Algorithms for outerplanar graph roots and graph roots of pathwidth at most 2 (2019)
Journal Article
Golovach, P., Heggernes, P., Kratch, D., Lima, P., & Paulusma, D. (2019). Algorithms for outerplanar graph roots and graph roots of pathwidth at most 2. Algorithmica, 81(7), 2795-2828. https://doi.org/10.1007/s00453-019-00555-y

Deciding if a graph has a square root is a classical problem, which has been studied extensively both from graph-theoretic and algorithmic perspective. As the problem is NP-complete, substantial effort has been dedicated to determining the complexity... Read More about Algorithms for outerplanar graph roots and graph roots of pathwidth at most 2.

Classifying k-Edge Colouring for H-free Graphs (2019)
Journal Article
Galby, E., Lima, P., Paulusma, D., & Ries, B. (2019). Classifying k-Edge Colouring for H-free Graphs. Information Processing Letters, 146, 39-43. https://doi.org/10.1016/j.ipl.2019.02.006

A graph is H-free if it does not contain an induced subgraph isomorphic to H. For every integer k and every graph H, we determine the computational complexity of k-Edge Colouring for H-free graphs.

Finding a small number of colourful components (2019)
Conference Proceeding
Bulteau, L., Dabrowski, K., Fertin, G., Johnson, M., Paulusma, D., & Vialette, S. (2019). Finding a small number of colourful components. In 30th Annual Symposium on Combinatorial Pattern Matching

Clique-width for hereditary graph classes (2019)
Journal Article
Dabrowski, K., Johnson, M., & Paulusma, D. (2019). Clique-width for hereditary graph classes. https://doi.org/10.1017/9781108649094.002

Clique-width is a well-studied graph parameter owing to its use in understanding algorithmic tractability: if the clique-width of a graph class G is bounded by a constant, a wide range of problems that are NP-complete in general can be shown to be po... Read More about Clique-width for hereditary graph classes.

Graph isomorphism for (H1,H2)-free graphs: an almost complete dichotomy (2019)
Conference Proceeding
Bonamy, M., Dabrowski, K. K., Johnson, M., & Paulusma, D. (2019). Graph isomorphism for (H1,H2)-free graphs: an almost complete dichotomy. In Z. Friggstad, J. Sack, & M. R. Salavatipour (Eds.), Algorithms and data structures : 16th International Symposium, WADS 2019, Edmonton, AB, Canada, August 5–7, 2019, proceedings (181-195). https://doi.org/10.1007/978-3-030-24766-9_14

We almost completely resolve the computational complexity of Graph Isomorphism for classes of graphs characterized by two forbidden induced subgraphs H1 and H2. Schweitzer settled the complexity of this problem restricted to (H1;H2)-free graphs for a... Read More about Graph isomorphism for (H1,H2)-free graphs: an almost complete dichotomy.

Surjective H-Colouring over reflexive digraphs (2018)
Journal Article
Larose, B., Martin, B., & Paulusma, D. (2018). Surjective H-Colouring over reflexive digraphs. ACM Transactions on Computation Theory, 11(1), Article 3. https://doi.org/10.1145/3282431

The Surjective H-Colouring problem is to test if a given graph allows a vertex-surjective homomorphism to a fixed graph H. The complexity of this problem has been well studied for undirected (partially) reflexive graphs. We introduce endo-triviality,... Read More about Surjective H-Colouring over reflexive digraphs.

Hereditary graph classes: when the complexities of coloring and clique cover coincide (2018)
Journal Article
Blanché, A., Dabrowski, K., Johnson, M., & Paulusma, D. (2019). Hereditary graph classes: when the complexities of coloring and clique cover coincide. Journal of Graph Theory, 91(3), 267-289. https://doi.org/10.1002/jgt.22431

graph is (H1;H2)-free for a pair of graphs H1;H2 if it contains no induced subgraph isomorphic to H1 or H2. In 2001, Král’, Kratochvíl, Tuza, and Woeginger initiated a study into the complexity of Colouring for (H1;H2)-free graphs. Since then, others... Read More about Hereditary graph classes: when the complexities of coloring and clique cover coincide.

