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Arsenii Sagdeev

Publications and source records attributed to Arsenii Sagdeev.

At least 19 recordsLinked to original sources
arXiv2026

Vertex-Ramsey theorems for Cartesian powers of graphs

For graphs $G,H$ and positive integers $r$ and $n$ we write $G^{\square n} \xrightarrow{r} H$ if every $r$-vertex-coloring of the Cartesian power $G^{\square n}$ of $G$ contains a monochromatic copy of $H$. Since chromatic number $χ$ of $G^{\square n}$ is the same as $χ(G)$, there is an $r$-vertex coloring of $G^{\square n}$ for $r=χ(G)$, such that each color class is an independent set. We prove that for $r<χ(G)$ there is a large class of graphs $H$ such that $G^{\square n} \xrightarrow{r} H$. These graphs are so-called layered graphs in a hypercube. We also show that for some graphs $G$, such as for example odd cycles or cliques, the class of layered graphs $H$ is the only one satisfying the above Ramsey property when $χ(G)/2 < r < χ(G)$. In addition, we prove a more general result relating Ramsey properties of $G$ and graphs $H$ such that $G^{\square n} \xrightarrow{r} H$. One of the technical tools is a Ramsey-type statement for discrete cubes $[m]^n$ that we call the Cube Layered Lemma, which is of independent interest. One of the original motivations for studying Ramsey properties of Cartesian powers of $G$ is the fact that $G^{\square n}$ is a unit distance graph if $G$ is a unit distance graph. This provides applications in Euclidean Ramsey theory.

math.CO↗
arXiv2026

Largest density of a layered subgraph of a hypercube

Let $L(t)$ denote the largest number of edges induced by $t$ vertices from two vertex layers of a hypercube. We show that $$\frac14 t\log_2 t+\frac18 t\log_2\log_2 t-O(t) \leq L(t) \leq \frac14 t\log_2 t+ (1+o(1))t\log_2\log_2 t.$$

math.CO↗
arXiv2026

Ordered Yao graphs: maximum degree, edge density, and clique numbers

For a positive integer $k$ and an ordered set of $n$ points in the plane, define its \textit{k-sector ordered Yao graphs} as follows. Divide the plane around each point into $k$ equal sectors and draw an edge from each point to its closest predecessor in each of the $k$ sectors. We analyze several natural parameters of these graphs. Our main results are as follows: I Let $d_k(n)$ be the maximum integer so that for every $n$-element point set in the plane, there exists an order such that the corresponding $k$-sector ordered Yao graph has maximum degree at least $d_k(n)$. We show that $d_k(n)=n-1$ if $k=4$ or $k \ge 6$, and provide some estimates for the remaining values of $k$. Namely, we show that $d_1(n) = Θ( \log {n} )$; $\frac{1}{2}(n-1) \le d_3(n) \le 5\left\lceil\frac{n}{6}\right\rceil-1$; $\frac{2}{3}(n-1) \le d_5(n) \le n-1$; II Let $e_k(n)$ be the minimum integer so that for every $n$-element point set in the plane, there exists an order such that the corresponding $k$-sector ordered Yao graph has at most $e_k(n)$ edges. Then $e_k(n)=\left\lceil\frac{k}{2}\right\rceil\cdot n-o(n)$. III Let $w_k$ be the minimum integer so that for every point set in the plane, there exists an order such that the corresponding $k$-sector ordered Yao graph has clique number at most $w_k$. Then $\lceil\frac{k}{2}\rceil \le w_k\le \lceil\frac{k}{2}\rceil+1$. All the orders mentioned above can be constructed effectively.

