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Elena Pavelescu

Publications and source records attributed to Elena Pavelescu.

At least 19 recordsLinked to original sources
arXiv2026

The total chord length of maximal outerplanar graphs

We consider embeddings of maximal outerplanar graphs whose vertices all lie on a cycle $\mathcal{C}$ bounding a face. Each edge of the graph that is not in $\mathcal{C}$, a chord, is assigned a length equal to the length of the shortest path in $\mathcal{C}$ between its endpoints. We define the total chord length of a graph as the sum of lengths of all its chords. For each order $n\ge 5$, we find outerplanar graphs whose total chord length is the minimum among all graphs of the same order, and graphs whose total chord length is the maximum among all graphs of the same order. We give a complete characterization of the graphs that attain the maximum total chord length. We show that every integer value in the interval determined by the minimum and maximum values is the total chord length of a maximal outerplanar graph of the same order.

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arXiv2026

New minor minimal non-apex graphs

A graph is apex if it becomes planar after the deletion of one vertex. The family of apex graphs is closed under taking minors, so it is characterized by a finite set of forbidden minors. Determining the finite set of forbidden minors for apex graphs remains an open question. In this paper, we list all forbidden minors for apex graphs with 12 or fewer vertices and all forbidden minors for apex graphs with 26 and fewer edges. We also present graphs outside of these ranges. We show that a graph with 13 vertices and minimal degree 6 is either apex or contains a $K_6$ minor, proving Jørgensen's conjecture for order 13.

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arXiv2024

Intrinsically knotted graphs and connected domination

We classify all the maximal linklessly embeddable graphs of order 12 and show that their complements are all intrinsically knotted. We derive results about the connected domination numbers of a graph and its complement. We provide an answer to an open question about the minimal order of a 3-non-compliant graph. We prove that the complements of knotlessly embeddable graphs of order at least 15 are all intrinsically knotted. We provide results on general $k$-non-compliant graphs and leave a set of open questions for further exploration of the subject.

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arXiv2024

Connected Domination in Plane Triangulations

A set of vertices of a graph $G$ such that each vertex of $G$ is either in the set or is adjacent to a vertex in the set is called a dominating set of $G$. If additionally, the set of vertices induces a connected subgraph of $G$ then the set is a connected dominating set of $G$. The domination number $γ(G)$ of $G$ is the smallest number of vertices in a dominating set of $G$, and the connected domination number $γ_c(G)$ of $G$ is the smallest number of vertices in a connected dominating set of $G$. We find the connected domination numbers for all triangulations of up to thirteen vertices. For $n\ge 15$, $n\equiv 0$ (mod 3), we find graphs of order $n$ and $γ_c=\frac{n}{3}$. We also show that the difference $γ_c(G)-γ(G)$ can be arbitrarily large.

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arXiv2022

The Complement Problem for Linklessly Embeddable Graphs

We find all maximal linklessly embeddable graphs of order up to 11, and verify that for every graph $G$ of order 11 either $G$ or its complement $cG$ is intrinsically linked. We give an example of a graph $G$ of order 11 such that both $G$ and $cG$ are $K_6$-minor free. We provide minimal order examples of maximal linklessly embeddable graphs that are not triangular or not 3-connected. We prove a Nordhaus-Gaddum type conjecture on the Colin de Verdière invariant for graphs on at most 11 vertices. We give a description of the programs used in the search.

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arXiv2022

The Extremal Function for Apex Graphs

McCarty and Thomas conjectured that a linklessly embeddable graph with $n\ge 7 $ vertices and $t$ triangles has at most $3n-9 +\frac{t}{3}$ edges. Thomas and Yoo proved this to be true for apex graphs. We give a shorter and simpler proof for the apex case.

