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S. Yu. Orevkov

Publications and source records attributed to S. Yu. Orevkov.

At least 19 recordsLinked to original sources
arXiv2026

On arrangements of plane real quartics with respect to three lines

We complete the classification of mutual arrangements of a smooth real algebraic or real pseudoholomorphic quartic curve and three lines under condition that each oval of the quartic intersects the union of the lines. This classification was started in a recent preprint by Maletto. There is one arrangement which is realizable pseudoholomorphically but not algebraically. It can be constructed in different ways, in particular, by a combinatorial patchworking on an irregular triangulation. This is the first example of a combinatorial patchworking which produces a PL curve in $RP^2$ whose arrangement relative to the coordinate axes is algebraically unrealizable.

math.AG↗
arXiv2026

On curves of degree 10 with 12 triple points

We construct an irreducible rational curve of degree 10 in $CP^2$ which has 12 triple points and a union of three rational quartics with 19 triple points. This gives counter-examples to a conjecture by Dimca, Harbourne, and Sticlaru. We also prove that there exists an analytic family $C_u$ of curves of degree 10 with 12 triple points which tends as $u\to 0$ to the union of the dual Hesse arrangement of lines (9 lines with 12 triple points) with an additional line. We hope that our approach to the proof of the latter fact could be of independent interest.

math.AG↗
arXiv2025

On mutual arrangements of a plane real curve relative to an $M$-quartic with an oval-snake

An oval $O$ of a plane real algebraic quartic curve $S$ is called a snake coiling around a real curve $C_k$ of degree $k$ if $O\cup\mathbb{R}C_k$ is isotopic to $O'\cup\mathbb{R}C_k$, where $O'$ is the boundary of a thickening of the embedded segment that transversally intersects $\mathbb{R}C_k$ at $2k$ points. In this article we prove that in this case $\mathbb{R}C_k\cup\mathbb{R}S$ is isotopic to $\mathbb{R}C_k\cup\mathbb{R}Q$, where $Q$ is a perturbation of the doubled conic. We prove analogs of this statement for real pseudoholomorphic curves under some additional assumptions.

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arXiv2024

Separating semigroup of genus 4 curves

A rational function on a real algebraic curve $C$ is called separating if it takes real values only at real points. Such a function defines a covering $\mathbb R C\to\mathbb{RP}^1$. Let $c_1,\dots,c_r$ be connected components of $\mathbb R C$. M. Kummer and K. Shaw defined the separating semigroup of $C$ as the set of all sequences $(d_1(f),\dots,d_r(f))$ where $f$ is a separating function and $d_i(f)$ is the degree of the restriction of $f$ to $c_i$. In the present paper we describe the separating semigroups of all genus 4 curves. For the proofs we consider the canonical embedding of $C$ into a quadric $X$ in $\mathbb P^3$ and apply Abel's theorem to 1-forms obtained as Poincar\'e residues at $C$ of certain meromorphic 2-forms on $X$.

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arXiv2024

Seifert forms and slice Euler characteristic of links

We define the Witt coindex of a link with non-trivial Alexander polynomial, as a concordance invariant from the Seifert form. We show that it provides an upper bound for the (locally flat) slice Euler characteristic of the link, extending the work of Levine on algebraically slice knots and Taylor on the genera of knots. Then we extend the techniques by Levine on isometric structures and characterize completely the forms of coindex $1$ under the condition that the symmetrized Seifert form is non-degenerate. We illustrate our results with examples where the coindex is used to show that a two-component link does not bound a locally flat cylinder in the four-ball, whereas any other known restriction does not show it.

math.GT↗
arXiv2023

C-boundary links up to six crossings

An oriented link is called $\mathbb C$-boundary if it is realizable as $(\partial B,A\cap\partial B)$ where $A$ is an algebraic curve in $\mathbb C^2$ and $B$ is an embedded $4$-ball. This notion was introduced by Michel Boileau and Lee Rudolph in 1995. In a recent joint paper with N.G. Kruzhilin we gave a complete classification of $\mathbb C$-boundaries with at most 5 crossings. In the present paper a more regular method of construction of $\mathbb C$-boundaries is proposed and the classification is extended up to 6 crossings.

math.GT↗
arXiv2022

Complete bipartite graphs flexible in the plane

A complete bipartite graph $K_{3,3}$, considered as a planar linkage with joints at the vertices and with rods as edges, in general admits only motions as a whole, i.e., is inflexible. Two types of its paradoxical mobility were found by Dixon in 1899. Later on, in a series of papers by different authors, the question of flexibility of $K_{m,n}$ was solved for almost all pairs $(m,n)$. In the present paper, we solve it for all complete bipartite graphs in the Euclidean plane as well as in the sphere and in the hyperbolic plane. We give independent self-contained proofs without extensive computations which are almost the same in the Euclidean, hyperbolic and spherical cases.

math.AG↗
arXiv2022

Diffusion orthogonal polynomials in 3-dimensional domains bounded by developable surfaces

The following problem is studied: describe the triplets $(\Omega,g,\mu)$, $\mu=\rho\,dx$, where $g= (g^{ij}(x))$ is the (co)metric associated with the symmetric second order differential operator $L(f) = \frac{1}{\rho}\sum_{ij} \partial_i (g^{ij} \rho\,\partial_j f)$ defined on a domain $\Omega$ of $\mathbb R^n$ and such that there exists an orthonormal basis of $\mathcal L^2(\mu)$ made of polynomials which are eigenvectors of $L$, and the basis is compatible with the filtration of the space of polynomials with respect to some weighted degree. In a joint paper with D. Bakry and M. Zani this problem was solved in dimension 2 for the usual degree. In the author's subsequent paper this problem was solved in dimension 2 for any weighted degree. In the present paper this problem is solved in dimension 3 for the usual degree under the condition that $\partial\Omega$ contains a piece of a tangent developable surface. The proof is based on Pl\"ucker-like formulas in the form given by Ragni Piene. All the found solutions are generalized for any dimension.

