arXiv subjects

A. Skopenkov

Publications and source records attributed to A. Skopenkov.

At least 55 records · Page 3Linked to original sources

arXiv2019

Embeddings of non-simply-connected 4-manifolds in 7-space. I. Classification modulo knots

We work in the smooth category. Let $N$ be a closed connected orientable 4-manifold with torsion free $H_1$, where $H_q:=H_q(N;Z)$. Our main result is a complete readily calculable classification of embeddings $N\to R^7$, up to the equivalence relation generated by isotopy and embedded connected sum with embeddings $S^4\to R^7$. Such a classification was already known only for $H_1=0$ by the work of Boéchat, Haefliger and Hudson from 1970. Our classification involves the Boéchat-Haefliger invariant $\varkappa(f)\in H_2$, Seifert bilinear form $λ(f):H_3\times H_3\to Z$ and $β$-invariant $β(f)$ which assumes values in a quotient of $H_1$ defined by values of $\varkappa(f)$ and $λ(f)$. In particular, for $N=S^1\times S^3$ we give a geometrically defined 1-1 correspondence between the set of equivalence classes of embeddings and an explicit quotient of the set $Z\oplus Z$. Our proof is based on development of Kreck modified surgery approach, involving some simpler reformulations, and also uses parametric connected sum.

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arXiv2019

Elements of mathematics in problems. Through olympiads and circles to profession

This is a collection of teaching materials used in several Russian universities, schools, and mathematical circles. Most problems are chosen in such a way that in the course of the solution and discussion a reader learns important mathematical ideas and theories. The materials can be used by pupils and students for self-study, and by teachers. This is an abridged pre-copyedit version of the published book submitted with the permission of the publisher. Each included individual material is self-contained and ready-for-use. Solutions to problems are not included intentionally. This collection consolidates updates of several arXiv submissions, e.g., arXiv:1305.2598.

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arXiv2019

How Fermat found extrema

We present a short elementary proof of the well-known criterion for a cubic polynomial to have three real roots. The proof is based on Fermat's approach to calculus for polynomials. This approach illustrates the idea of a derivative rigorously but without technical $\varepsilon$-$δ$ language. The note is accessible to high-school students. An English version is followed by a more detailed Russian version.

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arXiv2018

Eliminating Higher-Multiplicity Intersections, III. Codimension 2

We study conditions under which a finite simplicial complex $K$ can be mapped to $\mathbb R^d$ without higher-multiplicity intersections. An almost $r$-embedding is a map $f: K\to \mathbb R^d$ such that the images of any $r$ pairwise disjoint simplices of $K$ do not have a common point. We show that if $r$ is not a prime power and $d\geq 2r+1$, then there is a counterexample to the topological Tverberg conjecture, i.e., there is an almost $r$-embedding of the $(d+1)(r-1)$-simplex in $\mathbb R^d$. This improves on previous constructions of counterexamples (for $d\geq 3r$) based on a series of papers by M. Özaydin, M. Gromov, P. Blagojević, F. Frick, G. Ziegler, and the second and fourth present authors. The counterexamples are obtained by proving the following algebraic criterion in codimension 2: If $r\ge3$ and if $K$ is a finite $2(r-1)$-complex then there exists an almost $r$-embedding $K\to \mathbb R^{2r}$ if and only if there exists a general position PL map $f:K\to \mathbb R^{2r}$ such that the algebraic intersection number of the $f$-images of any $r$ pairwise disjoint simplices of $K$ is zero. This result can be restated in terms of cohomological obstructions or equivariant maps, and extends an analogous codimension 3 criterion by the second and fourth authors. As another application we classify ornaments $f:S^3 \sqcup S^3\sqcup S^3\to \mathbb R^5$ up to ornament concordance. It follows from work of M. Freedman, V. Krushkal and P. Teichner that the analogous criterion for $r=2$ is false. We prove a lemma on singular higher-dimensional Borromean rings, yielding an elementary proof of the counterexample.

