Fakult¨at fur¨ Mathematik und Wirtschafts- wissenschaften Institut fur¨ Optimierung und Operations Research Cycle decompositions and t-perfect graphs Dissertation zur Erlangung des Doktorgrades Dr. rer. nat. der Fakult¨at Mathematik und Wirtschaftswissenschaften der Universit¨at Ulm Vorgelegt von: Elke Monika Fuchs aus Ulm 2017 Amtierender Dekan: Prof. Dr. Alexander Lindner 1. Gutachter: Prof. Dr. Henning Bruhn-Fujimoto 2. Gutachter: Prof. Dr. Dieter Rautenbach Tag der Promotion: 26. September 2017 Contents 1 Summary1 2 Notation3 I t-perfection5 3 Perfection and t-perfection7 4 t-perfection in P5-free graphs 13 4.1 Introduction . 13 4.2 t-perfection in near-bipartite graphs . 14 4.3 Minimally t-imperfect antiwebs . 18 4.4 Harmonious cutsets . 21 4.5 P5-free graphs . 26 4.6 Colouring . 32 5 t-perfect quadrangulations of the projective plane 35 5.1 Introduction . 35 5.2 Reducing quadrangulations of the sphere and the projective plane 38 5.3 (Strong) t-perfection for quadrangulations of the projective plane 43 6 t-perfect triangulations of the projective plane 45 6.1 Introduction . 45 6.2 Loose odd wheels . 46 6.3 Triangulations . 46 6.4 Proof of Theorem 41 . 49 6.5 Irreducible Triangulations . 51 6.6 Even-contraction creating a loose odd wheel or an induced C7 .. 54 6.7 Even-contraction destroying an odd hole . 57 iii Contents II Minimum Cycle Decomposition and Haj´os’conjecture 69 7 Cycle decompositions of pathwidth-6 graphs 71 7.1 Introduction . 71 7.2 Reducible structures . 73 7.3 Recolouring Techniques . 75 7.4 Proofs for the reducible structures . 86 7.5 Path-decompositions . 90 Bibliography 95 iv 1 Summary This thesis contains results obtained during my time as a PhD student. It consists of two parts. The first part is about characterising t-perfect graphs — the second part deals with cycle decomposition of graphs and Haj´os’conjecture. At the beginning of the first part, there is an introduction to the field of t-perfect graphs (see Chapter 3). There we explain the concepts, give specific definitions and an overview on related work. Afterwards, each chapter contains the results of one manuscript I co-authored. The discussion of these results can also be found in the mentioned research papers and is borrowed from there (see [16], [31] and [30]). Part I deals with t-perfect graphs. A graph is called t-perfect if its stable set polytope is fully described by non-negativity, edge and odd-cycle constraints. This thesis contains several new approaches for characterising t-perfection: in Chapter 4, we characterise P5-free t-perfect graphs in terms of forbidden t- minors. Moreover, we show that P5-free t-perfect graphs can always be coloured with three colours, and that they can be recognised in polynomial time. Besides, in Chapter 5, we show that every quadrangulation of the sphere can be transformed into a 4-cycle by deletions of degree-2 vertices and by t-contractions at degree-3 vertices. We further show that a non-bipartite quadrangulation of the projective plane can be transformed into an odd wheel by t-contractions and deletions of degree-2 vertices. As a consequence, we can give a characterisation of quadrangulations of the projective plane concerning t-perfection. We deduce that a quadrangulation of the projective plane is (strongly) t-perfect if and only if the graph is bipartite. Furthermore, we prove in Chapter 6 that a triangulation of the projective plane is (strongly) t-perfect if and only if it is perfect and contains no K4. Part II includes results from [32]. It deals with the problem of finding a decom- position of the edge set of an Eulerian graph into cycles. Haj´osconjectured that a simple Eulerian graph on n vertices can be decomposed into at most b(n − 1)/2c cycles. The conjecture is only proved for graph classes in which every element contains vertices of degree 2 or 4. We develop new techniques to construct cycle decompositions. They work on the common neighbourhood of two degree-6 ver- 1 1 Summary tices. With these techniques we find structures that cannot occur in a minimal counterexample to Haj´os’conjecture and verify the conjecture for Eulerian graphs of pathwidth at most 6. 