Vertex covers by monochromatic pieces - A survey of results and problems

This survey is devoted to problems and results concerning covering the vertices of edge colored graphs or hypergraphs with monochromatic paths, cycles and other objects. It is an expanded version of the talk with the same title at the Seventh Cracow Conference on Graph Theory, held in Rytro in September 14-19, 2014.Comment: Discrete Mathematics, 201

TBSs in some minimum coverings

AbstractLet (X,B) be a (λKn,G)-covering with excess E and a blocking set T. Let Γ1, Γ2, …, Γs be all connected components of E with at least two vertices (note that s=0 if E=0̸). The blocking set T is called tight if further V(Γi)∩T≠0̸ and V(Γi)∩(X∖T)≠0̸ for 1≤i≤s. In this paper, we give a complete solution for the existence of a minimum (λKn,G)-covering admitting a blocking set (BS), or a tight blocking set (TBS) for any λ and when G=K3 and G=K3+e

The use of blocking sets in Galois geometries and in related research areas

Blocking sets play a central role in Galois geometries. Besides their intrinsic geometrical importance, the importance of blocking sets also arises from the use of blocking sets for the solution of many other geometrical problems, and problems in related research areas. This article focusses on these applications to motivate researchers to investigate blocking sets, and to motivate researchers to investigate the problems that can be solved by using blocking sets. By showing the many applications on blocking sets, we also wish to prove that researchers who improve results on blocking sets in fact open the door to improvements on the solution of many other problems

On cubic bridgeless graphs whose edge-set cannot be covered by four perfect matchings

The problem of establishing the number of perfect matchings necessary to cover the edge-set of a cubic bridgeless graph is strictly related to a famous conjecture of Berge and Fulkerson. In this paper we prove that deciding whether this number is at most 4 for a given cubic bridgeless graph is NP-complete. We also construct an infinite family $\cal F$ of snarks (cyclically 4-edge-connected cubic graphs of girth at least five and chromatic index four) whose edge-set cannot be covered by 4 perfect matchings. Only two such graphs were known. It turns out that the family $\cal F$ also has interesting properties with respect to the shortest cycle cover problem. The shortest cycle cover of any cubic bridgeless graph with $m$ edges has length at least $\tfrac43m$, and we show that this inequality is strict for graphs of $\cal F$. We also construct the first known snark with no cycle cover of length less than $\tfrac43m+2$.Comment: 17 pages, 8 figure

Evaluation of exhalation resistance and inspired carbon dioxide concentration in elastomeric half-mask respirators with modified or covered exhalation valves

