Forbidden Substructures Research Articles

The r-coloring and maximum stable set problem in hypergraphs with bounded matching number and edge size

Motivated by the analogous questions in graphs, we study the complexity of coloring and stable set problems in hypergraphs with forbidden substructures and bounded edge size. Letting ν(G) denote the maximum size of a matching in H, we obtain complete dichotomies for the complexity of the following problems parametrized by fixed r,k,s∈N:•r-Coloring in hypergraphs G with edge size at most k and ν(G)≤s;•r-Precoloring Extension in k-uniform hypergraphs G with ν(G)≤s;•r-Precoloring Extension in hypergraphs G with edge size at most k and ν(G)≤s;•Maximum Stable Set in k-uniform hypergraphs G with ν(G)≤s;•Maximum Weight Stable Set in k-uniform hypergraphs with ν(G)≤s; as well as partial results for r-Coloring in k-uniform hypergraphs ν(G)≤s. We then turn our attention to 2-Coloring in 3-uniform hypergraphs with forbidden induced subhypergraphs, and give a polynomial-time algorithm when restricting the input to hypergraphs excluding a fixed one-edge hypergraph. Finally, we consider linear 3-uniform hypergraphs (in which every two edges share at most one vertex), and show that excluding an induced matching in G implies that ν(G) is bounded by a constant; and that 3-coloring linear 3-uniform hypergraphs G with ν(G)≤532 is NP-hard.

Posets with interfaces as a model for concurrency

We introduce posets with interfaces (iposets) and generalise their standard serial composition to a new gluing composition. In the partial order semantics of concurrency, interfaces and gluing allow modelling events that extend in time and across components. Alternatively, taking a decompositional view, interfaces allow cutting through events, while serial composition may only cut through edges of a poset. We show that iposets under gluing composition form a category, which generalises the monoid of posets under serial composition up to isomorphism. They form a 2-category when a subsumption order and a lax tensor in the form of a non-commutative parallel composition are added, which generalises the interchange monoids used for modelling series-parallel posets. We also study the gluing-parallel hierarchy of iposets, which generalises the standard series-parallel one. The class of gluing-parallel iposets contains that of series-parallel posets and the class of interval orders, which are well studied in concurrency theory, too. We also show that it is strictly contained in the class of all iposets by identifying several forbidden substructures.

Large Cliques in Hypergraphs with Forbidden Substructures

A result due to Gyarfas, Hubenko, and Solymosi (answering a question of Erdős) states that if a graph G on n vertices does not contain K2,2 as an induced subgraph yet has at least $$c\left(\begin{array}{c}n 2\end{array}\right)$$ edges, then G has a complete subgraph on at least $$\frac{c^2}{10}n$$ vertices. In this paper we suggest a “higher-dimensional” analogue of the notion of an induced K2,2 which allows us to generalize their result to k-uniform hypergraphs. Our result also has an interesting consequence in discrete geometry. In particular, it implies that the fractional Helly theorem can be derived as a purely combinatorial consequence of the colorful Helly theorem.

Detecting wormhole attacks in 3D wireless ad hoc networks via 3D forbidden substructures

A wormhole in wireless ad hoc networks is a physically low-latency link that connects two topologically distant nodes. Thereby, an adversary can launch the wormhole attack by tunnelling recorded packets from one node to the other and retransmitting them in the network. Since the wormhole attack is independent of MAC layer protocols and immune to cryptographic techniques, it has been one of the most dangerous security threats to wireless ad hoc networks. At present, efficient algorithms have been applied to 2D networks to detect wormhole attacks by seeking for forbidden substructures. However, when we employ their defined forbidden substructures to detect wormhole attacks in 3D networks (which are more pervasive in reality), we encounter severe obstacles. Through an in-depth examination, we discover the existence of efficient 3D forbidden substructures by introducing maximum independent sets (MaxIS) into the network. Essentially different from 2D forbidden substructures, 3D forbidden substructures can hardly be intuitively perceived. Driven by above understandings, we design a MaxIS-based wormhole detection algorithm for 3D networks using only connectivity information. Furthermore, we conduct thorough theoretical analyses and illustrate that our algorithm is able to detect almost 100% wormhole attacks even in a wireless network with very poor connectivity.

