Search

Tree-cut width is a parameter that has been introduced as an attempt to obtain an analogue of treewidth for edge cuts. Unfortunately, in spite of its desirable structural properties, it turned out that tree-cut width falls short as an edge-cut based alternative to treewidth in algorithmic aspects. This has led to the very recent introduction of a simple edge-based parameter called edge-cut width [WG 2022], which has precisely the algorithmic applications one would expect from an analogue of treewidth for edge cuts, but does not have the desired structural properties. In this paper, we study a variant of tree-cut width obtained by changing the threshold for so-called thin nodes in tree-cut decompositions from 2 to 1. We show that this "slim tree-cut width" satisfies all the requirements of an edge-cut based analogue of treewidth, both structural and algorithmic, while being less restrictive than edge-cut width. Our results also include an alternative characterization of slim tree-cut width via an easy-to-use spanning-tree decomposition akin to the one used for edge-cut width, a characterization of slim tree-cut width in terms of forbidden immersions as well as approximation algorithm for computing the parameter. [ABSTRACT FROM AUTHOR]

The recently introduced graph parameter tree-cut width plays a similar role with respect to immersions as the graph parameter treewidth plays with respect to minors. In this paper, we provide the first algorithmic applications of tree-cut width to hard combinatorial problems. Treecut width is known to be lower-bounded by a function of treewidth, but it can be much larger and hence has the potential to facilitate the efficient solution of problems that are not known to be fixedparameter tractable (FPT) when parameterized by treewidth. We introduce the notion of nice treecut decompositions and provide FPT algorithms for the showcase problems Capacitated Vertex Cover, Capacitated Dominating Set, and Imbalance parameterized by the tree-cut width of an input graph. On the other hand, we show that List Coloring, Precoloring Extension, and Boolean CSP (the last parameterized by the tree-cut width of the incidence graph) are W[1]-hard and hence unlikely to be FPT when parameterized by tree-cut width. [ABSTRACT FROM AUTHOR]

In 1990, Thomas proved that every graph admits a tree decomposition of minimum width that additionally satisfies a certain vertex-connectivity condition called leanness. This result had many uses and has been extended to several other decompositions. In this paper, we consider tree-cut decompositions, that have been introduced by Wollan (2015) as a possible edge-version of tree decompositions. We show that every graph admits a tree-cut decomposition of minimum width that additionally satisfies an edge-connectivity condition analogous to Thomas' leanness. [ABSTRACT FROM AUTHOR]

This paper revisits the classical edge-disjoint paths (EDP) problem, where one is given an undirected graph G and a set of terminal pairs P and asks whether G contains a set of pairwise edge-disjoint paths connecting every terminal pair in P. Our aim is to identify structural properties (parameters) of graphs which allow the efficient solution of EDP without restricting the placement of terminals in P in any way. In this setting, EDP is known to remain NP-hard even on extremely restricted graph classes, such as graphs with a vertex cover of size 3. We present three results which use edge-separator based parameters to chart new islands of tractability in the complexity landscape of EDP. Our first and main result utilizes the fairly recent structural parameter tree-cut width (a parameter with fundamental ties to graph immersions and graph cuts): we obtain a polynomial-time algorithm for EDP on every graph class of bounded tree-cut width. Our second result shows that EDP parameterized by tree-cut width is unlikely to be fixed-parameter tractable. Our final, third result is a polynomial kernel for EDP parameterized by the size of a minimum feedback edge set in the graph. [ABSTRACT FROM AUTHOR]

Suppose scrF is a finite family of graphs. We consider the following meta-problem, called scrF -Immersion Deletion: given a graph G and integer k, decide whether the deletion of at most k edges of G can result in a graph that does not contain any graph from scrF as an immersion. This problem is a close relative of the scrF -Minor Deletion problem studied by Fomin et al. [Proceedings of FOCS, IEEE, 2012, pp. 470--479], where one deletes vertices in order to remove all minor models of graphs from scrF. We prove that whenever all graphs from scrF are connected and at least one graph of scrF is planar and subcubic, then the scrF -Immersion Deletion problem admits a constant-factor approximation algorithm running in time scrO (m3 cdot n3 cdot logm), a linear kernel that can be computed in time scrO (m4 cdot n3 cdot logm), and a scrO (2scrO (k) +m4 cdot n3 cdot logm)-time fixed-parameter algorithm, where n,m count the vertices and edges of the input graph. These results mirror the findings of Fomin et al., who obtained a similar set of algorithmic results for scrF -Minor Deletion, under the assumption that at least one graph from scrF is planar. An important difference is that we are able to obtain a linear kernel for scrF -Immersion Deletion, while the exponent of the kernel of Fomin et al. for scrF -Minor Deletion depends heavily on the family scrF. In fact, this dependence is unavoidable under plausible complexity assumptions, as proven by Giannopoulou et al. [ACM Trans. Algorithms, 13 (2017), p. 35]. This reveals that the kernelization complexity of scrF -Immersion Deletion is quite different from that of scrF -Minor Deletion. [ABSTRACT FROM AUTHOR]

The tree-cut width of a graph is a graph parameter defined by Wollan (J Combin Theory, Ser B, 110:47-66, 2015) with the help of tree-cut decompositions. In certain cases, tree-cut width appears to be more adequate than treewidth as an invariant that, when bounded, can accelerate the resolution of intractable problems. While designing algorithms for problems with bounded tree-cut width, it is important to have a parametrically tractable way to compute the exact value of this parameter or, at least, some constant approximation of it. In this paper we give a parameterized 2-approximation algorithm for the computation of tree-cut width; for an input n-vertex graph G and an integer w, our algorithm either confirms that the tree-cut width of G is more than w or returns a tree-cut decomposition of G certifying that its tree-cut width is at most 2 w, in time $$2^{O(w^2\log w)} \cdot n^2$$ . Prior to this work, no constructive parameterized algorithms, even approximated ones, existed for computing the tree-cut width of a graph. As a consequence of the Graph Minors series by Robertson and Seymour, only the existence of a decision algorithm was known. [ABSTRACT FROM AUTHOR]

We continue and extend previous work on the parameterized complexity analysis of the NP-hard Stable Roommates with Ties and Incomplete Lists problem, thereby strengthening earlier results both on the side of parameterized hardness as well as on the side of fixed-parameter tractability. Other than for its famous sister problem Stable Marriage which focuses on a bipartite scenario, Stable Roommates with Incomplete Lists allows for arbitrary acceptability graphs whose edges specify the possible matchings of each two agents (agents are represented by graph vertices). Herein, incomplete lists and ties reflect the fact that in realistic application scenarios the agents cannot bring all other agents into a linear order. Among our main contributions is to show that it is W[1]-hard to compute a maximum-cardinality stable matching for acceptability graphs of bounded treedepth, bounded tree-cut width, and bounded disjoint paths modulator number (these are each time the respective parameters). Moreover, we obtain that 'only' asking for perfect stable matchings or the mere existence of a stable matching is fixed-parameter tractable with respect to tree-cut width but not with respect to treedepth. On the positive side, we also provide fixed-parameter tractability results for the parameter feedback edge set number. [ABSTRACT FROM AUTHOR]