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arXiv2017

A simple proof of the isoperimetric theorem for the hyperbolic plane

In this pedagogical note we present a short proof of the following main result of arxiv.org/abs/0911.5319, and clarify its relation to the isoperimetric problem. On the hyperbolic plane consider triangles ABC with fixed lengths of AB and AC. The maximal area of these triangles is attained for the triangle ABC such that \angle A=\angle B+\angle C. The proof is essentially the same as in the above-cited paper. However, since the result is beautiful and important, a short proof cleared of unnecessary details could be interesting for a reader. The note is accessible for students familiar with elementary hyperbolic geometry. (This note is based on referee reports to the above paper, see www.mccme.ru/mmks)

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arXiv2017

On the metastable Mabillard-Wagner conjecture

The purpose of this note is to attract attention to the following conjecture (metastable $r$-fold Whitney trick) by clarifying its status as not having a complete proof, in the sense described in the paper. Assume that $D=D_1\sqcup\ldots\sqcup D_r$ is disjoint union of $r$ disks of dimension $s$, $f:D\to B^d$ a proper PL map such that $f\partial D_1\cap\ldots\cap f\partial D_r=\emptyset$, $rd\ge (r+1)s+3$ and $d\ge s+3$. If the map $$f^r:\partial(D_1\times\ldots\times D_r)\to (B^d)^r-\{(x,x,\ldots,x)\in(B^d)^r\ |\ x\in B^d\}$$ extends to $D_1\times\ldots\times D_r$, then there is a PL map $\overline f:D\to B^d$ such that $$\overline f=f \quad\text{on}\quad D_r\cup\partial D\quad\text{and}\quad \overline fD_1\cap\ldots\cap \overline fD_r=\emptyset.$$

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arXiv2016

Algebraic topology from geometric viewpoint

This book is expository and is in Russian. It is shown how in the course of solution of interesting geometric problems (close to applications) naturally appear main notions of algebraic topology (homology groups, obstructions and invariants, characteristic classes). Thus main ideas of algebraic topology are presented with minimal technicalities. Familiarity of a reader with basic notions of topology (such as 2-dimensional manifolds and vector fields) is desirable, although definitions are given at the beginning. The book is accessible to undergraduates and could also be an interesting easy reading for professional mathematicians.

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arXiv2015

How do autodiffeomorphisms act on embeddings

We work in the smooth category. The following problem was suggested by E. Rees in 2002: describe the precomposition action of self-diffeomorphisms of S^p x S^q on the set of isotopy classes of embeddings S^p x S^q -> R^m. Let g : S^p x S^q -> R^m be an embedding such that g |_{a x S^q} : a x S^q -> R^m - g (b x S^q) is null-homotopic for some pair of different points a,b in S^p. Theorem. If h is an autodiffeomorphism of S^p x S^q identical on a neighborhood of a x S^q for some a\in S^p and p R^m, g : S^p x S^q -> R^m isotopy classes of embeddings. As a corollary we obtain that under certain conditions for orientation-preserving embeddings s : S^p x D^q -> N the S^p-parametric embedded connected sum f#_sg depends only on f,g and the homology class of s|_{S^p x 0}.

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arXiv2015

When Any Group of N Elements is Cyclic?

We give a simple proof of the well-known fact: any group of n elements is cyclic if and only if n and ϕ(n) are coprime. This note is accessible for students familiar with permutations and basic number theory. No knowledge of abstract group theory is required; a few necessary notions are introduced in the course of the proof. The note could also be an interesting easy reading for mature mathematicians.

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arXiv2015

Existence proofs in combinatorics using independence

This note is purely expository and is in Russian. We show how to prove interesting combinatorial results using the local Lovasz lemma. The note is accessible for students having basic knowledge of combinatorics; the notion of independence is defined and the Lovasz lemma is stated and proved. Our exposition follows `Probabilistic methods' of N. Alon and J. Spencer. The main difference is that we show how the proof could have been invented. The material is presented as a sequence of problems, which is peculiar not only to Zen monasteries but also to advanced mathematical education; most problems are presented with hints or solutions.

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arXiv2014

Some more proofs from the Book: solvability and insolvability of equations in radicals

This paper is purely expository. We present short elementary proofs of * the Gauss Theorem on constructibility of regular polygons; * the existence of a cubic equation unsolvable in real radicals; * the existence of a quintic equation unsolvable in complex radicals (Galois Theorem). The statements of these celebrated results are simple and well-known. However, their proofs given in most textbooks rely upon much unmotivated material and are far from being economic. We do not use the terms `Galois group' or even `group'. The paper is accessible for students familiar with polynomials and complex numbers, and could be an interesting easy reading for professional mathematicians. Short English version is followed by an extended Russian version where before presenting the proofs we illustrate the main ideas by sequences of problems with hints or solutions.