Canonizing structural Ramsey theorems

At the beginning of 1950s Erdős and Rado suggested the investigation of the Ramsey-type results where the number of colors is not finite. This marked the birth of the so-called canonizing Ramsey theory. In 1985 Prömel and Voigt made the first step towards the structural canonizing Ramsey theory when they proved the canonical Ramsey property for the class of finite linearly ordered hypergraphs, and the subclasses thereof defined by forbidden substructures. Building on their results in this paper we provide several new structural canonical Ramsey results. We prove the canonical Ramsey theorem for the class of all finite linearly ordered tournaments, the class of all finite posets with linear extensions and the class of all finite linearly ordered metric spaces. We conclude the paper with the canonical version of the celebrated Nešetřil–Rödl Theorem. In contrast to the “classical” Ramsey-theoretic approach, in this paper we advocate the use of category theory to manage the complexity of otherwise technically overwhelming proofs typical in canonical Ramsey theory.

River landscapes and optimal channel networks

We study tree structures termed optimal channel networks (OCNs) that minimize the total gravitational energy loss in the system, an exact property of steady-state landscape configurations that prove dynamically accessible and strikingly similar to natural forms. Here, we show that every OCN is a so-called natural river tree, in the sense that there exists a height function such that the flow directions are always directed along steepest descent. We also study the natural river trees in an arbitrary graph in terms of forbidden substructures, which we call k-path obstacles, and OCNs on a d-dimensional lattice, improving earlier results by determining the minimum energy up to a constant factor for every [Formula: see text] Results extend our capabilities in environmental statistical mechanics.

Perfect elimination orderings for symmetric matrices

We introduce a new class of structured symmetric matrices by extending the notion of perfect elimination ordering from graphs to weighted graphs or matrices. This offers a common framework capturing common vertex elimination orderings of monotone families of chordal graphs, Robinsonian matrices and ultrametrics. We give a structural characterization for matrices that admit perfect elimination orderings in terms of forbidden substructures generalizing chordless cycles in graphs.

A Structural Characterization for Certifying Robinsonian Matrices

A symmetric matrix is Robinsonian if its rows and columns can be simultaneously reordered in such a way that entries are monotone nondecreasing in rows and columns when moving toward the diagonal. The adjacency matrix of a graph is Robinsonian precisely when the graph is a unit interval graph, so that Robinsonian matrices form a matrix analogue of the class of unit interval graphs. Here we provide a structural characterization for Robinsonian matrices in terms of forbidden substructures, extending the notion of asteroidal triples to weighted graphs. This implies the known characterization of unit interval graphs and leads to an efficient algorithm for certifying that a matrix is not Robinsonian.

On a special class of median algebras

In this short note we consider a class of median algebras, called (1, 2 : 3)-semilattices, that is pertaining to cluster analysis. Such median algebras arise from a natural generalization of conservativeness, and their description is given in terms of forbidden substructures.

The one-dimensional Euclidean domain: finitely many obstructions are not enough

We show that one-dimensional Euclidean preference profiles can not be characterized in terms of finitely many forbidden substructures. This result is in strong contrast to the case of single-peaked and single-crossing preference profiles, for which such finite characterizations have been derived in the literature.

Forbidden substructure for interval digraphs/bigraphs

An interval matrix is the adjacency matrix of an interval digraph or equivalently the biadjacency matrix of an interval bigraph. In this paper we investigate the forbidden substructures of an interval bigraph. Our method finds hitherto existing forbidden substructures for interval matrices, and via a more concise statement, as well as a new example showing that these substructures are not exhaustive.

All cartesian closed categories of quasicontinuous domains consist of domains

Quasicontinuity is a generalisation of Scott's notion of continuous domain, introduced in the early 80s by Gierz, Lawson and Stralka. In this paper we ask which cartesian closed full subcategories exist in qCONT, the category of all quasicontinuous domains and Scott-continuous functions. The surprising, and perhaps disappointing, answer turns out to be that all such subcategories consist entirely of continuous domains. In other words, there are no new cartesian closed full subcategories in qCONT beyond those already known to exist in CONT.To prove this, we reduce the notion of meet-continuity for dcpos to one which only involves well-ordered chains. This allows us to characterise meet-continuity by “forbidden substructures”. We then show that each forbidden substructure has a non-quasicontinuous function space.

Conservative Median Algebras and Semilattices

We characterize conservative median algebras and semilattices by means of forbidden substructures and by providing their representation as chains. Moreover, using a dual equivalence between median algebras and certain topological structures, we obtain descriptions of the median-preserving mappings between products of finitely many chains.

