arXiv subjects

Laurentiu Ploscaru

Publications and source records attributed to Laurentiu Ploscaru.

4 recordsLinked to original sources

arXiv2026

Many Antipodal Pairs Force Many Neighboring Pairs

Let $X=\{x_1,\dots,x_n\}\subset \mathbb{R}^2$ be a finite set of points of diameter at most $1$. It is natural to expect that if many pairs $(x_i,x_j)$ lie at distance close to $1$ from each other, then some clustering phenomenon must occur, implying that a significant number of these pairs are also very close to each other. %For $0<\varepsilon<1$, we call a pair $(x_i,x_j)$ $\varepsilon$-antipodal if $\|x_i-x_j\|\ge 1-\varepsilon$ and $\varepsilon$-neighboring if $\|x_i-x_j\|\le \varepsilon$. We prove that there exists a universal constant $c>0$ such that for all $0<\varepsilon<1$, whenever $n$ is large enough, we have: \[ \big|\{(i,j):\|x_i-x_j\|\le \varepsilon\}\big| \geq c\cdot \varepsilon^{1/2}\cdot \big|\{(i,j):\|x_i-x_j\|\geq 1-\varepsilon\}\big|. \] This confirms a recent conjecture of Steinerberger, who asked whether the $\varepsilon^{1/2}$ ratio is the best possible. We also study a two-parameter version of Steinerberger's question by considering the number of pairs at distance at most $\varepsilon_1$ and at distance at least $1-\varepsilon_2$. We show that in this case the optimal ratio is $\varepsilon_1^2\cdot\varepsilon_2^{-3/2}$. The proof proceeds by introducing an auxiliary graph associated with the set $X$ and reducing the problem to bounding the largest eigenvalue of its adjacency matrix. Our main result is the outcome of human--AI interactions using ChatGPT 5.4.

math.MG↗

arXiv2024

Distinct degrees and homogeneous sets II

Given an $n$-vertex graph $G$, let $\hom (G)$ denote the size of a largest homogeneous set in $G$ and let $f(G)$ denote the maximal number of distinct degrees appearing in an induced subgraph of $G$. The relationship between these parameters has been well studied by several researchers over the last 40 years, beginning with Erdős, Faudree and Sós in the Ramsey regime when $\hom (G) = O(\log n)$. Our main result here proves that any $n$-vertex graph $G$ with $\hom (G) \leq n^{1/2}$ satisfies \begin{align*} f(G) \geq \sqrt[3]{\frac {n^2}{\hom (G)} } \cdot n^{-o(1)}. \end{align*} This confirms a conjecture of the authors from a previous work, in which we addressed the $\hom (G) \geq n^{1/2}$ regime. Together, these provide the complete extremal relationship between these parameters (asymptotically), showing that any $n$-vertex graph $G$ satisfies \begin{align*} \max \Big ( f(G) \cdot \hom (G), \sqrt {f(G) ^3 \cdot \hom (G) } \Big ) \geq n^{1-o(1)}. \end{align*} This relationship is tight (up to the $n^{-o(1)}$ term) for all possible values of $\hom (G)$, from $Ω(\log n )$ to $n$, as demonstrated by appropriately generated Erdős $-$ Renyi random graphs.

math.CO↗

arXiv2022

Distinct degrees and homogeneous sets

In this paper we investigate the extremal relationship between two well-studied graph parameters: the order of the largest homogeneous set in a graph $G$ and the maximal number of distinct degrees appearing in an induced subgraph of $G$, denoted respectively by $\hom (G)$ and $f(G)$. Our main theorem improves estimates due to several earlier researchers and shows that if $G$ is an $n$-vertex graph with $\hom (G) \geq n^{1/2}$ then $f(G) \geq \big ( {n}/{\hom (G)} \big )^{1 - o(1)}$. The bound here is sharp up to the $o(1)$-term, and asymptotically solves a conjecture of Narayanan and Tomon. In particular, this implies that $\max \{ \hom (G), f(G) \} \geq n^{1/2 -o(1)}$ for any $n$-vertex graph $G$,which is also sharp. The above relationship between $\hom (G)$ and $f(G)$ breaks down in the regime where $\hom (G) < n^{1/2}$. Our second result provides a sharp bound for distinct degrees in biased random graphs, i.e. on $f\big (G(n,p) \big )$. We believe that the behaviour here determines the extremal relationship between $\hom (G)$ and $f(G)$ in this second regime. Our approach to lower bounding $f(G)$ proceeds via a translation into an (almost) equivalent probabilistic problem, and it can be shown to be effective for arbitrary graphs. It may be of independent interest.

math.CO↗

arXiv2022

A bipartite version of the Erdős $-$ McKay conjecture

An old conjecture of Erdős and McKay states that if all homogeneous sets in an $n$-vertex graph are of order $O(\log n)$ then the graph contains induced subgraphs of each size from $\{0,1,\ldots, Ω(n^2)\}$. We prove a bipartite analogue of the conjecture: if all balanced homogeneous sets in an $n \times n$ bipartite graph are of order $O(\log n)$ then the graph contains induced subgraphs of each size from $\{0,1,\ldots, Ω(n^2)\}$.

math.CO↗