2 2 Notation In general, we follow the notation of Diestel [23], where also any missing facts and defintions about graphs may be found. In the following, we give a summary of all the important definitions in this thesis. A graph is a pair G = (V, E) of sets with E ⊆ [V ]2. The elements of E are 2- element subsets of V . The set V contains the vertices of the graph G, the elements of E are its edges. To be more precise, we sometimes write V (G) and E(G) for the vertex and the edge set of a graph G. All the graphs in this thesis are finite, simple and undirected. The number of neighbours of a vertex v in V is the degree deg(v) of v. If the degree of a vertex v equals k, we call v a degree-k vertex. A path of length l between two vertices x0 and xl is a sequence of l + 1 distinct vertices x0x1 . xl such that xixi+1 ∈ E for 0 ≤ i ≤ l − 1. A path on k vertices is denoted by Pk. We say that a graph is Pk-free if it does not contain a path on k vertices as induced subgraph. A cycle C is a path x0 . xl such that x0xl ∈ E and l ≥ 2. We write C as x0 . xlx0 and call C a l-cycle. The complete graph on n vertices is denoted by Kn. Moreover, we write G for the complement of a graph G. A graph is called bipartite if V can be partitioned into two classes such that no two vertices of one class are adjacent. A vertex colouring of a graph is a map from V to a set of colours such that the colours of two adjacent vertices are different. The chromatic number χ(G) of a graph G is the smallest number of colours needed to colour its vertices. The colourings considered in Part I are always vertex colourings of graphs. In Part II, we define and use a specific edge colouring. An independent set or a stable set is a set of vertices such that no two of its elements are adjacent. A clique K is a set of vertices such that every two vertices are adjacent in K. The clique number ω(G) of a graph G is the size of a largest clique in G. Other important definitions can be found in the following chapters. 3 2 Notation 4 Part I t-perfection 5 3 Perfection and t-perfection Perfect graphs have been one of the most discussed graph classes in the last 60 years. They were introduced by Berge in [8]. Berge called a graph perfect if for every induced subgraph the chromatic number and the size of a largest clique coincide. For instance, it is easy to prove that bipartite graphs are perfect. Berge observed that for every already known class of perfect graphs the complementary class is also perfect. He conjectured that this holds in general. This conjecture was proved by Lov´asz,and it is often refered to as the Weak Perfect Graph Conjecture. Theorem 1 (Lov´asz[47]). The complement of a perfect graph is perfect. Furthermore, Berge considered subgraphs that are not contained in perfect graphs. He thought about a structural characterisation of perfect graphs. He knew that odd holes are imperfect (since the largest clique in an odd hole is of size 2, but the chromatic number of an odd hole is 3). An odd hole Ck is an induced odd cycle on k ≥ 5 vertices. An odd anti-hole Ck is the complement of an odd hole (see Figure 3.1 for examples). 2 C5 = C5 C7 C7 = C7 Figure 3.1: The odd holes and antiholes on 5 and 7 vertices He was also able to prove the imperfection of odd antiholes, which is also im- plied by Theorem 1. Berge’s well-known conjecture was that odd holes and odd antiholes are the forbidden subgraphs of imperfect graphs. In the celebrated proof of the so called Strong Perfect Graph Theorem, Chudnovsky, Robertson, Seymour and Thomas proved Berge’s conjecture. Theorem 2 (Chudnovsky, Robertson, Seymour and Thomas [18]). A simple graph G is perfect if and only if G contains no odd hole or odd anti-hole. 7 3 Perfection and t-perfection A remarkable observation of perfect graphs is that there is a relation to poly- hedra, although the two characterisations mentioned before are graph-theoretical. The stable set polytope SSP(G) ⊆ RV of G is defined as the convex hull of the characteristic vectors of stable subsets of V . The characteristic vector of a subset V S of the set V is the vector χS ∈ {0, 1} with χS(v) = 1 if v ∈ S and 0 otherwise. Each characteristic vector x ∈ RV (G) of a stable set in a graph G satisfies x ≥ 0, X xv ≤ 1 for every clique K in G. (3.1) v∈V (K) Results of Lov´asz[47], Fulkerson [33] and Chv´atal[21] showed that G is perfect if and only if (3.1) suffices to describe SSP(G). Summing up, there are three quite different views on perfect graphs — a view in terms of colouring, a structural and a polyhedral view. Modification of the inequalities (3.1) leads to generalisations of perfection.