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On Different Graph Covering Numbers Structural, Extremal and Algorithmic Results More on Different Graph Covering Numbers Structural, Extremal and Algorithmic Results Master Thesis of Peter Stumpf At the Department of Informatics Institute of Theoretical Computer Science Reviewers: Prof. Dr. Dorothea Wagner Prof. Dr. Peter Sanders Prof. Maria Axenovich Ph.D. Advisors: Torsten Ueckerdt Marcel Radermacher Time Period: 23.02.2017 – 22.08.2017 KIT – University of the State of Baden-Wuerttemberg and National Laboratory of the Helmholtz Association www.kit.edu Statement of Authorship I hereby declare that this document has been composed by myself and describes my own work, unless otherwise acknowledged in the text. Karlsruhe, 22nd August 2017 iii Abstract A covering number measures how “difficult” it is to cover all edges of a host graph with guest graphs of a given guest class. E.g., the global covering number, which has received the most attention, is the smallest number k such that the host graph is the union of k guest graphs from the guest class. In this thesis, we consider the recent framework of the global, the local and the folded covering number (Knauer and Ueckerdt, Discrete Mathematics 339 (2016)). The local covering number relaxes the global covering number by counting the guest graphs only locally at every vertex. And the folded covering number relaxes the local covering number even further, by allowing several vertices in the same guest graph to be identified, at the expense of counting these with multiplicities. More precisely, we consider a cover of a host graph H with regards to a guest class G to be a finite set of guest graphs S = {G1,...,Gm} ⊂ G paired with an edge-surjective homomorphism φ: V (G1 ∪· ... ∪· Gm) → V (H). The cover is called guest-injective, if for i = 1, . , m its restricted homomorphism φ|Gi is vertex-injective. The global covering number is thus the smallest number k such that there is a guest-injective cover with |S| = k. The local covering number is the smallest number k such that there is a guest-injective cover with |φ−1(v)| ≤ k for every vertex v ∈ H. And the folded covering number is the smallest number k such that there is a (not necessarily guest-injective) cover with |φ−1(v)| ≤ k for every vertex v ∈ H. In this thesis we investigate the relations between these numbers. As one result we show that the local covering number of any shift graph with regards to bipartite graphs is at most 2, while the corresponding global covering numbers can be arbitrarily large. This is the first known separation of local and global covering number with a subgraph-hereditary guest class. This concludes the study of separation results, as a separation of folded and local covering number with such a guest class was already known, as well as the fact that such a separation is not possible for union-closed topological-minor-closed guest classes. Furthermore, we investigate for a, b ∈ N0 with b < 2a the classes of (a, b)-sparse graphs as guest classes. For these the local and the global covering number always coincide. This generalizes existing results for forests and pseudo-forests. We prove that the global covering number with regards to these (a, b)-sparse graphs always matches a fairly simple lower bound given by a variation of the host graph’s maximum average degree. We moreover provide an efficient algorithm to calculate corresponding optimal covers. While most attention in the given framework focuses on properties for the guest classes, we also investigate graph classes of host graphs without fixing the guest class. Namely, we introduce a property called cover resistance. We call a class H of host graphs (f/l/g)-cover resistant, if the (folded/local/global) covering numbers for these host graphs usually become arbitrarily large. I.e., it is (f/l/g)-cover resistant, if for each union-closed induced-hereditary guest class G we have host graphs in H with arbitrarily large (folded/local/global) covering numbers, unless G contains H. For the classes of host graphs the relaxations of folded and local covering number are inverse, i.e., the f-cover resistance implies l-cover resistance which in turn implies g-cover resistance. We give examples for each of these resistances. As a result of our investigation, we characterize the induced-hereditary guest classes with bounded folded covering number as those containing all bipartite graphs. We further show the class of all graphs is the only induced-hereditary guest class with bounded local covering number. v Deutsche Zusammenfassung Eine Überdeckungszahl misst wie “schwer” es ist alle Kanten eines Gastgebergraphen mit Gastgraphen einer gegebenen Gastklasse zu überdecken. Die globale Überdeck- ungszahl ist z.B. die kleinste Zahl k für die der Gastgebergraph die Vereinigung von k Gastgraphen ist. Sie hat bisher am meisten Aufmerksamkeit erhalten. In dieser Arbeit betrachten wir ein junges Rahmenkonzept welches die globale, die lokale und die gefaltete Überdeckungszahl umfasst. (Knauer und Ueckerdt, Discrete Mathematics 339 (2016)). Die locale Überdeckungszahl relaxiert die globale Überdeckungszahl, indem sie die Gäste nur lokal an jedem einzelnem Knoten zählt. Und die gefaltete Überdeckungszahl wiederum relaxiert die lokale Überdeckungszahl, indem sie es zulässt mehrere Knoten des gleichen Gastgraphen zu identifizieren, die dafür jedoch auch mehrfach gezählt werden. Genauer gesagt betrachten wir eine Überdeckung eines Gastgebergraphen H bezüglich einer Gastklasse G als Paar einer endliche Menge von Gastgraphen S = {G1,...,Gm} ⊂ G zusammen mit einem Kanten-surjektiven Homomorphismus φ: V (G1 ∪· ...