Facial circuits of planar graphs and context-free languages

Formulas of monadic second-order logic can be used to specify graph transductions, i.e., multivalued functions from graphs to graphs. We obtain in this way classes of graph transductions, called monadic second-order definable graph transductions (or more simply definable transductions) that are closed under composition and preserve the two known classes of context-free sets of graphs, namely the class of Hyperedge Replacement (HR) and the class of Vertex Replacement (VR) sets. These two classes can be characterized in terms of definable transductions and recognizable sets of finite trees. These characterizations are independent of the rewriting mechanisms used to define the HR and VR grammars. When restricted to words, the definable transductions are strictly more powerful than the rational transductions with finite image; they do not preserve context-free languages. We also describe the sets of discrete (edgeless) labeled graphs that are the images of HR and VR sets under definable transductions: this gives a version of Parikh's Theorem (i.e., the characterization of the commutative images of context-free languages) which extends the classical one and applies to HR and VR sets of graphs.

Vertex-minors, monadic second-order logic, and a conjecture by Seese

We establish that every equational graph can be characterized, up to isomorphism, by a formula of monadic second-order logic. It follows that the isomorphism of two equational graphs is decidable. Equational graphs can be used to describe the behaviour of recursive applicative program schemes. We obtain a sufficient and decidable condition for the equivalence of these program schemes.

Model-checking is one of the most powerful techniques for verifying systems and programs, which since the pioneering results by Knapik et al., Ong, and Kobayashi, is known to be applicable to functional programs with higher-order types against properties expressed by formulas of monadic second-order logic. What happens when the program in question, in addition to higher-order functions, also exhibits algebraic effects such as probabilistic choice or global store? The results in the literature range from those, mostly positive, about nondeterministic effects, to those about probabilistic effects, in the presence of which even mere reachability becomes undecidable. This work takes a fresh and general look at the problem, first of all showing that there is an elegant and natural way of viewing higher-order programs producing algebraic effects as ordinary higher-order recursion schemes. We then move on to consider effect handlers, showing that in their presence the model checking problem is bound to be undecidable in the general case, while it stays decidable when handlers have a simple syntactic form, still sufficient to capture so-called generic effects . Along the way, we hint at how a general specification language could look like, this way justifying some of the results in the literature, and deriving new ones.

The notion of word-representable graphs has been extensively studied. It is well known that the set of word-representable graphs are exactly the graphs whose edges can be ordered in a semi-transitive manner. Thus the set of word-representable graphs is decidable. This paper gives an alternative and simpler proof of decidability of word-representable graphs. The second part of the paper introduces a notion called multi-word-representability. Many classes of graphs - planar graphs, interval graphs, split graphs, co-bipartite graphs and line graphs - are shown to be two word-representable. An upper bound on the number of words needed to represent $$k-$$ colourable graphs has also been calculated.

Tutte sets in graphs II: The complexity of finding maximum Tutte sets

We study an approximation algorithm with a performance guarantee to solve a new $$\mathcal {NP}$$ -hard optimization problem on planar graphs. The problem, which is referred to as the minimum hub cover problem, has recently been introduced to the literature to improve query processing over large graph databases. Planar graphs also arise in various graph query processing applications, such as; biometric identification, image classification, object recognition, and so on. Our algorithm is based on a well-known graph decomposition technique that partitions the graph into a set of outerplanar graphs and provides an approximate solution with a proven performance ratio. We conduct a comprehensive computational experiment to investigate the empirical performance of the algorithm. Computational results demonstrate that the empirical performance of the algorithm surpasses its guaranteed performance. We also apply the same decomposition approach to develop a decomposition-based heuristic, which is much more efficient than the approximation algorithm in terms of computation time. Computational results also indicate that the efficacy of the decomposition-based heuristic in terms of solution quality is comparable to that of the approximation algorithm.

Zum basisproblem der nicht in die projektive ebene einbettbaren graphen, I

Perfectly matched sets in graphs: Parameterized and exact computation

Let α2(G), γ(G) and γc(G) be the 2-independence number, the domination number, and the connected domination number of a graph G respectively. Then α2(G) ≤ γ (G) ≤ γc(G). In this paper , we present a simple heuristic for Minimum Connected Dominating Set in graphs. When running on a graph G excluding Km (the complete graph of order m) as a minor, the heuristic produces a connected dominating set of cardinality at most 7α2(G) - 4 if m = 3, or at most [Formula: see text] if m ≥ 4. In particular, if running on a planar graph G, the heuristic outputs a connected dominating set of cardinality at most 15α2(G) - 5.

Every graph can be represented uniquely in a hierarchical way by means of its modular decomposition. We establish that the modular decomposition of a linearly ordered graph is definable in monadic second-order (MS) logic in the graph itself. The modular decomposition does not depend on the linear order of the given graph. A set of graphs is recognizable w.r.t. the operations associated with graph substitution iff it is definable by a formula of an extension of MS logic based on the use of auxilliary linear orderings. This paper is an extended abstract: complete proofs can be found in [7].

Cycles through all finite vertex sets in infinite graphs

We examine the simultaneous embedding with xed edges problem for two planar graphs G1 and G2 with the focus on their intersection S = G1\G2. In particular, we will present the complete set of intersection graphs S that guarantee a simultaneous embedding with xed edges for (G1;G2). More formally, we dene the subset ISEFE of all planar graphs as follows: A graph S lies in ISEFE if every pair of planar graphs (G1;G2) with intersection S = G1 G2 has a simultaneous embedding with xed edges. We will characterize this set by a detailed study of topological embeddings and nally give a complete list of graphs in this set as our main result of this paper.

3 - Polynomial Algorithms—Matroids

Suppose that an n-vertex graph has a distinct labeling with the integers {1, . . . ,n}. Such a graph is radial level planar if it admits a crossings-free drawing under two constraints. First, each vertex lies on a concentric circle such that the radius of the circle equals the label of the vertex. Second, each edge is drawn with a radially monotone curve. We characterize the set of unlabeled radial level planar(URLP) graphs that are radial level planar in terms of 7 and 15 forbidden subdivisions depending on whether the graph is disconnected or connected, respectively. We also provide linear-time drawing algorithms for any URLP graph.