"This study explores the impact of modifying or covering the exhalation valves of an EHMR on exhaled breathing resistance and inspired CO2. Two approaches were considered: (1) modifying EHMRs to filter the exhaled breath through the inhalation filters; (2) covering EHMR exhalation valves with surgical masks. Nine EHMR configurations approved by NIOSH were included in this study, where the "configurations" were the combination of a facepiece, filter, and other required components. These nine EHMR configurations were selected from three different respirator manufacturers and five different facepiece models. Four of the five facepiece models were assessed in both N95 and P100 filter configurations, and one facepiece model was assessed in a P100 filter configuration only. Exhalation resistance and inspired CO2 level were measured in each of the nine EHMR configurations under four study conditions, for a total of 72 measurements: (1) No EHMR modifications or coverings - experimental control; (2) EHMR modified to filter exhaled breath through inhalation filters; (3) EHMR exhalation valves covered with Level 1 surgical mask (per ASTM F2100); and (4) EHMR exhalation valves covered with Level 3 surgical mask (per ASTM F2100). Both exhalation resistance and inspired CO2 level were measured in accordance with NIOSH Standard Testing Procedures. For the exhalation resistance measurement, three samples were each donned one time (n=3) on a manikin, which was made to simulate exhalation at a constant airflow rate of 85 L/min. The initial resistance was then measured. For the inspired CO2 level measurement, one sample was donned three times each (n=3) on a manikin, which was made to simulate breathing at a rate of 14.5 breaths/min and a tidal volume of 0.724 L. The simulated exhaled breath consisted of 5% CO2, and the inspired CO2 concentration was measured at 40 Hz and averaged over three respiratory cycles. Test results showed that all EHMR configurations under all test conditions met the NIOSH exhalation resistance performance requirement of not exceeding 25 mmH2O. Eight of nine configurations modified to filter the exhaled breath showed measurable increases in mean exhalation resistance compared to their respective controls, increasing from between 4.40 and 9.48 mmH2O in the controls to between 8.21 and 18.5 mmH2O in the modified configurations. All configurations covered with Level 1 or Level 3 surgical masks also showed measurable increases in mean exhalation resistance compared to their respective controls, increasing from between 4.40 and 9.48 mmH2O in the controls to between 7.11 and 17.6 mmH2O in the covered configurations. The lowest of the three inspired CO2 level measurements for all EHMR configurations under all test conditions was less than 2%, with one observation of one configuration modified to filter the exhaled breath exceeding 2%. All nine EHMR configurations modified to filter the exhaled breath showed measurable increases in mean inspired CO2 levels compared to their respective controls, increasing from between 0.47% and 1.03% in the controls to between 1.25% and 1.94% in the modified configurations, while the inspired CO2 levels of EHMRs covered with Level 1 or Level 3 surgical masks did not consistently increase across all configurations, changing from between 0.47% and 1.03% in the controls to 0.80% and 1.26% in the covered configurations. Findings indicate that EHMRs can meet the NIOSH exhalation resistance performance requirement after being (1) modified to filter the exhaled breath, (2) covered with a Level 1 surgical mask, or (3) covered with a Level 3 surgical mask. Findings indicate that modifying EHMRs to filter the exhaled breath increases inspired CO2 levels, potentially increasing user discomfort but unlikely to cause serious physiological symptoms in healthy users, as all but one measurement was below 2%. Covering EHMR exhalation valves with surgical masks is unlikely to increase discomfort due to elevated inspired CO2 levels nor to cause serious physiological symptoms, as measurements were well below 2%. NIOSH's performance requirements provide an approval pathway for new EHMR designs without exhalation valves, which may be desirable when considering source control. In fact, while this study was in progress, several EHMR models without exhalation valves and an EHMR with a filtered exhalation valve accessory were approved. Since this study showed that modifying the EHMRs to filter the exhaled breath generally increased both exhalation resistance and inspired CO2 levels, and the magnitude of those increases varied among EHMR configurations, future research on the comfort and tolerability of new and modified EHMRs without exhalation valves needs to inform and improve designs appropriate for use in healthcare." - NIOSHTIC-2NIOSHTIC no. 20064300Suggested Citation: NIOSH [2022]. Evaluation of exhalation resistance and inspired carbon dioxide concentration in elastomeric half-mask respirators with modified or covered exhalation valves. By Strickland KT, Fernando R, Schall J, Walbert G, Brannen J. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2022-109. https://doi.org/10.26616/NIOSHPUB20221092022-109.pdf?id=10.26616/NIOSHPUB202210920221079