∪· Gm) → V (H). Wir bezeichnen die Überdeckung als Gast-injektiv, wenn für i = 1, . , m der eingeschränkte Homomoprhismus φ|Gi Knoten-injektiv ist. In diesem Formalismus ist die globale Überdeckungszahl die kleinste Zahl k, sodass es eine Gast-injektive Überdeckung mit |S| = k gibt. Die lokale Überdeckungszahl ist die kleinste Zahl k, sodass es eine Gast-injektive Überdeckung gibt mit |φ−1(v)| ≤ k für jeden Knoten v ∈ H. Und die gefaltete Überdeckungszahl ist die kleinste Zahl k, sodass es eine (nicht notwendigerweise Gast-injektive) Überdeckung gibt mit |φ−1(v)| ≤ k für jeden Knoten v ∈ H. In dieser Arbeit untersuchen wir die Beziehungen unter diesen Überdeckungszahlen. Eines unserer Resultate ist, dass bezüglich bipartiter Graphen die lokale Überdeck- ungszahl jedes Shift Graphs höchstens 2 beträgt, während die entsprechende globale Überdeckungszahl beliebig groß wird. Dies stellt die erste bekannte Separierung der lokalen und der globalen Überdeckungszahl mit einer Subgraph-hereditären Gastklasse dar. Damit schließen wir die Studie der Separierungen ab, da eine entsprechende Separierung der gefalteten und der lokalen Überdeckungszahl bereits bekannt war, und wir zudem wissen, dass solche Separierungen nicht mit Gastklassen möglich sind, die abgeschlossen unter disjunkter Vereinigung und dem Nehmen von topologischen Minoren sind. Wir untersuchen außerdem für a, b ∈ N0 mit b < 2a die Klasse der (a, b)-dünnbesetzten Graphen als Gastklasse. Für diese Klassen fallen die lokale und die globale Überdeck- ungszahl zusammen. Damit verallgemeinern wir bestehende Resultate für Wälder und Pseudowälder. Wir zeigen dass die globale Überdeckungszahl bezüglich dieser (a, b)-dünnbesetzten Graphen immer mit einer sehr einfachen unteren Schranke zusam- menfällt, die als Variation des maximalen Durschnittsgrades des Gastgebergraphens gegeben is. Darüberhinaus beschreiben wir einen effizienten Algorithmus mit dem man entsprechende optimale Überdeckungen erhält. Während der größte Teil der Aufmerksamkeit im gegebenen Rahmenkonzept auf Eigen- schaften der Gastklassen liegt, untersuchen wir auch Graphklassen von Gastgeber- graphen ohne die Gastklasse festzulegen. Genauer gesagt führen wir die Eigenschaft der Überdeckungsresistenz ein. Eine Klasse H heißt (f,l,g)-überdeckungsresistent, wenn die (gefaltete/lokale/globale) Überdeckungszahl für diese Gastgebergraphen gewöhnlich sehr groß werden. Genauer, die Klasse is (f,l,g)-überdeckungsresistent, wenn wir für jede Gastklasse G, die abgeschlossen unter disjunkter Vereinigung und induziert-hereditär ist, Gastgebergraphen mit beliebig großen Überdeckungszahlen finden, es sei denn G enthält H bereits. Für die Gastgeberklassen ist die Reihenfolge vi der Relaxierungen der gefalteten und der lokalen Überdeckungszahl umgekehrt. Das heißt, die f-Überdeckungsresistenz impliziert die l-Überdeckungsresistenz, welche wiederung die g-Überdeckungsresistenz impliziert. Wir geben für jede dieser Re- sistenzen Beispiele. Als Resultat erhalten wir die Charakterisierung der induziert- hereditären Gastklassen mit beschränkter gefalteter Überdeckungszahl als jene Klassen, die alle bipartiten Graphen enthalten. Wir zeigen außerdem, dass die Klasse aller Graphen die einzige Gastklasse mit beschränkter lokaler Überdeckungszahl ist. vii Contents 1 Introduction1 2 Preliminaries5 2.1 Hypergraphs . .7 2.2 Hyper Ramsey Theory . .7 2.3 Matroids . .7 2.4 Directed Graphs . .7 2.5 Graph Classes . .7 2.6 Easier Notation . .8 3 Covering Numbers9 3.1 The Union Covering Number . 11 3.2 Basic Results . 13 4 Separability and Non-Separability 15 4.1 Separation of Local- and Union-Covering Number with Regards to a Subgraph- Hereditary Guest Class . 16 4.1.1 Shift Graphs are u-Cover Resistant . 19 4.2 Non-Separability for Host Classes of Bounded χ ................ 20 4.3 Non-Separability of (a, b)-Sparse Graphs . 22 4.3.1 Covers with Regards to (a, b)-Sparse Graphs . 23 5 Computing (a, b)-Sparse Covers 31 5.1 Detecting (a, b)-Sparse Graphs . 31 5.2 Computing Optimal Global (a, b)-Sparse Covers . 38 6 Guests of Bounded Degree 55 7 Cover Resistance 57 7.1 Motivation . 57 7.2 Cover Resistance . 57 7.3 Relation to Induced Ramsey Theory . 58 7.4 Constructing Cover Resistant Host Classes . 63 7.5 Relations Between Different Cover Resistances . 65 7.6 Negative Results . 68 7.7 f-Cover Resistance . 70 7.8 The Class of All Graphs is l-Cover Resistant . 72 8 Conclusion 73 8.1 Separability . 73 8.2 The Guest Class of (a, b)-Sparse Graphs . 74 8.3 Cover Resistance . 74 ix Contents Bibliography 77 x 1. Introduction In general a covering number states how “difficult” it is to cover all edges of a host graph H by guest graphs from a given guest class G.
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