Searcharxiv⌕ Search

arXiv subjects

Marc Coppens

Publications and source records attributed to Marc Coppens.

At least 19 recordsLinked to original sources
arXiv2026

Smoothness results for the schemes of special divisors on general k-gonal curves

For a general $k$-gonal curve $C$ with a morphism $f: C \rightarrow \mathbb{P}^1$ of degree $k$, we consider the refinement of the Brill-Noether schemes $W^r_d(C)$ by means of the Brill-Noether degeneracy schemes $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$. The schemes $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$ as sets are closures of subsets $\Sigma_{\overrightarrow {e}}(C,f)$ of $\Pic (C)$ and as a scheme $\Sigma_{\overrightarrow {e}}(C,f)$ is a smooth open subscheme of $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$. In this paper we describe naturally defined open subsets of $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$ in general strictly containing $\Sigma_{\overrightarrow {e}}(C,f)$ such that $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$ is smooth along them. As an application we describe all invertible sheaves $L$ on $C$ having an injective Petri map. Some of those sets $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$ are the irreducible components of $W^r_d(C)$. In those cases we prove $W^r_d(C)$ is smooth at a point $L$ of those larger open subsets of $\overline{\Sigma}_{\overrightarrow {e}}(C,f)$ unless $L$ belongs to at least two irreducible components of $W^r_d(C)$ (such points exist). On the other hand in general the singular locus of the schemes $W^r_d(C)$ is not equal to the complement of the union of $W^{r+1}_d(C)$ and the intersections of two different components of $W^r_d(C)$.

math.AG↗
arXiv2025

A picture of the irreducible components of $W^r_d(C)$ for a general $k$-gonal curve $C$

Based on results on Hurwitz-Brill-Noether theory obtained by H. Larson we give a picture of the irreducible components of $W^r_d(C)$ for a general $k$-gonal curve of genus $g$. This picture starts from irreducible components of $W^r_d(C)$ restricted to an open subset of $Pic (C)$ satisfying Brill-Noether theory as in the case of a general curve of genus $g$. We obtain some degeneracy loci associated to a morphism of locally-free sheaves on them of the expected dimension. All the irreducible components of the schemes $W^r_d(C)$ are translates of their closures in $Pic (C)$. We complete the proof that the schemes $W^r_d(C)$ are generically smooth in case $C$ is a general $k$-gonal curve (claimed but not completely proved before). We obtain some results on the tangent spaces to the splitting degeneracy loci for an arbitrary $k$-gonal curve and we obtain some new smoothness results in case $C$ is a general $k$-gonal curve.

math.AG↗
arXiv2024

A study of general Martens-special chains of cycles

For a general Martens-special chain of cycles $\Gamma$ of type $k$ we prove that the gonality is equal to $k+2$. Although $\dim (W^1_{k+2} (\Gamma))=k$ we prove that $w^1_{k+2}(\Gamma)=0$. We also compute the gonality sequence of $\Gamma$ and we prove it is divisorial complete. We prove that a general Martens-special discrete chain of cycles $G$ of type $k$ has the same gonality sequence.

math.AG↗
arXiv2023

A study of H. Martens' Theorem on chains of cycles

Let $\Gamma$ be a chain of cycles of genus $g$. Let $d$,$r$ be integers with $1 \leq r \leq g-2$ and $2r\leq d \leq g-3+r$. Then $w^r_d(\Gamma)=d-2r$ implies $\Gamma$ is hyperelliptic. For each $g \geq 2r+3$ there exist non-hyperelliptic chains of cycles satisfying $w^r_{g-2+r}(\Gamma)=g-2-r$. In the case of algebraic curves such equality implies the curve is hyperelliptic. In particular we obtain the existence of chains of cycles $\Gamma$ such that $w^r_{g-2+r}(\Gamma) \neq w^1_{g-r}(\Gamma)$ in case $r \geq 2$. In the case of algebraic curves such numbers are equal because of the Riemann-Roch Theorem.

math.CO↗
arXiv2020

The scrollar invariants of k-gonal curves having a nodal model on a smooth quadric having its nodes on few lines

We determine the scrollar invariants of the normalization $C$ of a nodal curve $\Gamma$ of type $(k,a)$ on a smooth quadric $\mathbb{P}^1 \times \mathbb{P}^1$ associated to the $g^1_k$ defined by the pencil of lines of type $(0,1)$ in case all nodes are contained in at most $k-1$ lines of type $(1,0)$. This result is very much related to results obtained by E. Ballico, but in this paper the proof follows directly from an easy lemma. Also a result of E. Ballico on the existence of curves with prescribed scrollar invariant is a consequence of that lemma making the arguments much shorter.

math.AG↗
arXiv2017

The uniqueness of Weierstrass points with semigroup and related subgroups

Assume $a$ and $b=na+r$ with $n \geq 1$ and $0 $ then $C$ is called a $C_{a;b}$-curve. In case $r \neq a-1$ and $b \neq a+1$ we prove $C$ has no other point $Q \neq P$ having Weierstrass semigroup equal to $ $. We say the Weierstrass semigroup $ $ occurs at most once. The curve $C_{a;b}$ has genus $(a-1)(b-1)/2$ and the result is generalized to genus $g<(a-1)(b-1)/2$. We obtain a lower bound on $g$ (sharp in many cases) such that all Weierstrass semigroups of genus $g$ containing $ $ occur at most once.