Minimal Ramsey graphs, orthogonal Latin squares, and hyperplane coverings

This thesis consists of three independent parts. The first part of the thesis is concerned with Ramsey theory. Given an integer $q\geq 2$, a graph $G$ is said to be \emph{$q$-Ramsey} for another graph $H$ if in any $q$-edge-coloring of $G$ there exists a monochromatic copy of $H$. The central line of research in this area investigates the smallest number of vertices in a $q$-Ramsey graph for a given $H$. In this thesis, we explore two different directions. First, we will be interested in the smallest possible minimum degree of a minimal (with respect to subgraph inclusion) $q$-Ramsey graph for a given $H$. This line of research was initiated by Burr, Erdős, and Lovász in the 1970s. We study the minimum degree of a minimal Ramsey graph for a random graph and investigate how many vertices of small degree a minimal Ramsey graph for a given $H$ can contain. We also consider the minimum degree problem in a more general asymmetric setting. Second, it is interesting to ask how small modifications to the graph $H$ affect the corresponding collection of $q$-Ramsey graphs. Building upon the work of Fox, Grinshpun, Liebenau, Person, and Szabó and Rödl and Siggers, we prove that adding even a single pendent edge to the complete graph $K_t$ changes the collection of 2-Ramsey graphs significantly. The second part of the thesis deals with orthogonal Latin squares. A {\em Latin square of order $n$} is an $n\times n$ array with entries in $[n]$ such that each integer appears exactly once in every row and every column. Two Latin squares $L$ and $L'$ are said to be {\em orthogonal} if, for all $x,y\in [n]$, there is a unique pair $(i,j)\in [n]^2$ such that $L(i,j) = x$ and $L'(i,j) = y$; a system of {\em $k$ mutually orthogonal Latin squares}, or a {\em $k$-MOLS}, is a set of $k$ pairwise orthogonal Latin squares. Motivated by a well-known result determining the number of different Latin squares of order $n$ log-asymptotically, we study the number of $k$-MOLS of order $n$. Earlier results on this problem were obtained by Donovan and Grannell and Keevash and Luria. We establish new upper bounds for a wide range of values of $k = k(n)$. We also prove a new, log-asymptotically tight, bound on the maximum number of other squares a single Latin square can be orthogonal to. The third part of the thesis is concerned with grid coverings with multiplicities. In particular, we study the minimum number of hyperplanes necessary to cover all points but one of a given finite grid at least $k$ times, while covering the remaining point fewer times. We study this problem for the grid $\mathbb{F}_2^n$, determining the number exactly when one of the parameters $n$ and $k$ is much larger than the other and asymptotically in all other cases. This generalizes a classic result of Jamison for $k=1$. Additionally, motivated by the recent work of Clifton and Huang and Sauermann and Wigderson for the hypercube $\set{0,1}^n\subseteq\mathbb{R}^n$, we study hyperplane coverings for different grids over $\mathbb{R}$, under the stricter condition that the remaining point is omitted completely. We focus on two-dimensional real grids, showing a variety of results and demonstrating that already this setting offers a range of possible behaviors.Diese Dissertation besteht aus drei unabh\"angigen Teilen. Der erste Teil beschäftigt sich mit Ramseytheorie. Für eine ganze Zahl $q\geq 2$ nennt man einen Graphen \emph{$q$-Ramsey} f\"ur einen anderen Graphen $H$, wenn jede Kantenf\"arbung mit $q$ Farben einen einfarbigen Teilgraphen enthält, der isomorph zu $H$ ist. Das zentrale Problem in diesem Gebiet ist die minimale Anzahl von Knoten in einem solchen Graphen zu bestimmen. In dieser Dissertation betrachten wir zwei verschiedene Varianten. Als erstes, beschäftigen wir uns mit dem kleinstm\"oglichen Minimalgrad eines minimalen (bezüglich Teilgraphen) $q$-Ramsey-Graphen f\"ur einen gegebenen Graphen $H$. Diese Frage wurde zuerst von Burr, Erd\H{o}s und Lov\'asz in den 1970er-Jahren studiert. Wir betrachten dieses Problem f\"ur einen Zufallsgraphen und untersuchen, wie viele Knoten kleinen Grades ein Ramsey-Graph f\"ur gegebenes $H$ enthalten kann. Wir untersuchen auch eine asymmetrische Verallgemeinerung des Minimalgradproblems. Als zweites betrachten wir die Frage, wie sich die Menge aller $q$-Ramsey-Graphen f\"ur $H$ verändert, wenn wir den Graphen $H$ modifizieren. Aufbauend auf den Arbeiten von Fox, Grinshpun, Liebenau, Person und Szabó und Rödl und Siggers beweisen wir, dass bereits der Graph, der aus $K_t$ mit einer h\"angenden Kante besteht, eine sehr unterschiedliche Menge von 2-Ramsey-Graphen besitzt im Vergleich zu $K_t$. Im zweiten Teil geht es um orthogonale lateinische Quadrate. Ein \emph{lateinisches Quadrat der Ordnung $n$} ist eine $n\times n$-Matrix, gef\"ullt mit den Zahlen aus $[n]$, in der jede Zahl genau einmal pro Zeile und einmal pro Spalte auftritt. Zwei lateinische Quadrate sind \emph{orthogonal} zueinander, wenn f\"ur alle $x,y\in[n]$ genau ein Paar $(i,j)\in [n]^2$ existiert, sodass es $L(i,j) = x$ und $L'(i,j) = y$ gilt. Ein \emph{k-MOLS der Ordnung $n$} ist eine Menge von $k$ lateinischen Quadraten, die paarweise orthogonal sind. Motiviert von einem bekannten Resultat, welches die Anzahl von lateinischen Quadraten der Ordnung $n$ log-asymptotisch bestimmt, untersuchen wir die Frage, wie viele $k$-MOLS der Ordnung $n$ es gibt. Dies wurde bereits von Donovan und Grannell und Keevash und Luria studiert. Wir verbessern die beste obere Schranke f\"ur einen breiten Bereich von Parametern $k=k(n)$. Zusätzlich bestimmen wir log-asymptotisch zu wie viele anderen lateinischen Quadraten ein lateinisches Quadrat orthogonal sein kann. Im dritten Teil studieren wir, wie viele Hyperebenen notwendig sind, um die Punkte eines endlichen Gitters zu überdecken, sodass ein bestimmter Punkt maximal $(k-1)$-mal bedeckt ist und alle andere mindestens $k$-mal. Wir untersuchen diese Anzahl f\"ur das Gitter $\mathbb{F}_2^n$ asymptotisch und sogar genau, wenn eins von $n$ und $k$ viel größer als das andere ist. Dies verallgemeinert ein Ergebnis von Jamison für den Fall $k=1$. Au{\ss}erdem betrachten wir dieses Problem f\"ur Gitter im reellen Vektorraum, wenn der spezielle Punkt überhaupt nicht bedeckt ist. Dies ist durch die Arbeiten von Clifton und Huang und Sauermann und Wigderson motiviert, die den Hyperwürfel $\set{0,1}^n\subseteq \mathbb{R}^n$ untersucht haben. Wir konzentrieren uns auf zwei-dimensionale Gitter und zeigen, dass schon diese sich sehr unterschiedlich verhalten können

