In a graph G, a k-attack A is any set of at most k vertices and l-defense D is a set of at most l vertices. We say that defense D counters attack A if each a in A can be matched to a distinct defender d in D with a equal to d or a adjacent to d in G. In the defensive domination problem, we are interested in deciding, for a graph G and positive integers k and l given on input, if there exists an l-defense that counters every possible k-attack on G. Defensive domination is a natural resource allocation problem and can be used to model network robustness and security, disaster response strategies, and redundancy designs. The defensive domination problem is naturally in the complexity class $Σ^P_2$. The problem was known to be NP-hard in general, and polynomial-time algorithms were found for some restricted graph classes. In this note we prove that the defensive domination problem is $Σ^P_2$-complete. We also introduce a natural variant of the defensive domination problem in which the defense is allowed to be a multiset of vertices. This variant is also $Σ^P_2$-complete, but we show that it admits a polynomial-time algorithm in the class of interval graphs. A similar result was known for the original setting in the class of proper interval graphs.

In this article we prove that the minimum-degree greedy algorithm, with adversarial tie-breaking, is a ( 2 / 3 ) -approximation for the Maximum Independent Set problem on interval graphs. We show that this is tight, even on unit interval graphs of maximum degree 3. We show that on chordal graphs, the greedy algorithm is a ( 1 / 2 ) -approximation and that this is again tight. These results contrast with the known (tight) approximation ratio of 3 Δ + 2 of the greedy algorithm for general graphs of maximum degree Δ .

Abstract The planar slope number $${{\,\textrm{psn}\,}}(G)$$ psn ( G ) of a planar graph G is the minimum number of edge slopes in a planar straight-line drawing of G. It is known that $${{\,\textrm{psn}\,}}(G) \in O(c^{\Delta })$$ psn ( G ) ∈ O ( c Δ ) for every planar graph G of maximum degree $$\Delta $$ Δ . This upper bound has been improved to $$O(\Delta ^5)$$ O ( Δ 5 ) if G has treewidth three, and to $$O(\Delta )$$ O ( Δ ) if G has treewidth two. In this paper we prove $${{\,\textrm{psn}\,}}(G) \le \max \{4,\Delta \}$$ psn ( G ) ≤ max { 4 , Δ } when G is a Halin graph, and thus has treewidth three. Furthermore, we present the first polynomial upper bound on the planar slope number for a family of graphs having treewidth four. Namely we show that $$O(\Delta ^2)$$ O ( Δ 2 ) slopes suffice for nested pseudotrees.

Abstract. An obstacle representation of a graph [Formula: see text] consists of a set of pairwise disjoint simply connected closed regions and a one-to-one mapping of the vertices of [Formula: see text] to points such that two vertices are adjacent in [Formula: see text] if and only if the line segment connecting the two corresponding points does not intersect any obstacle. The obstacle number of a graph is the smallest number of obstacles in an obstacle representation of the graph in the plane such that all obstacles are simple polygons. It is known that the obstacle number of each [Formula: see text]-vertex graph is [Formula: see text] [M. Balko, J. Cibulka, and P. Valtr, Discrete Comput. Geom., 59 (2018), pp. 143–164] and that there are [Formula: see text]-vertex graphs whose obstacle number is [Formula: see text] [V. Dujmović and P. Morin, Electron. J. Combin., 22 (2015), 3.1]. We improve this lower bound to [Formula: see text] for simple polygons and to [Formula: see text] for convex polygons. To obtain these stronger bounds, we improve known estimates on the number of [Formula: see text]-vertex graphs with bounded obstacle number, solving a conjecture by Dujmović and Morin. We also show that if the drawing of some [Formula: see text]-vertex graph is given as part of the input, then for some drawings [Formula: see text] obstacles are required to turn them into an obstacle representation of the graph. Our bounds are asymptotically tight in several instances. We complement these combinatorial bounds by two complexity results. First, we show that computing the obstacle number of a graph [Formula: see text] is fixed-parameter tractable in the vertex cover number of [Formula: see text]. Second, we show that, given a graph [Formula: see text] and a simple polygon [Formula: see text], it is NP-hard to decide whether [Formula: see text] admits an obstacle representation using [Formula: see text] as the only obstacle.

Abstract A morph is a continuous transformation between two representations of a graph. We consider the problem of morphing between contact representations of a plane graph. In an $${\mathcal {F}}$$ F -contact representation of a plane graph G, vertices are realized by internally disjoint elements from a family $${\mathcal {F}}$$ F of connected geometric objects. Two such elements touch if and only if their corresponding vertices are adjacent. These touchings also induce the same embedding as in G. In a morph between two $${\mathcal {F}}$$ F -contact representations we insist that at each time step (continuously throughout the morph) we have an $${\mathcal {F}}$$ F -contact representation. We focus on the case when $$\mathcal {F}$$ F is the family of triangles in $$\mathbb {R}^2$$ R 2 that are the lower-right half of axis-parallel rectangles. Such RT-representations exist for every plane graph and right triangles are one of the simplest families of shapes supporting this property. Moreover, they naturally correspond to 3-orientations. Thus, they provide a natural case to study regarding morphs of contact representations of plane graphs. We characterize the pairs of RT-representations admitting a morph between each other via the respective 3-orientations. Our characterization leads to a polynomial-time algorithm to decide whether there is a morph between two RT-representations of an n-vertex plane triangulation, and, if so, computes a morph with $${\mathcal {O}}(n^2)$$ O ( n 2 ) steps. Each of these steps is a linear morph moving the endpoints of each triangle at constant speed along straight-line trajectories. Our characterization also implies that for 4-connected plane triangulations there is a morph between every pair of RT-representations where the “top-most” triangle in both representations corresponds to the same vertex.