math.CO↗
arXiv2026

Cutting corners

We say that a subset $M$ of $\mathbb R^n$ is exponentially Ramsey if there are $ε>0$ and $n_0$ such that $χ(\mathbb R^n,M)\ge(1+ε)^n$ for any $n>n_0$, where $χ(\mathbb R^n,M)$ stands for the minimum number of colors in a coloring of $\mathbb R^n$ such that no copy of $M$ is monochromatic. One important result in Euclidean Ramsey theory is due to Frankl and Rödl, and states the following (under some mild extra conditions): if both $N_1$ and $N_2$ are exponentially Ramsey then so is $N_1\times N_2$. Applied several times to two-point sets, this result implies that any subset of a `hyperrectangle' is exponentially Ramsey. However, generally, such `embeddings' result in very inefficient bounds on the aforementioned $ε$. In this paper, we present another way of combining exponentially Ramsey sets, which gives much better estimates in some important cases. In particular, we show that the chromatic number of $\mathbb R^n$ with a forbidden equilateral triangle satisfies $χ(\mathbb R^n,\triangle)\ge\big(1.0742...+o(1)\big)^n$, greatly improving upon the previous constant $1.0144$. We also obtain similar strong results for regular simplices of larger dimensions, as well as for related geometric Ramsey-type questions in Manhattan norm. We then show that the same technique implies several interesting corollaries in other combinatorial problems. In particular, we give an explicit upper bound on the size of a family $\mathcal F\subset2^{[n]}$ that contains no weak $k$-sunflowers, i.e. no collection of $k$ sets with pairwise intersections of the same size. This bound improves upon previously known results for all $k\ge4$. Finally, we also present a simple deduction of the (other) celebrated Frankl--Rödl theorem from an earlier result of Frankl and Wilson. It gives probably the shortest known proof of Frankl and Rödl result with the most efficient bounds.

math.CO↗
arXiv2025

Ramsey problems for graphs in Euclidean spaces and Cartesian powers

Given a graph $H$, let $χ_H(\mathbb{R}^n)$ be the smallest positive integer $r$ such that there exists an $r$-coloring of $\mathbb{R}^n$ with no monochromatic unit-copy of $H$, that is a set of $|V(H)|$ vertices of the same color such that any two vertices corresponding to an edge of $H$ are at distance one. This Ramsey-type function extends the famous Hadwiger--Nelson problem on the chromatic number $χ(\mathbb{R}^n)=χ_{K_2}(\mathbb{R}^n)$ of the space from a complete graph $K_2$ on two vertices to an arbitrary graph $H$. It also extends the classical Euclidean Ramsey problem for congruent monochromatic subsets to the family of those defined by a specific subset of unit distances. Among others, we show that $χ_H(\mathbb{R}^n)=χ(\mathbb{R}^n)$ for any even cycle $H$ of length $8$ or at least $12$ as well as for any forest and that $χ_H(\mathbb{R}^n)=\lceilχ(\mathbb{R}^n)/2\rceil$ for any sufficiently long odd cycle. Our main tools and results, which are of independent interest, establish that Cartesian powers enjoy Ramsey-type properties for graphs with favorable Turán-type characteristics, such as zero hypercube Turán density. In addition, we prove induced variants of these results, find bounds on $χ_H(\mathbb{R}^n)$ for growing dimensions $n$, and prove a canonical-type result. We conclude with many open problems. One of these is to determine $χ_{C_4}(\mathbb{R}^2)$, for a cycle $C_4$ on four vertices.

math.CO↗
arXiv2025

Lattice and Non-lattice Piercing of Axis-Parallel Rectangles

Given a family ${\mathcal F}$ of shapes in the plane, we study what is the lowest possible density of a point set $P$ that pierces (``intersects'', ``hits'') all translates of each shape in ${\mathcal F}$. For instance, if ${\mathcal F}$ consists of two axis-parallel rectangles the best known piercing set, i.e., one with the lowest density, is a lattice. Given a finite family ${\mathcal F}$ of axis-parallel rectangles, we present an algorithm for finding an optimal ${\mathcal F}$-piercing lattice. The algorithm runs in time polynomial in the number of rectangles and the maximum aspect ratio of the rectangles in the family. No prior algorithms for this problem were known. On the other hand, we show that for every $n \geq 3$, there exists a family of $n$ axis-parallel rectangles for which the best piercing density achieved by a lattice is separated by a positive (constant) gap from the optimal piercing density for the respective family. Finally, we show that the best lattice can be sometimes worse by $20\%$ than the optimal piercing set.

