Searcharxiv⌕ Search

arXiv subjects

Sergey Finashin

Publications and source records attributed to Sergey Finashin.

At least 19 recordsLinked to original sources
arXiv2026

Recognising conjugacy classes of Dehn twists on $\mathbb D_3$ via Dynnikov coordinates

We describe in terms of Dynnikov coordinates the three orbits of the pure mapping class group action on the set of essential curves in a 3-punctured disc $\mathbb D_3$. For any essential curve $\gamma\subset\mathbb D_3$ we present an efficient algorithm to untwist $\gamma$ into one of the 3 basic representatives of the orbits. In turn, we give transformation formulas between the Dynnikov and torus $\mathbb Z^2$-coordinates for multicurves. Besides proving minimality of the algorithm, we give an explicit minimal length formula in terms of the associated even continued fractions.

math.GT↗
arXiv2025

Monodromy factorizations of lines on del Pezzo surfaces

We give a list of monodromy factorizations in the pure mapping class group $PMod(T_{d+1})$ of a torus with d+1 marked points that represent lines on a del Pezzo surface Y of degree $d\le4$. These factorizations are lifts of a certain fixed monodromy factorization in $PMod(T_d)$ that represents Y. In the case d=1, discussed in more detail, we give an explicit correspondence between such factorizations and the 240 roots of $E_8=K^\perp$, the orthogonal complement in $H_2(Y)$ of the canonical class.

math.AG↗
arXiv2024

The real Mordell-Weil group of rational elliptic surfaces and real lines on del Pezzo surfaces of degree $K^2=1$

We undertake a study of topological properties of the real Mordell-Weil group $\operatorname{MW}_{\mathbb R}$ of real rational elliptic surfaces $X$ which we accompany by a related study of real lines on $X$ and on the "subordinate" del Pezzo surfaces $Y$ of degree 1. We give an explicit description of isotopy types of real lines on $Y_{\mathbb R}$ and an explicit presentation of $\operatorname{MW}_{\mathbb R}$ in the mapping class group $\operatorname{Mod}(X_{\mathbb R})$. Combining these results we establish an explicit formula for the action of $\operatorname{MW}_{\mathbb R}$ in $H_1(X_{\mathbb R})$.

math.AG↗
arXiv2023

On Affine Real Cubic Surfaces

We prove that the space of affine, transversal at infinity, non-singular real cubic surfaces has 15 connected components. We give a topological criterion to distinguish them and show also how these 15 components are adjacent to each other via wall-crossing.

math.AG↗
arXiv2022

Combined count of real rational curves of canonical degree 2 on real del Pezzo surfaces with $K^2=1$

We propose two systems of "intrinsic" signs for counting such curves. In both cases the result acquires an exceptionally strong invariance property: it does not depend on the choice of a surface. One of our counts includes all divisor classes of canonical degree 2 and gives in total 30. The other one excludes the class $-2K$, but adds up the results of counting for a pair of real structures that differ by Bertini involution. This count gives 96.

math.AG↗
arXiv2022

On wall-crossing invariance of certain sums of Welschinger numbers

We continue our quest for real enumerative invariants not sensitive to changing the real structure and extend the construction we uncovered previously for counting curves of anti-canonical degree $\leqslant 2$ on del Pezzo surfaces with $K^2=1$ to curves of any anti-canonical degree and on any del Pezzo surfaces of degree $K^2\leqslant 3$.

math.AG↗
arXiv2021

Two kinds of real lines on real del Pezzo surfaces of degree 1

We show how the real lines on a real del Pezzo surface of degree 1 can be split into two species, elliptic and hyperbolic, via a certain distinguished, intrinsically defined, Pin-structure on the real locus of the surface. We prove that this splitting is invariant under real automorphisms and real deformations of the surface, and that the difference between the total numbers of hyperbolic and elliptic lines is always equal to 16.

math.AG↗
arXiv2020

A glimpse into Rokhlin's Signature Divisibility Theorem

This paper was conceived as an addendum to the note "Rokhlin's signature theorems" (by O.Viro and the authors of this paper). In the main section we give an overview of Rokhlin's proof of his famous theorem on divisibility of signature by 16. In the appendix we retrace some of further developments that show how this theorem became a cornerstone in the contemporary theory of manifolds.