Polychromatic Colorings on the Integers

We show that for any set $S\subseteq \mathbb{Z}$, $|S|=4$ there exists a 3-coloring of $\mathbb{Z}$ in which every translate of $S$ receives all three colors. This implies that $S$ has a codensity of at most $1/3$, proving a conjecture of Newman [D. J. Newman, Complements of finite sets of integers, Michigan Math. J. 14 (1967) 481--486]. We also consider related questions in $\mathbb{Z}^d$, $d\geq 2$.Comment: 16 pages, improved presentatio

Subspace coverings with multiplicities

We study the problem of determining the minimum number $f(n,k,d)$ of affine subspaces of codimension $d$ that are required to cover all points of $\mathbb{F}_2^n\setminus \{\vec{0}\}$ at least $k$ times while covering the origin at most $k-1$ times. The case $k=1$ is a classic result of Jamison, which was independently obtained by Brouwer and Schrijver for $d = 1$. The value of $f(n,1,1)$ also follows from a well-known theorem of Alon and F\"uredi about coverings of finite grids in affine spaces over arbitrary fields. Here we determine the value of this function exactly in various ranges of the parameters. In particular, we prove that for $k \ge 2^{n-d-1}$ we have $f(n,k,d)=2^d k - \left \lfloor \frac{k}{2^{n-d}} \right \rfloor$, while for $n > 2^{2^d k-k-d+1}$ we have $f(n,k,d)= n + 2^dk-d-2$, and also study the transition between these two ranges. While previous work in this direction has primarily employed the polynomial method, we prove our results through more direct combinatorial and probabilistic arguments, and also exploit a connection to coding theory.Comment: 15 page