Linear Time LexDFS on Chordal Graphs

Lexicographic Depth First Search (LexDFS) is a special variant of a Depth First Search (DFS), which was introduced by Corneil and Krueger in 2008. While this search has been used in various applications, in contrast to other graph searches, no general linear time implementation is known to date. In 2014, K\"ohler and Mouatadid achieved linear running time to compute some special LexDFS orders for cocomparability graphs. In this paper, we present a linear time implementation of LexDFS for chordal graphs. Our algorithm is able to find any LexDFS order for this graph class. To the best of our knowledge this is the first unrestricted linear time implementation of LexDFS on a non-trivial graph class. In the algorithm we use a search tree computed by Lexicographic Breadth First Search (LexBFS)

Konvexität in Graphen: lineare Knotenordnungen und Graphensuchen

We study convexities designed to characterise some of the most fundamental classes of graphs. To this end, we present some known results on this topic in a slightly different form, so as to give a homogeneous representation of a very disparate field. Furthermore, we present some new results on the Caratheodory number of interval graphs and also give a more or less exhaustive account of everything that is known in this context on AT-free graphs, including new results on characterising linear vertex orders and the structure of the intervals of this class. We introduce the new class of bilateral AT-free graphs which is motivated by the linear order characterisation and the convexity used to describe AT-free graphs. We discuss their relation to other known classes and consider the complexity of recognition. Furthermore, as a consequence of notions from abstract convexity we present algorithmic results with regards to some natural subclasses of these. As an application of notion of an extreme vertex of a convex geometry, we discuss structural aspects of avoidable vertices in graphs, which form a generalisation of simplicial vertices. This includes a characterisation of avoidable vertices as simplicial vertices in some minimal triangulation of the graph and a new proof of the existence result. Furthermore, we discuss the algorithmic issues regarding the problem of efficient computation of avoidable vertices in a given graph. This is complemented by an algorithmic application of the concept of avoidable vertices to the maximum weight clique problem, by identifying a rather general class of graphs in which every avoidable vertex is bisimplicial. This leads to a polynomial-time algorithm for the maximum weight clique problem in this class of graphs. Implications of this approach for digraphs are also discussed. All of these results lead to a conjecture concerning the generalisation of avoidable vertices to avoidable paths and we prove this conjecture for paths of length less or equal to two. Finally, we analyse the properties of many different and widely used forms of graph search. Here, we discuss the problem of recognising whether a given vertex can be the last vertex visited by some fixed graph search. Moreover, we present some new aspects of the problem of deciding whether a given spanning tree of a graph is a graph search tree of a particular type of search. We generalise the concept of such trees to many well-known searches and give a broad analysis of the computational complexity of this problem. Both of these discussions are motivated by the use of graph searches in the context of computing properties of convexity.In dieser Arbeit betrachten wir Konvexitäten, die entworfen wurden, um einige der grundlegendsten Graphenklassen zu charakterisieren. Dazu präsentieren wir einige bekannte Resultate zu diesem Thema in einer abgeänderten Form, um eine homogene Darstellung eines diversen Felds zu bieten. Außerdem, geben wir neue Resultate über die Caratheodory Zahl von Intervallgraphen, sowie einen weitestgehend vollständigen Überblick über alle Ergebnisse bezüglich der charakterisierenden Konvexität von AT-freien Graphen, welcher auch neue Ergebnisse über charakterisierende Knotenordnungen und die Struktur der Intervalle dieser Klasse umfasst. Wir führen die neue Klasse der bilateral AT-freien Graphen ein, welche durch die charakterisierende Knotenordnung und Konvexität der AT-freien Graphen motiviert ist. Wir diskutieren das Verhältnis dieser Graphen zu anderen Unterklassen der AT-freien Graphen und untersuchen die Komplexität ihrer Erkennung. Außerdem geben wir einige algorithmische Ergebnisse zu Unterklassen von bilateral AT-freien Graphen, welche aus der Analyse ihrer Konvexität folgen. Als Anwendung des Begriffs eines Extremknoten einer konvexen Geometrie diskutieren wir einige strukturelle Aspekte von vermeidbaren Knoten, welche eine Verallgemeinerung der simplizialen Knoten darstellen. Dies beinhaltet eine Charakterisierung von vermeidbaren Knoten als simpliziale Knoten einer minimalen Triangulierung eines Graphen, sowie einen neuen Beweis über deren Existenz. Wir analysieren die algorithmischen Aspekte des Erkennungsproblems von vermeidbaren Knoten eines gegebenen Graphen. Diese Ergebnisse verwenden wir, um das Konzept eines vermeidbaren Knotens zur Berechnung von Cliquen maximalen Gewichts algorithmisch auszunutzen, indem eine Klasse ermittelt wird, für die jeder vermeidbare Knoten bisimplizial ist. Dies führt zu einem Polynomialzeitalgorithmus zur Berechnung einer Clique maximalen Gewichts auf dieser Klasse. Die Konsequenzen dieses Ansatzes werden auch für gerichtete Graphen analysiert. Alle diese Ergebnisse geben den Anlass zu einer Vermutung, die die Existenz vermeidbarer Knoten zu der Existenz vermeidbarer Pfade ausweitet; diese Vermutung wird für Pfade der Länge 1 und 2 bewiesen. Schließlich betrachten wir die Eigenschaften einiger unterschiedlicher und häufig genutzter Graphensuchen. Wir diskutieren das Problem der Erkennung von Endknoten dieser Suchen. Außerdem präsentieren wir neue Ergebnisse über die Erkennung von Suchbäumen verschiedener Graphensuchen. Wir verallgemeinern das Konzept solcher Suchbäume, um weitere komplexere Suchstrategien abzufangen, und betrachten die Komplexität der Erkennung solcher Bäume. Diese Untersuchungen sind motiviert durch die häufige Verwendung von Graphensuchen, um Eigenschaften von Konvexitäten algorithmisch zu ermitteln

