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Marek Janasz

Publications and source records attributed to Marek Janasz.

15 recordsLinked to original sources
arXiv2026

Simplicial arrangements in real projective three-space revisited

In this paper we study irreducible simplicial arrangements of projective planes in $\mathbb{P}^{3}(\mathbb{R})$ from combinatorial and projective geometry viewpoints. We first formulate a simpliciality criterion in terms of incidences between rank-two and rank-three flats, together with equivalent formulations using face numbers, reduced restrictions, and characteristic polynomials. We also relate the classical planar restriction data of Grünbaum-Shephard to Ziegler multirestrictions. Our principal result concerns the rank-four special-vertex property: among the irreducible crystallographic Coxeter arrangements of rank four, the arrangements of types $A_4$ and $B_4$ admit a special vertex, whereas those of types $D_4$ and $F_4$ do not. A simplicial deletion chain inside $B_4$ supplies further irreducible examples with a special vertex. Finally, we compare rank-flat, Purdy-type, and Grünbaum-Shephard defects for these arrangements.

math.CO↗
arXiv2026

On $\mathscr{M}$-arrangements of conics and lines with ordinary singularities

In this paper we study combinatorial aspects of reduced plane curves known as $\mathscr{M}$-curves. This notion is a natural generalization of maximizing plane curves, which are well-known in the theory of algebraic surfaces. We focus on $\mathscr{M}$-arrangements of conics and lines with ordinary singularities of multiplicity at most four. We provide numerical constraints on their existence, especially in terms of weak combinatorics, and then study in detail the case of arrangements consisting of one conic and lines. We also construct a new example with one conic and eleven lines, prove boundedness results for real arrangements of this type, and record a regularity consequence for the associated Milnor algebra and module of Jacobian syzygies.

math.AG↗
arXiv2026

On the boundedness of some real line arrangements of type at most one

In this note, we show that real line arrangements of type at most one, admitting only intersection points of multiplicity at most five, satisfy certain boundedness properties. In particular, we prove that a free real arrangement of $d$ lines with intersection multiplicities bounded by $5$ can have at most $522$ lines and consequently there exist only finitely many combinatorial types of such arrangements.

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arXiv2026

On homological properties of some Cynk-Szemberg octic hyperplane arrangements

In this paper we study Cynk-Szemberg octic hyperplane arrangements from the perspective of homological properties of their derivation modules. In particular, we define the notion of the type of hyperplane arrangements that will be used in our characterization of rigid Cynk-Szemberg octic hyperplane arrangements. Moreover, we deliver a combinatorial non-freeness criterion for essential hyperplane arrangements in $\mathbb{C}^{4}$.

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arXiv2025

Unexpected hypersurfaces of type $(d+k,d)$

Unexpected hypersurfaces arise when vanishing in points of a set $Z$ and higher-order vanishing along a general linear subspace fails to impose the expected number of independent conditions on forms of a fixed degree. The phenomenon was first observed for planar curves by Cook, Harbourne, Migliore and Nagel. This paper shows a syzygy-based construction of, possibly unexpected, hypersurfaces of degree $d+k$ in $\mathbb{P}^n$, vanishing along a codimension two general linear subspace with multiplicity $d$; thus generalizing the work of Trok and the previous work of the last two authors. Our framework unifies the classical planar cases with higher-dimensional examples, including Trok's construction. We give a sufficient criterion for unexpectedness (via the splitting behaviour the syzygy bundles of the powers of the Jacobian ideal, associated with the hyperplane arrangement dual to $Z$) and provide explicit examples in $\mathbb{P}^3$ and $\mathbb{P}^4$.

math.AG↗
arXiv2025

On free line arrangements with double, triple and quadruple points

We show that there are only finitely many combinatorial types of free real line arrangements with only double, triple and quadruple intersection points, and we enlist all admissible weak-combinatorics of them. Then we classify all real $M$-line arrangements. In particular, we show that real $M$-line arrangements are simplicial.

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arXiv2025

On the existence of maximizing curves of odd degrees

In this paper we provide the non-existence criterion for the so-called maximizing curves of odd degrees. Furthermore, in the light of our criterion, we define a new class of plane curves that generalizes the notion of maximizing curves which we call as $M$-curves.

math.AG↗
arXiv2024

On arrangements of smooth plane quartics and their bitangents

In the present paper, we revisit the geometry of smooth plane quartics and their bitangents from several perspectives. First, we study in detail the weak combinatorics of arrangements of bitangents associated with highly symmetric quartic curves. We consider quartic curves from the point of view of the order of their automorphism groups, in order to establish a lower bound on the number of quadruple intersection points for arrangements of bitangents associated with smooth plane quartics, which are smooth members of Ciani's pencil. We then construct new examples of $3$-syzygy reduced plane curves using smooth plane quartics and their bitangents.

math.AG↗
arXiv2024

On symmetric plane quartic curves

In the present paper we study the geometry of plane quartics with large automorphism groups. We show results devoted to smooth plane quartics that are invariant under the action of the elementary abelian group of type $[2,2,2]$, and we study geometric properties of the smooth plane quartic having automorphism group of order $48$.

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arXiv2022

On plane conic arrangements with nodes and tacnodes

In the present paper, we study arrangements of smooth plane conics having only nodes and tacnodes as the singularities. We provide an interesting estimation on the number of nodes and tacnodes that depends only on a linear function of the number of conics. Based on that result, we obtain a new upper bound on the number of tacnodes which turns out to be better than Miyaoka's bound for a large enough number of conics. We also study the freeness and nearly freeness of such arrangements providing a detailed description.

math.AG↗
arXiv2020

On Seshadri constants and point-curve configurations

In the note we study the multipoint Seshadri constants of $\mathcal{O}_{\mathbb{P}^{2}_{\mathbb{C}}}(1)$ centered at singular loci of certain curve arrangements in the complex projective plane. Our first aim is to show that the values of Seshadri constants can be approximated with use of a combinatorial invariant which we call the configurational Seshadri constant. We study specific examples of point-curve configurations for which we provide actual values of the associated Seshadri constants. In particular, we provide an example based on Hesse point-conic configuration for which the associated Seshadri constant is computed by a line. This shows that multipoint Seshadri constants are not purely combinatorial.

math.AG↗
arXiv2018

New phenomena in the containment problem for simplicial arrangements

In this note we consider two simplicial arrangements of lines and ideals $I$ of intersection points of these lines. There are $127$ intersection points in both cases and the numbers $t_i$ of points lying on exactly $i$ configuration lines (points of multiplicity $i$) coincide. We show that in one of these examples the containment $I^{(3)} \subseteq I^2$ holds, whereas it fails in the other. We also show that the containment fails for a subarrrangement of $21$ lines. The interest in the containment relation between $I^{(3)}$ and $I^2$ for ideals of points in $¶^2$ is motivated by a question posted by Huneke around $2000$. Configurations of points with $I^{(3)} \not\subseteq I^2$ are quite rare. Our example reveals two particular features: All points are defined over $\Q$ and all intersection points of lines are involved. In examples studied by now only points with multiplicity $i\geq 3$ were considered. The novelty of our arrangements lies in the geometry of the element in $I^{(3)}$ which witness the noncontainment in $I^2$. In all previous examples such an element was a product of linear forms. Now, in both cases there is an irreducible curve of higher degree involved.

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