We obtain new parameterized algorithms for the classical problem of determining whether a directed acyclic graph admits an upward planar drawing. Our results include a new fixed-parameter algorithm parameterized by the number of sources, an XP-algorithm parameterized by treewidth, and a fixed-parameter algorithm parameterized by treedepth. All three algorithms are obtained using a novel framework for the problem that combines SPQR tree-decompositions with parameterized techniques. Our approach unifies and pushes beyond previous tractability results for the problem on series-parallel digraphs, single-source digraphs and outerplanar digraphs.

In an upward-planar L-drawing of a directed acyclic graph (DAG) each edge $e$ is represented as a polyline composed of a vertical segment with its lowest endpoint at the tail of $e$ and of a horizontal segment ending at the head of $e$. Distinct edges may overlap, but not cross. Recently, upward-planar L-drawings have been studied for $st$-graphs, i.e., planar DAGs with a single source $s$ and a single sink $t$ containing an edge directed from $s$ to $t$. It is known that a plane $st$-graph, i.e., an embedded $st$-graph in which the edge $(s,t)$ is incident to the outer face, admits an upward-planar L-drawing if and only if it admits a bitonic $st$-ordering, which can be tested in linear time. We study upward-planar L-drawings of DAGs that are not necessarily $st$-graphs. On the combinatorial side, we show that a plane DAG admits an upward-planar L-drawing if and only if it is a subgraph of a plane $st$-graph admitting a bitonic $st$-ordering. This allows us to show that not every tree with a fixed bimodal embedding admits an upward-planar L-drawing. Moreover, we prove that any acyclic cactus with a single source (or a single sink) admits an upward-planar L-drawing, which respects a given outerplanar embedding if there are no transitive edges. On the algorithmic side, we consider DAGs with a single source (or a single sink). We give linear-time testing algorithms for these DAGs in two cases: (i) when the drawing must respect a prescribed embedding and (ii) when no restriction is given on the embedding, but it is biconnected and series-parallel.

We obtain new polynomial kernels and compression algorithms for Path Cover and Cycle Cover, the well-known generalizations of the classical Hamiltonian Path and Hamiltonian Cycle problems. Our choice of parameterization is strongly influenced by the work of Biró, Hujter, and Tuza, who in 1992 introduced $H$-graphs, intersection graphs of connected subgraphs of a subdivision of a fixed (multi-)graph $H$. In this work, we turn to proper $H$-graphs, where the containment relationship between the representations of the vertices is forbidden. As the treewidth of a graph measures how similar the graph is to a tree, the size of graph $H$ is the parameter measuring the closeness of the graph to a proper interval graph. We prove the following results. Path Cover admits a kernel of size $\mathcal{O}(\|H\|^8)$, where $\|H\|$ is the size of graph $H$. In other words, we design an algorithm that for an $n$-vertex graph $G$ and integer $k\geq 1$, in time polynomial in $n$ and $\|H\|$, outputs a graph $G'$ of size $\mathcal{O}(\|H\|^8)$ and $k'\leq |V(G')|$ such that the vertex set of $G$ is coverable by $k$ vertex-disjoint paths if and only if the vertex set of $G'$ is coverable by $k'$ vertex-disjoint paths. Hamiltonian Cycle admits a kernel of size $\mathcal{O}(\|H\|^8)$. Cycle Cover admits a polynomial kernel. We prove it by providing a compression of size $\mathcal{O}(\|H\|^{10})$ into another \sf NP-complete problem, namely, Prize Collecting Cycle Cover, that is, we design an algorithm that, in time polynomial in $n$ and $\|H\|$, outputs an equivalent instance of Prize Collecting Cycle Cover of size $\mathcal{O}(\|H\|^{10})$. In all our algorithms we assume that a proper $H$-decomposition is given as a part of the input.