math.AG↗
arXiv2015

A metric graph satisfying $w^1_4=1$ that cannot be lifted to a curve satisfying $\dim (W^1_4)=1$

For all integers $g \geq 6$ we prove the existence of a metric graph $G$ with $w^1_4=1$ such that $G$ has Clifford index 2 and there is no tropical modification $G'$ of $G$ such that there exists a finite harmonic morphism of degree 2 from $G'$ to a metric graph of genus 1. Those examples show that dimension theorems on the space classifying special linear systems for curves do not all of them have immediate translation to the theory of divisors on metric graphs.

math.AG↗
arXiv2014

Free divisors on metric graphs

On a metric graph we introduce the notion of a free divisor as a replacement for the notion of a base point free complete linear system on a curve. By means of an example we show that the Clifford inequality is the only obstruction for the existence of very special free divisors on a graph. This is different from the situation of base point free linear systems on curves. It gives rise to the existence of many types of divisors on graphs that cannot be lifted to curves maintaining the rank and it also shows that classifications made for linear systems of some fixed small positive Clifford index do not hold (exactly the same) on graphs.

math.AG↗
arXiv2013

Clifford's Theorem for graphs

Let $\Gamma$ be a metric graph having a linear system $g^r_{2r}$ for some $2 \leq r \leq g-2$ then $\Gamma$ has a linear system $g^1_2$. This is similar to the well-known Clifford's Theorem from the theory of linear systems on smooth projective curves.

math.AG↗
arXiv2012

Pencils on separating (M-2)-curves

A separating ($M-2$)-curve is a smooth geometrically irreducible real projective curve $X$ such that $X(\mathbb{R})$ has $g-1$ connected components and $X(\mathbb{C})\setminus X(\mathbb{R})$ is disconnected. Let $T_g$ be a Teichm\"uller space of separating ($M-2$)-curves of genus $g$. We consider two partitions of $T_g$, one by means of a concept of special type, the other one by means of the separating gonality. We show that those two partitions are very closely related to each other. As an application we obtain the existence of real curves having isolated real linear systems $g^1_{g-1}$ for all $g\geq 4$.

math.AG↗
arXiv2011

The separating gonality of a separating real curve

A smooth real curve is called separating in case the complement of the real locus inside the complex locus is disconnected. This is the case if there exists a morphism to the projective line whose inverse image of the real locus of the projective line is the real locus of the curve. Such morphism is called a separating morphism. The minimal degree of a separating morphism is called the separating gonality. The separating gonality cannot be less than the number s of the connected components of the real locus of the curve. A theorem of Ahlfors implies this separating gonality is at most the g+1 with g the genus of the curve. A better upper bound depending on s is proved by Gabard. In this paper we prove that there are no more restrictions on the values of the separating gonality.

math.AG↗
arXiv2011

Pencils on real curves

We consider coverings of real algebraic curves to real rational algebraic curves. We show the existence of such coverings having prescribed topological degree on the real locus. From those existence results we prove some results on Brill-Noether Theory for pencils on real curves. For coverings having topological degree 0 we introduce the covering number k and we prove the existence of coverings of degree 4 with prescribed covering number.

math.AG↗
arXiv2010

Linear pencils on graphs and on real curves

A degeneration of curves gives rise to an interesting relation between linear systems on curves and on graphs. In this paper, we consider the case of linear pencils and as an application, we obtain some results on pencils on real curves.

math.AG↗
arXiv2009

Linear systems on graphs with a real structure

A degeneration of a smooth projective curve to a strongly stable curve gives rise to a specialization map from divisors on curves to divisors on graphs. In this paper we show that this specialization behaves well under the presence of real structures. In particular we study real linear systems on graphs with a real structure and we prove results on them comparable to results in the classical theory of real curves. We also consider generalizations to metric graphs and tropical curves.

math.AG↗
arXiv2009

Singular hypersurfaces possessing infinitely many star points

We prove that a component of the closure of the set of star points on a hypersurface X of degree d>2 in N-dimensional projective space is linear. Afterwards, we focus on the case where the component is of maximal dimension N-2 and the case where X is a surface (i.e. N=3).

math.AG↗
arXiv2009

Star points on smooth hypersurfaces

A point P on a smooth hypersurface X of degree d in an N-dimensional projective space is called a star point if and only if the intersection of X with the embedded tangent space T_P(X) is a cone with vertex P. This notion is a generalization of total inflection points on plane curves and Eckardt points on smooth cubic surfaces in three-dimensional projective space. We generalize results on the configuration space of total inflection points on plane curves to star points. We give a detailed description of the configuration space for hypersurfaces with two or three star points. We investigate collinear star points and we prove that the number of star points on a smooth hypersurface is finite.

math.AG↗