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arXiv2014

Basic differential geometry as a sequence of interesting problems

This book is expository and is in Russian (sample English translation of two pages is given). It is shown how in the course of solution of interesting geometric problems (close to applications) naturally appear different notions of curvature, which distinguish given geometry from the 'ordinary' one. Direct elementary definitions of these notions are presented. The book is accessible for students familiar with analysis of several variables, and could be an interesting easy reading for professional mathematicians. The material is presented as a sequence of problems, which is peculiar not only to Zen monasteries but also to serious mathematical education.

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arXiv2013

A two-page disproof of the Borsuk partition conjecture

It is presented the simplest known disproof of the Borsuk conjecture stating that if a bounded subset of n-dimensional Euclidean space contains more than n points, then the subset can be partitioned into n+1 nonempty parts of smaller diameter. The argument is due to N. Alon and is a remarkable application of combinatorics and algebra to geometry. This note is purely expository and is accessible for students.

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arXiv2013

Yet another proof from the Book: the Gauss theorem on regular polygons

This note is purely expository. The statement of the Gauss theorem on the constructibility of regular polygons by means of compass and ruler is simple and well-known. However, its proofs given in most textbooks rely upon much unmotivated material and are far from being economic. In this note a short elementary proof of the Gauss theorem is presented. The note is accessible for students familiar with polynomials and complex numbers, and could be an interesting easy reading for professional mathematicians.

math.HO↗

arXiv2013

A simple proof of the Abel-Ruffini theorem

This paper is purely expositional. The statement of the Abel-Ruffini theorem on unsolvability of equations using radicals is simple and well-known. We sketch an elementary proof of this theorem. We do not use the terms 'field extension', 'Galois group' and even 'group'. However, our presentation is a good way to learn (or recall) starting idea of the Galois theory. Our exposition follows `Mathematical Omnibus' of S. Tabachnikov and D.B. Fuchs (in English, http://www.math.psu.edu/tabachni/Books/taba.pdf). The main difference is that we show how the proof could have been invented. The paper is accessible for students familiar with complex numbers, and could be an interesting easy reading for mature mathematicians. The material is presented as a sequence of problems, which is peculiar not only to Zen monasteries but also to advanced mathematical education; most problems are presented with hints or solutions.

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arXiv2012

On the Kuratowski graph planarity criterion

This paper is purely expositional. The statement of the Kuratowski graph planarity criterion is simple and well-known. However, its classical proof is not easy. In this paper we present the Makarychev proof (with further simplifications by Prasolov, Telishev, Zaslavski and the author) which is possibly the simplest. In the Rusian version before the proof we present all the necessary definitions, and afterwards we state some close results on graphs and more general spaces. The paper is accessible for students familiar with the notion of a graph, and could be an interesting easy reading for mature mathematicians.

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arXiv2012

Embeddings of homology equivalent manifolds with boundary

We prove a theorem on equivariant maps implying the following two corollaries: (1) Let N and M be compact orientable n-manifolds with boundaries such that M\subset N, the inclusion M\to N induces an isomorphism in integral cohomology, both M and N have (n-d-1)-dimensional spines and m > max {n+2, (3n+1-d)/2} . Then the restriction-induced map E^m(N)\to E^m(M) is bijective. Here E^m(X) is the set of embeddings X\to R^m up to isotopy (in the PL or smooth category). (2) For a 3-manifold N with boundary whose integral homology groups are trivial and such that N\not\cong D^3 (or for its special 2-spine N) there exists an equivariant map from the deleted product of N to S^2, although N does not embed into R^3. The second corollary completes the answer to the following question: for which pairs (m,n) for each n-polyhedron N the existence of an equivariant map from the deleted product of N to S^{m-1} implies the embeddability of N into R^m? An answer was known for each pair (m,n) except (3,3) and (3,2).

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arXiv2011

Ambient homogeneity (in Russian)

This note is purely expository. A subset N of the plane is affine ambient homogeneous if for each x,y in N there exists an affine transformation taking x to y and N to itself. The result of D. Repovs, E. V. Scepin and the author on such subsets is presented, together with discussion, corollaries and generalizations. At the end some non-elementary corollaries are given (including a simple proof of the smooth version of the Hilbert-Smith conjecture on topological groups). Most part of the text is accessible to undergraduates familiar with the notion of continuity. The text could be an interesting easy reading for mature mathematicians.

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