We investigate the parameterized complexity of the recognition problem for the proper H-graphs. The H-graphs are the intersection graphs of connected subgraphs of a subdivision of a multigraph H, and the properness means that the containment relationship between the representations of the vertices is forbidden. The class of H-graphs was introduced as a natural (parameterized) generalization of interval and circular-arc graphs by Biró, Hujter, and Tuza in 1992, and the proper H-graphs were introduced by Chaplick et al. in WADS 2019 as a generalization of proper interval and circular-arc graphs. For these graph classes, H may be seen as a structural parameter reflecting the distance of a graph to a (proper) interval graph, and as such gained attention as a structural parameter in the design of efficient algorithms. We show the following results. - For a tree T with t nodes, it can be decided in 2^{𝒪(t² log t)} ⋅ n³ time, whether an n-vertex graph G is a proper T-graph. For yes-instances, our algorithm outputs a proper T-representation. This proves that the recognition problem for proper H-graphs, where H required to be a tree, is fixed-parameter tractable when parameterized by the size of T. Previously only NP-completeness was known. - Contrasting to the first result, we prove that if H is not constrained to be a tree, then the recognition problem becomes much harder. Namely, we show that there is a multigraph H with 4 vertices and 5 edges such that it is NP-complete to decide whether G is a proper H-graph.

In a fan-planar drawing of a graph an edge can cross only edges with a common end-vertex. In this paper, we study fan-planar drawings that use h (horizontal) layers and are proper, i.e., edges connect adjacent layers. We show that if the embedding of the graph is fixed, then testing the existence of such drawings is fixed-parameter tractable in h, via a reduction to a similar result for planar graphs by Dujmović et al. If the embedding is not fixed, then we give partial results for h = 2: It was already known how to test the existence of fan-planar proper 2-layer drawings for 2-connected graphs, and we show here how to test this for trees. Along the way, we exhibit other interesting results for graphs with a fan-planar proper h-layer drawing; in particular we bound their pathwidth and show that they have a bar-1-visibility representation.

We consider the problem of morphing between contact representations of a plane graph. In a contact representation of a plane graph, vertices are realized by internally disjoint elements from a family of connected geometric objects. Two such elements touch if and only if their corresponding vertices are adjacent. These touchings also induce the same embedding as in the graph. In a morph between two contact representations we insist that at each time step (continuously throughout the morph) we have a contact representation of the same type. We focus on the case when the geometric objects are triangles that are the lower-right half of axis-parallel rectangles. Such RT-representations exist for every plane graph and right triangles are one of the simplest families of shapes supporting this property. Thus, they provide a natural case to study regarding morphs of contact representations of plane graphs. We study piecewise linear morphs, where each step is a linear morph moving the endpoints of each triangle at constant speed along straight-line trajectories. We provide a polynomial-time algorithm that decides whether there is a piecewise linear morph between two RT-representations of a plane triangulation, and, if so, computes a morph with a quadratic number of linear morphs. As a direct consequence, we obtain that for 4-connected plane triangulations there is a morph between every pair of RT-representations where the "top-most" triangle in both representations corresponds to the same vertex. This shows that the realization space of such RT-representations of any 4-connected plane triangulation forms a connected set.

In their seminal work, Mustafa and Ray [30] showed that a wide class of geometric set cover (SC) problems admit a PTAS via local search - this is one of the most general approaches known for such problems. Their result applies if a naturally defined "exchange graph" for two feasible solutions is planar and is based on subdividing this graph via a planar separator theorem due to Frederickson [17]. Obtaining similar results for the related maximum coverage problem (MC) seems non-trivial due to the hard cardinality constraint. In fact, while Badanidiyuru, Kleinberg, and Lee [4] have shown (via a different analysis) that local search yields a PTAS for two-dimensional real halfspaces, they only conjectured that the same holds true for dimension three. Interestingly, at this point it was already known that local search provides a PTAS for the corresponding set cover case and this followed directly from the approach of Mustafa and Ray. In this work we provide a way to address the above-mentioned issue. First, we propose a color-balanced version of the planar separator theorem. The resulting subdivision approximates locally in each part the global distribution of the colors. Second, we show how this roughly balanced subdivision can be employed in a more careful analysis to strictly obey the hard cardinality constraint. More specifically, we obtain a PTAS for any "planarizable" instance of MC and thus essentially for all cases where the corresponding SC instance can be tackled via the approach of Mustafa and Ray. As a corollary, we confirm the conjecture of Badanidiyuru, Kleinberg, and Lee [4] regarding real halfspaces in dimension three. We feel that our ideas could also be helpful in other geometric settings involving a cardinality constraint.

For a graph G, a function $$\psi $$ is called a bar visibility representation of G when for each vertex $$v \in V(G)$$ , $$\psi (v)$$ is a horizontal line segment (bar) and $$uv \in E(G)$$ iff there is an unobstructed, vertical, $$\varepsilon $$ -wide line of sight between $$\psi (u)$$ and $$\psi (v)$$ . Graphs admitting such representations are well understood (via simple characterizations) and recognizable in linear time. For a directed graph G, a bar visibility representation $$\psi $$ of G, additionally, for each directed edge (u, v) of G, puts the bar $$\psi (u)$$ strictly below the bar $$\psi (v)$$ . We study a generalization of the recognition problem where a function $$\psi '$$ defined on a subset $$V'$$ of V(G) is given and the question is whether there is a bar visibility representation $$\psi $$ of G with $$\psi |V' = \psi '$$ . We show that for undirected graphs this problem together with closely related problems are $$\mathsf {NP}$$ -complete, but for certain cases involving directed graphs it is solvable in polynomial time.