cs.CG↗
arXiv2025

Maximizing the Maximum Degree in Ordered Nearest Neighbor Graphs

For an ordered point set in a Euclidean space or, more generally, in an abstract metric space, the ordered Nearest Neighbor Graph is obtained by connecting each of the points to its closest predecessor by a directed edge. We show that for every set of $n$ points in $\mathbb{R}^d$, there exists an order such that the corresponding ordered Nearest Neighbor Graph has maximum degree at least $\log{n}/(4d)$. Apart from the $1/(4d)$ factor, this bound is the best possible. As for the abstract setting, we show that for every $n$-element metric space, there exists an order such that the corresponding ordered Nearest Neighbor Graph has maximum degree $Ω(\sqrt{\log{n}/\log\log{n}})$.

math.CO↗
arXiv2025

Canonical theorems in geometric Ramsey theory

In Euclidean Ramsey Theory usually we are looking for monochromatic configurations in the Euclidean space, whose points are colored with a fixed number of colors. In the canonical version, the number of colors is arbitrary, and we are looking for an `unavoidable' set of colorings of a finite configuration, that is a set of colorings with the property that one of them always appears in any coloring of the space. This set definitely includes the monochromatic and the rainbow colorings. In the present paper, we prove the following two results of this type. First, for any acute triangle $T$, and any coloring of $\mathbb{R}^3$, there is either a monochromatic or a rainbow copy of $T$. Second, for every $m$, there exists a sufficiently large $n$ such that in any coloring of $\mathbb{R}^n$, there exists either a monochromatic or a rainbow $m$-dimensional unit hypercube. In the maximum norm, $\ell_{\infty}$, we have a much stronger statement. For every finite $M$, there exits an $n$ such that in any coloring of $\mathbb{R}_\infty^n$, there is either a monochromatic or a rainbow isometric copy of $M$.

math.CO↗
arXiv2025

Packing Density of Sets With Only Two Nonmixed Gaps

For a finite set of integers such that the first few gaps between its consecutive elements equal $a$, while the remaining gaps equal $b$, we study dense packings of its translates on the line. We obtain an explicit lower bound on the corresponding optimal density, conjecture its tightness, and prove it in case one of the gap lengths, $a$ or $b$, appears only once. This is equivalent to a Motzkin problem on the independence ratio of certain integer distance graphs.

math.CO↗
arXiv2025

Faces in girth-saturated graphs on surfaces

What is the maximum length ${\rm f}_{\rm max}(\ell, Σ)$ of a facial cycle of an inclusion-maximal graph with girth at least $\ell$ embedded on a given surface $Σ$? If $Σ=\mathcal{P}$ is a plane, we show that $3\ell-11\leq {\rm f}_{\rm max}(\ell, \mathcal{P})\leq 8\ell-13$. We also prove that ${\rm f}_{\rm max}(\ell, Σ)$ is bounded for any integer $\ell$ and any closed surface $Σ$. For a fixed $Σ$, we show that $Ω(\ell) ={\rm f}_{\rm max}(\ell, Σ) = O(\ell^2)$, while for a fixed $\ell\ge 6$, ${\rm f}_{\rm max}(\ell, Σ)=Θ(g)$, where $g$ is the genus of $Σ$.