Spherical tilings by congruent quadrangles: Forbidden cases and substructures

In this article we show the non-existence of a class of spherical tilings by congruent quadrangles. We also prove several forbidden substructures for spherical tilings by congruent quadrangles. These are results that will help to complete of the classification of spherical tilings by congruent quadrangles.

Decidability of Definability

AbstractFor a fixed countably infinite structure Γ with finite relational signature τ, we study the following computational problem: input are quantifier-free τ-formulas ϕ0, ϕ1, …, ϕn that define relations R0, R1, …, Rn over Γ. The question is whether the relation R0 is primitive positive definable from R1, …, Rn, i.e., definable by a first-order formula that uses only relation symbols for R1, …, Rn, equality, conjunctions, and existential quantification (disjunction, negation, and universal quantification are forbidden).We show decidability of this problem for all structures Γ that have a first-order definition in an ordered homogeneous structure Δ with a finite relational signature whose age is a Ramsey class and determined by finitely many forbidden substructures. Examples of structures Γ with this property are the order of the rationals, the random graph, the homogeneous universal poset, the random tournament, all homogeneous universal C-relations, and many more. We also obtain decidability of the problem when we replace primitive positive definability by existential positive, or existential definability. Our proof makes use of universal algebraic and model theoretic concepts, Ramsey theory, and a recent characterization of Ramsey classes in topological dynamics.

Recognition and Characterization of Chronological Interval Digraphs

Interval graphs admit elegant structural characterizations and linear time recognition algorithms; on the other hand, the usual interval digraphs lack a forbidden structure characterization as well as a low-degree polynomial time recognition algorithm. In this paper we identify another natural digraph analogue of interval graphs that we call ”chronological interval digraphs”. By contrast, the new class admits both a forbidden structure characterization and a linear time recognition algorithm. Chronological interval digraphs arise by interpreting the standard definition of an interval graph with a natural orientation of its edges. Specifically, $G$ is a chronological interval digraph if there exists a family of closed intervals $I_v$, $v \in V(G)$, such that $uv$ is an arc of $G$ if and only if $I_u$ intersects $I_v$ and the left endpoint of $I_u$ is not greater than the left endpoint of $I_v$. (Equivalently, if and only if $I_u$ contains the left endpoint of $I_v$.)We characterize chronological interval digraphs in terms of vertex orderings, in terms of forbidden substructures, and in terms of a novel structure of so-called $Q$-paths. The first two characterizations exhibit strong similarity with the corresponding characterizations of interval graphs. The last characterization leads to a linear time recognition algorithm.

A characterization of the single-crossing domain

We characterize single-crossing preference profiles in terms of two forbidden substructures, one of which contains three voters and six (not necessarily distinct) alternatives, and one of which contains four voters and four (not necessarily distinct) alternatives. We also provide an efficient way to decide whether a preference profile is single-crossing.

Series Parallel Digraphs with Loops

In the conversion of finite automata to regular expressions, an exponential blowup in size can generally not be avoided. This is due to graph-structural properties of automata which cannot be directly encoded by regular expressions and cause the blowup combinatorially. In order to identify these structures, we generalize the class of arc-series-parallel digraphs beyond the acyclic case. The resulting digraphs are shown to be reversibly encoded by linear-sized regular expressions. Also, a characterization of this new class by a set of seven forbidden substructures is given. Automata that require expressions of superlinear size must contain some of these substructures.

A note on the parameterized complexity of unordered maximum tree orientation

In the Unordered Maximum Tree Orientation problem, a set P of paths in a tree and a parameter k is given, and we want to orient the edges in the tree such that all but at most k paths in P become directed paths. This is a more difficult variant of a well-studied problem in computational biology where the directions of paths in P are already given. We show that the parameterized complexity of the unordered version is between Edge Bipartization and Vertex Bipartization, and we give a characterization of orientable path sets in trees by forbidden substructures, which are cycles of a certain kind.

Equitable defective coloring of sparse planar graphs

A graph has an equitable, defective k -coloring (an ED- k -coloring) if there is a k -coloring of V ( G ) that is defective (every vertex shares the same color with at most one neighbor) and equitable (the sizes of all color classes differ by at most one). A graph may have an ED- k -coloring, but no ED- ( k + 1 ) -coloring. In this paper, we prove that planar graphs with minimum degree at least 2 and girth at least 10 are ED- k -colorable for any integer k ≥ 3 . The proof uses the method of discharging. We are able to simplify the normally lengthy task of enumerating forbidden substructures by using Hall’s Theorem, an unusual approach.