math.GT↗
arXiv2020

Rokhlin's signature theorems

This note is written for a book dedicated to outstanding St-Petersburg mathematicians and timed to the ICM-2022 in St-Petersburg. In accordance with the plan of ICM-organizers, we try to tell about one of the most prominent Rokhlin's achievements in an accessible form and respecting the allowed volume.

math.HO↗
arXiv2020

Chirality of Real Non-singular Cubic Fourfolds and Their Pure Deformation Classification

In our previous works we have classified real non-singular cubic hypersurfaces in the 5-dimensional projective space up to equivalence that includes both real projective transformations and continuous variations of coefficients preserving the hypersurface non-singular. Here, we perform a finer classification giving a full answer to the chirality problem: which of real non-singular cubic hypersurfaces can not be continuously deformed to their mirror reflection.

math.AG↗
arXiv2019

First homology of a real cubic is generated by lines

We suggest a short proof of O.Benoist and O.Wittenberg theorem (arXiv:1907.10859) which states that for each real non-singular cubic hypersurface $X$ of dimension $\ge 2$ the real lines on $X$ generate the whole group $H_1(X(\Bbb R);\Bbb Z/2)$.

math.AG↗
arXiv2019

Segre indices and Welschinger weights as options for invariant count of real lines

In our previous paper we have elaborated a certain signed count of real lines on real projective n-dimensional hypersurfaces of degree 2n-1. Contrary to the honest "cardinal" count, it is independent of the choice of a hypersurface, and by this reason provides a strong lower bound on the honest count. In this count the contribution of a line is its local input to the Euler number of a certain auxiliary vector bundle. The aim of this paper is to present other, in a sense more geometric, interpretations of this local input. One of them results from a generalization of Segre species of real lines on cubic surfaces and another from a generalization of Welschinger weights of real lines on quintic threefolds.

math.AG↗
arXiv2019

Deformation classes of real Cayley M-octads

We study 8-point configurations in the real projective space forming an intersection locus of three quadrics and containing no coplanar quadruples. We found that there exists precisely 8 mirror-pairs of deformation classes of such configurations. We describe also the mutual position of these 8 pairs and find the real monodromy groups acting on the 8-point configurations, for each deformation class.

math.AG↗
arXiv2019

Deformation classification of real non-singular cubic threefolds with a marked line

We prove that the space of pairs $(X,l)$ formed by a real non-singular cubic hypersurface $X\subset P^4$ with a real line $l\subset X$ has 18 connected components and give for them several quite explicit interpretations. The first one relates these components to the orbits of the monodromy action on the set of connected components of the Fano surface $F_\mathbb{R}(X)$ formed by real lines on $X$. For another interpretation we associate with each of the 18 components a well defined real deformation class of real non-singular plane quintic curves and show that this deformation class together with the real deformation class of $X$ characterizes completely the component.

math.AG↗
arXiv2017

Topology of Real Schlafli Six-Line Configurations on Cubic Surfaces and in $\mathbb{RP}^3$

A famous configuration of 27 lines on a non-singular cubic surface in $\mathbb P^3$ contains remarkable subconfigurations, and in particular the ones formed by six pairwise disjoint lines. We study such six-line configurations in the case of real cubic surfaces from topological viewpoint, as configurations of six disjoint lines in the real projective 3-space, and show that the condition that they lie on a cubic surface implies a very special property of {\it homogeneity}. This property distinguish them in the list of 11 deformation types of configurations formed by six disjoint lines in $\mathbb{RP}^3$.

math.AG↗
arXiv2015

Abundance of 3-planes on real projective hypersurfaces

We show that a generic real projective $n$-dimensional hypersurface of odd degree $d$, such that $4(n-2)=\binom{d+3}3$, contains "many" real 3-planes, namely, in the logarithmic scale their number has the same rate of growth, $d^3\log d$, as the number of complex 3-planes. This estimate is based on the interpretation of a suitable signed count of the 3-planes as the Euler number of an appropriate bundle.

math.AG↗
arXiv2015

Apparent contours of nonsingular real cubic surfaces

We give a complete deformation classification of real Zariski sextics, that is of generic apparent contours of nonsingular real cubic surfaces. As a by-product, we observe a certain "reversion" duality in the set of deformation classes of these sextics.

math.AG↗