Avoidable vertices and edges in graphs

A vertex in a graph is simplicial if its neighborhood forms a clique. We consider three generalizations of the concept of simplicial vertices: avoidable vertices (also known as \textit{OCF}-vertices), simplicial paths, and their common generalization avoidable paths, introduced here. We present a general conjecture on the existence of avoidable paths. If true, the conjecture would imply a result due to Ohtsuki, Cheung, and Fujisawa from 1976 on the existence of avoidable vertices, and a result due to Chv\'atal, Sritharan, and Rusu from 2002 the existence of simplicial paths. In turn, both of these results generalize Dirac's classical result on the existence of simplicial vertices in chordal graphs. We prove that every graph with an edge has an avoidable edge, which settles the first open case of the conjecture. We point out a close relationship between avoidable vertices in a graph and its minimal triangulations, and identify new algorithmic uses of avoidable vertices, leading to new polynomially solvable cases of the maximum weight clique problem in classes of graphs simultaneously generalizing chordal graphs and circular-arc graphs. Finally, we observe that the proved cases of the conjecture have interesting consequences for highly symmetric graphs: in a vertex-transitive graph every induced two-edge path closes to an induced cycle, while in an edge-transitive graph every three-edge path closes to a cycle and every induced three-edge path closes to an induced cycle

Gemischt-ganzzahlige Programmierung für die additive Fertigung

Since the beginning of its development in the 1950s, mixed integer programming (MIP) has been used for a variety of practical application problems, such as sequence optimization. Exact solution techniques for MIPs, most prominently branch-and-cut techniques, have the advantage (compared to heuristics such as genetic algorithms) that they can generate solutions with optimality certificates. The novel process of additive manufacturing opens up a further perspective for their use. With the two common techniques, Wire Arc Additive Manufacturing (WAAM) and Laser Powder Bed Fusion (LPBD), the sequence in which a given component geometry must be manufactured can be planned. In particular, the heat transfer within the component must be taken into account here, since excessive temperature gradients can lead to internal stresses and warpage after cooling. In order to integrate the temperature, heat transfer models (heat conduction, heat radiation) are integrated into a sequencing model. This leads to the problem class of MIPDECO: MIPs with partial differential equations (PDEs) as further constraints. We present these model approaches for both manufacturing techniques and carry out test calculations for sample geometries in order to demonstrate the feasibility of the approach.Die gemischt-ganzzahlige Programmierung (GGP) wurde seit Beginn ihrer Entwicklung in den 1950er Jahren für eine Vielzahl praktischer Anwendungsprobleme, wie beispielsweise die Reihenfolgeoptimierung, eingesetzt. Exakte Lösungstechniken für GGPe, allen voran Verzweige-und-Begrenze-Techniken, haben (im Vergleich zu Heuristiken wie Genetischen Algorithmen) den Vorteil, dass sie Lösungen mit Optimalitätszertifikaten generieren können. Das neuartige Verfahren der Additiven Fertigung eröffnet eine weitere Perspektive für deren Einsatz. Mit den beiden gängigen Verfahren "Additive Fertigung mit Drahtlichtbogen" und "Laser-Pulverbett-Schmelzen" lässt sich die Reihenfolge planen, in der eine vorgegebene Bauteilgeometrie gefertigt werden sollte. Dabei ist insbesondere der Wärmeübergang innerhalb des Bauteils zu berücksichtigen, da zu hohe Temperaturgradienten beim Abkühlen zu Eigenspannungen und Verformungen führen können. Zur Integration der Temperatur werden Wärmeübergangsmodelle (Wärmeleitung, Wärmestrahlung) in ein mathematisches Reihenfolgemodell integriert. Dies führt zur Problemklasse von GGPPDGL: GGPe mit partiellen Differentialgleichungen (PDGLn) als weitere Nebenbedingungen. Wir stellen diese Modellansätze für beide Fertigungstechniken vor und führen Testrechnungen für Probegeometrien durch, um die Machbarkeit des Ansatzes zu demonstrieren

On the end-vertex problem of graph searches

End vertices of graph searches can exhibit strong structural properties and are crucial for many graph algorithms. The problem of deciding whether a given vertex of a graph is an end-vertex of a particular search was first introduced by Corneil, K\"ohler and Lanlignel in 2010. There they showed that this problem is in fact NP-complete for LBFS on weakly chordal graphs. A similar result for BFS was obtained by Charbit, Habib and Mamcarz in 2014. Here, we prove that the end-vertex problem is NP-complete for MNS on weakly chordal graphs and for MCS on general graphs. Moreover, building on previous results, we show that this problem is linear for various searches on split and unit interval graphs

Recognizing graph search trees

Graph searches and the corresponding search trees can exhibit important structural properties and are used in various graph algorithms. The problem of deciding whether a given spanning tree of a graph is a search tree of a particular search on this graph was introduced by Hagerup and Nowak in 1985, and independently by Korach and Ostfeld in 1989 where the authors showed that this problem is efficiently solvable for DFS trees. A linear time algorithm for BFS trees was obtained by Manber in 1990. In this paper we prove that the search tree problem is also in P for LDFS, in contrast to LBFS, MCS, and MNS, where we show NP-completeness. We complement our results by providing linear time algorithms for these searches on split graphs