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arXiv2022

Constructions stemming from non-separating planar graphs and their Colin de Verdière invariant

A planar graph $G$ is said to be non-separating if there exists an embedding of $G$ in $\mathbb{R}^2$ such that for any cycle $\mathcal{C}\subset G$, all vertices of $G\setminus \mathcal{C}$ are within the same connected component of $\mathbb{R}^2\setminus \mathcal{C}$. Dehkordi and Farr classified the non-separating planar graphs as either outerplanar graphs, subgraphs of wheel graphs, or subgraphs of elongated triangular prisms. We use maximal non-separating planar graphs to construct examples of maximal linkless graphs and maximal knotless graphs. We show that for a maximal non-separating planar graph $G$ with $n\ge 7$ vertices, the complement $cG$ is $(n-7)-$apex. This implies that the Colin de Verdière invariant of the complement $cG$ satisfies $μ(cG) \le n-4$. We show this to be an equality. As a consequence, the conjecture of Kotlov, Lovàsz, and Vempala that for a simple graph $G$, $μ(G)+μ(cG)\ge n-2$ is true for 2-apex graphs $G$ for which $G-\{u,v\}$ is planar non-separating. It also follows that complements of non-separating planar graphs of order at least nine are intrinsically linked. We prove that the complements of non-separating planar graphs $G$ of order at least ten are intrinsically knotted.

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arXiv2020

New bounds on maximal linkless graphs

We construct a family of maximal linklessly embeddable graphs on $n$ vertices and $3n-5$ edges for all $n\ge 10$, and another family on $n$ vertices and $m< \frac{25n}{12}-\frac{1}{4}$ edges for all $n\ge 13$. The latter significantly improves the lowest edge-to-vertex ratio for any previously known infinite family. We construct a family of graphs showing that the class of maximal linklessly embeddable graphs differs from the class of graphs that are maximal without a $K_6$ minor studied by L. Jorgensen. We give necessary and sufficient conditions for when the clique sum of two maximal linklessly embeddable graphs over $K_2$, $K_3$, or $K_4$ is a maximal linklessly embeddable graph, and use these results to prove our constructions yield maximal linklessly embeddable graphs.

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arXiv2018

Complete Minors of Self-Complementary Graphs

We show that any self-complementary graph with $n$ vertices contains a $K_{\lfloor \frac{n+1}{2}\rfloor}$ minor. We derive topological properties of self-complementary graphs.

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arXiv2018

Hadwiger numbers of self-complementary graphs

The Hadwiger number of a graph $G$, denoted by $h(G)$, is the order of the largest complete minor of $G$. A graph is said to be self-complementary if it is isomorphic to its complement. We prove that for all $n\equiv 0,1 (\text{mod 4})$ and for all $ \lfloor \dfrac{n+1}{2} \rfloor \le h \le \lfloor \dfrac{3n}{5}\rfloor $, there exists a self-complementary graph $G$ with $n$ vertices whose Hadwiger number is $h$.

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arXiv2018

Escher squares and lattice links

We give a shorter and simpler proof of the result of [2], which gives a necessary and sufficient condition for when a lattice diagram is the projection of a lattice link.

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arXiv2015

The total Thurston-Bennequin number of complete and complete bipartite Legendrian graphs

We study the Thurston-Bennequin number of complete and complete bipartite Legendrian graphs. We define a new invariant called the total Thurston-Bennequin number of the graph. We show that this invariant is determined by the Thurston-Bennequin numbers of 3-cycles for complete graphs and by the Thurston-Bennequin number of 4-cycles for complete bipartite graphs. We discuss the consequences of these results for K_4, K_5 and K_{3,3}.

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arXiv2013

Legendrian theta-graphs

In this article we give necessary and sufficient conditions for two triples of integers to be realized as the Thurston-Bennequin number and the rotation number of a Legendrian theta-graph with all cycles unknotted. We show that these invariants are not enough to determine the Legendrian class of a topologically planar theta-graph. We define the transverse push-off of a Legendrian graph and we determine its self linking number for Legendrian theta-graphs. In the case of topologically planar theta-graphs, we prove that the topological type of the transverse push-off is that of a pretzel link.

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