math.CO↗
arXiv2024

A Stopping Game on Zero-Sum Sequences

We introduce and analyze a natural game formulated as follows. In this one-person game, the player is given a random permutation $A=(a_1,\dots, a_n)$ of a multiset $M$ of $n$ reals that sum up to $0$, where each of the $n!$ permutation sequences is equally likely. The player only knows the value of $n$ beforehand. The elements of the sequence are revealed one by one and the player can stop the game at any time. Once the process stops, say, after the $i$th element is revealed, the player collects the amount $\sum_{j=i+1}^{n} a_j$ as his/her payoff and the game is over (the payoff corresponds to the unrevealed part of the sequence). Three online algorithms are given for maximizing the expected payoff in the binary case when $M$ contains only $1$'s and $-1$'s. $\texttt{Algorithm 1}$ is slightly suboptimal, but is easier to analyze. Moreover, it can also be used when $n$ is only known with some approximation. $\texttt{Algorithm 2}$ is exactly optimal but not so easy to analyze on its own. $\texttt{Algorithm 3}$ is the simplest of all three. It turns out that the expected payoffs of the player are $Θ(\sqrt{n})$ for all three algorithms. In the end, we address the general problem and deal with an arbitrary zero-sum multiset, for which we show that our $\texttt{Algorithm 3}$ returns a payoff proportional to $\sqrt{n}$, which is worst case-optimal.

cs.DM↗
arXiv2024

General penny graphs are at most 43/18-dense

We prove that among $n$ points in the plane in general position, the shortest distance occurs at most $43n/18$ times, improving upon the upper bound of $17n/7$ obtained by Tóth in 1997.

math.CO↗
arXiv2024

Monochromatic infinite sets in Minkowski planes

We prove that for any $\ell_p$-norm in the plane with $1<p<\infty$ and for every infinite $\mathcal{M} \subset \mathbb{R}^2$, there exists a two-colouring of the plane such that no isometric copy of $\mathcal{M}$ is monochromatic. On the contrary, we show that for every polygonal norm (that is, the unit ball is a polygon) in the plane, there exists an infinite $\mathcal{M} \subset \mathbb{R}^2$ such that for every two-colouring of the plane there exists a monochromatic isometric copy of $\mathcal{M}$.

math.CO↗
arXiv2024

Max-norm Ramsey Theory

Given a metric space $\mathcal{M}$ that contains at least two points, the chromatic number $χ\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ is defined as the minimum number of colours needed to colour all points of an $n$-dimensional space $\mathbb{R}^n_{\infty}$ with the max-norm such that no isometric copy of $\mathcal{M}$ is monochromatic. The last two authors have recently shown that the value $χ\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ grows exponentially for all finite $\mathcal{M}$. In the present paper we refine this result by giving the exact value $χ_{\mathcal{M}}$ such that $χ\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right) = (χ_{\mathcal{M}}+o(1))^n$ for all 'one-dimensional' $\mathcal{M}$ and for some of their Cartesian products. We also study this question for infinite $\mathcal{M}$. In particular, we construct an infinite $\mathcal{M}$ such that the chromatic number $χ\left(\mathbb{R}^n_{\infty}, \mathcal{M} \right)$ tends to infinity as $n \rightarrow \infty$.

math.CO↗
arXiv2023

A note on Borsuk's problem in Minkowski spaces

In 1993, Kahn and Kalai famously constructed a sequence of finite sets in $d$-dimensional Euclidean spaces that cannot be partitioned into less than $(1.203\ldots+o(1))^{\sqrt{d}}$ parts of smaller diameter. Their method works not only for the Euclidean, but for all $\ell_p$-spaces as well. In this short note, we observe that the larger the value of $p$, the stronger this construction becomes.

math.MG↗
arXiv2022

Two-colorings of normed spaces without long monochromatic unit arithmetic progressions

Given a natural $n$, we construct a two-coloring of $\mathbb{R}^n$ with the maximum metric satisfying the following. For any finite set of reals $S$ with diameter greater than $5^{n}$ such that the distance between any two consecutive points of $S$ does not exceed one, no isometric copy of $S$ is monochromatic. As a corollary, we prove that any normed space can be two-colored such that all sufficiently long unit arithmetic progressions contain points of both colors.

math.MG↗