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arXiv2022

Counting lattice triangulations: Fredholm equations in combinatorics

Let $f(m,n)$ be the number of primitive lattice triangulations of $m\times n$ rectangle. We compute the limits $\lim_n f(m,n)^{1/n}$ for $m=2$ and $3$. For $m=2$ we obtain the exact value of the limit which is equal to $(611+\sqrt{73})/36$. For $m=3$, we express the limit in terms of certain Fredholm's integral equation on generating functions. This provides a polynomial time algorithm for computation of the limit with any given precision (polynomial with respect the the number of computed digits).

math.CO↗
arXiv2022

Compactification of the space of branched coverings of the two-dimensional sphere

For a closed oriented surface $ Σ$ we define its degenerations into singular surfaces that are locally homeomorphic to wedges of disks. Let $X_{Σ,n}$ be the set of isomorphism classes of orientation preserving $n$-fold branched coverings $ Σ\rightarrow S^2 $ of the two-dimensional sphere. We complete $X_{Σ,n}$ with the isomorphism classes of mappings that cover the sphere by the degenerations of $ Σ$. In case $ Σ=S^2$, the topology that we define on the obtained completion $\bar{X}_{Σ,n}$ coincides on $X_{S^2,n}$ with the topology induced by the space of coefficients of rational functions $ P/Q $, where $ P,Q $ are homogeneous polynomials of degree $ n $ on $ \mathbb{C}\mathrm{P}^1\cong S^2$. We prove that $\bar{X}_{Σ,n}$ coincides with the Diaz-Edidin-Natanzon-Turaev compactification of the Hurwitz space $H(Σ,n)\subset X_{Σ,n}$ consisting of isomorphism classes of branched coverings with all critical values being simple.

math.GT↗
arXiv2020

Algebraically unrealizable complex orientations of plane real pseudoholomorphic curves

We prove two inequalities for the complex orientations of a separating (Type I) non-singular real algebraic curve in $RP^2$ of any odd degree. We also construct a separating non-singular pseudoholomorphic curve in $RP^2$ of any degree congruent to 9 mod 12 which does not satisfies one of these inequalities. Therefore the oriented isotopy type of the real locus of each of these curves is algebraically unrealizable.

math.AG↗
arXiv2020

Homomorphisms of commutator subgroups of braid groups with small number of strings

For any $n$, we describe all endomorphisms of the braid group $B_n$ and of its commutator subgroup $B'_n$, as well as all homomorphisms $B'_n\to B_n$. These results are new only for small $n$ because endomorphisms of $B_n$ are already described by Castel for $n\ge 6$, and homomorphisms $B'_n\to B_n$ and endomorphisms of $B'_n$ are already described by Kordek and Margalit for $n\ge 7$. We use very different approaches for $n=4$ and for $n\ge 5$.

math.GR↗
arXiv2020

Signatures of iterated torus links

We compute the multivariate signatures of any Seifert link (that is a union of some fibers in a Seifert homology sphere), in particular, of the union of a torus link with one or both of its cores (cored torus link). The signatures of cored torus links are used in Degtyarev-Florens-Lecuona splicing formula for computation of multivariate signatures of cables over links. We use Neumann's computation of equivariant signatures of such links. For signatures of torus links with the core(s) we also rewrite the Neumann's formula in terms of integral points in a certain parallelogram, similar to Hirzebruch's formula for signatures of torus links (without cores) via integral points in a rectangle.

math.GT↗
arXiv2020

Plane algebraic curves in fancy balls

Boileau and Rudolph called a link $L$ in the $3$-sphere a $\bf C$-boundary if it can be realized as the intersection of an algebraic curve $A$ in $\bf C^2$ with the boundary of a smooth embedded $4$-ball $B$. They showed that some links are not $\bf C$-boundaries. We say that $L$ is a strong $\bf C$-boundary if $A\setminus B$ is connected. In particular, all quasipositive links are strong $\bf C$-boundaries. In this paper we give examples of non-quasipositive strong $\bf C$-boundaries and non-strong $\bf C$-boundaries. We give a complete classification of (strong) $\bf C$-boundaries with at most 5 crossings.

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arXiv2019

Quasipositive braids and connected sums

We prove that the connected sum of two links is quasipositive if and onlyif each summand is quasipositive. The prove is based on the filling disk technique

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arXiv2018

Irreducibility of lemniscates

We prove that lemniscates (i.e., sets of the form $|P(z)|=1$ where $P$ is a complex polynomial) are irreducible real algebraic curves.

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arXiv2013

On modular computation of Groebner bases with integer coefficients

Let $I_1\subset I_2\subset\dots$ be an increasing sequence of ideals of the ring $\Bbb Z[X]$, $X=(x_1,\dots,x_n)$ and let $I$ be their union. We propose an algorithm to compute the Gr\"obner base of $I$ under the assumption that the Gr\"obner bases of the ideal $\Bbb Q I$ of the ring $\Bbb Q[X]$ and the the ideals $I\otimes(\Bbb Z/m\Bbb Z)$ of the rings $(\Bbb Z/m\Bbb Z)[X]$ are known. Such an algorithmic problem arises, for example, in the construction of Markov and semi-Markov traces on cubic Hecke algebras.

math.AC↗