth plane curves

Given a smooth curve defined over a field $k$ that admits a non-singular plane model over $\overline{k}$, a fixed separable closure of $k$, it does not necessarily have a non-singular plane model defined over the field $k$. We determine under which conditions this happens and we show an example of such phenomenon. Now, even assuming that such a smooth plane model exists, we wonder about the existence of non-singular plane models over $k$ for its twists. We characterize twists possessing such models and use such characterization to improve, for the particular case of smooth plane curves, the algorithm to compute twists of non-hyperelliptic curves wrote recently down by the third author. We also show an example of a twist not admitting such non-singular plane model. As a consequence, we get explicit equations for a non-trivial Brauer-Severi surface. Finally, we obtain a theoretical result to compute all the twists of smooth plane curves with cyclic automorphism group having a $k$-model whose automorphism group is generated by a diagonal matrix. Some examples are also provided.

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arXiv2015

On the locus of smooth plane curves with a fixed automorphism group

In this paper, we study some aspects of the irreducibility of $\widetilde{M_g^{Pl}(G)}$ and its interrelation with the existence of "normal forms", i.e. non-singular plane equations (depending on a set of parameters) such that a specialization of the parameters gives a certain non-singular plane model associated to the elements of $\widetilde{M_g^{Pl}(G)}$. In particular, we introduce the concept of being equation strongly irreducible (ES-Irreducible) for which the locus $\widetilde{M_g^{Pl}(G)}$ is represented by a single "normal form". Henn, and Komiya-Kuribayashi, observed that $\widetilde{M_3^{Pl}(G)}$ is ES-Irreducible. In this paper we prove that this phenomena does not occur for any odd $d>4$. More precisely, let $\mathbb{Z}/m\mathbb{Z}$ be the cyclic group of order $m$, we prove that $\widetilde{M_g^{Pl}(\mathbb{Z}/(d-1)\mathbb{Z})}$ is not ES-Irreducible for any odd integer $d\geq5$, and the number of its irreducible components is at least two. Furthermore, we conclude the previous result when $d=6$ for the locus $\widetilde{M_{10}^{Pl}(\mathbb{Z}/3\mathbb{Z})}$. Lastly, we prove the analogy of these statements when $K$ is any algebraically closed field of positive characteristic $p$ such that $p>(d-1)(d-2)+1$.

math.AG↗
arXiv2015

Automorphism groups of non-singular plane curves of degree 5

Henn and Komiya-Kuribayashi listed, independently, the groups $G$ for which $\widetilde{M_3^{Pl}(G)}$ is non-empty. In this paper, we determine the loci $\widetilde{M_6^{Pl}(G)}$, corresponding to non-singular degree $5$ projective plane curves, which are non-empty. Also, we present the analogy of Henn's results for quartic curves concerning non-singular plane model equations associated to these loci.

math.AG↗
arXiv2015

Plane non-singular curves with an element of "large" order in its automorphism group

In this note we determine, for an arbitrary but a fixed degree $d$, an algorithm to list the possible values $m$ for which $M_g^{Pl}(\mathbb{Z}/m)$ is non-empty where $\mathbb{Z}/m$ denotes the cyclic group of order $m$. In particular, we prove that $m$ should divide one of the integers: $d-1$, $d$, $d^2-3d+3$, $(d-1)^2$, $d(d-2)$ or $d(d-1)$. Secondly, consider a curve $δ\in M_g^{Pl}$ with $g=(d-1)(d-2)/2$ such that $Aut(δ)$ has an element of "very large" order, in the sense that this element is of order $d^2-3d+3$, $(d-1)^2$, $d(d-2)$ or $d(d-1)$. Then we investigate the groups $G$ for which $δ\in\widetilde{M_g^{Pl}(G)}$ and also we determine the locus $\widetilde{M_g^{Pl}(G)}$ in these situations. Moreover, we work with the same question when $Aut(δ)$ has an element of "large" order $\ell d$, $\ell (d-1)$ or $\ell(d-2)$ with $\ell\geq 2$ an integer.

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