On the parameterized complexity of (k,s)-SAT (2018)
Journal Article
Paulusma, D., & Szeider, S. (2019). On the parameterized complexity of (k,s)-SAT. Information Processing Letters, 43, 34-36. https://doi.org/10.1016/j.ipl.2018.11.005

Let (k, s)-SAT be the k-SAT problem restricted to formulas in which each variable occurs in at most s clauses. It is well known that (3, 3)-SAT is trivial and (3, 4)-SAT is NP-complete. Answering a question posed by Iwama and Takaki (DMTCS 1997), Ber... Read More about On the parameterized complexity of (k,s)-SAT.

Colouring (Pr+Ps)-free graphs (2018)
Conference Proceeding
Klimošová, T., Malík, J., Masařík, T., Novotná, J., Paulusma, D., & Slívová, V. (2018). Colouring (Pr+Ps)-free graphs. In W. Hsu, D. Lee, & C. Liao (Eds.), 29th International Symposium on Algorithms and Computation (ISAAC 2018) (5:1-5:13). https://doi.org/10.4230/lipics.isaac.2018.5

The k-Colouring problem is to decide if the vertices of a graph can be coloured with at most k colours for a fixed integer k such that no two adjacent vertices are coloured alike. If each vertex u must be assigned a colour from a prescribed list L(u)... Read More about Colouring (Pr+Ps)-free graphs.

Critical vertices and edges in H-free graphs (2018)
Journal Article
Paulusma, D., Picouleau, C., & Ries, B. (2019). Critical vertices and edges in H-free graphs. Discrete Applied Mathematics, 257, 361-367. https://doi.org/10.1016/j.dam.2018.08.016

A vertex or edge in a graph is critical if its deletion reduces the chromatic number of the graph by one. We consider the problems of deciding whether a graph has a critical vertex or edge, respectively. We give a complexity dichotomy for both proble... Read More about Critical vertices and edges in H-free graphs.

Computing small pivot-minors (2018)
Conference Proceeding
Dabrowski, K. K., Dross, F., Jeong, J., Kanté, M. M., Kwon, O., Oum, S., & Paulusma, D. (2018). Computing small pivot-minors. In A. Brandstädt, E. Köhler, & K. Meer (Eds.), Graph-Theoretic Concepts in Computer Science, 44th International Workshop, WG 2018, Cottbus, Germany, June 27–29, 2018 ; proceedings (125-138). https://doi.org/10.1007/978-3-030-00256-5_11

A graph G contains a graph H as a pivot-minor if H can be obtained from G by applying a sequence of vertex deletions and edge pivots. Pivot-minors play an important role in the study of rank-width. However, so far, pivot-minors have only been studied... Read More about Computing small pivot-minors.

Connected Vertex Cover for (sP1+P5)-free graphs (2018)
Conference Proceeding
Johnson, M., Paesani, G., & Paulusma, D. (2018). Connected Vertex Cover for (sP1+P5)-free graphs. In A. Brandstädt, E. Köhler, & K. Meer (Eds.), Graph-theoretic concepts in computer science : 44th International Workshop, WG 2018, Cottbus, Germany, June 27-29, 2018, Proceedings (279-291). https://doi.org/10.1007/978-3-030-00256-5_23

The Connected Vertex Cover problem is to decide if a graph G has a vertex cover of size at most k that induces a connected subgraph of G. This is a well-studied problem, known to be NP-complete for restricted graph classes, and, in particular, for H-... Read More about Connected Vertex Cover for (sP1+P5)-free graphs.

Simple games versus weighted voting games (2018)
Conference Proceeding
Hof, F., Kern, W., Kurz, S., & Paulusma, D. (2018). Simple games versus weighted voting games. In X. Deng (Ed.), Algorithmic game theory : 11th International Symposium, SAGT 2018, Beijing, China, September 11-14, 2018, Proceedings (69-81). https://doi.org/10.1007/978-3-319-99660-8_7

A simple game (N, v) is given by a set N of n players and a partition of 2N into a set L of losing coalitions L with value v(L)=0 that is closed under taking subsets and a set W of winning coalitions W with v(W)=1 . Simple games with α=minp≥0maxW∈W,L... Read More about Simple games versus weighted voting games.

Contraction and Deletion Blockers for Perfect Graphs and H -free Graphs (2018)
Journal Article
Diner, Ö., Paulusma, D., Picouleau, C., & Ries, B. (2018). Contraction and Deletion Blockers for Perfect Graphs and H -free Graphs. Theoretical Computer Science, 746, 49-72. https://doi.org/10.1016/j.tcs.2018.06.023

We study the following problem: for given integers d, k and graph G, can we reduce some fixed graph parameter π of G by at least d via at most k graph operations from some fixed set S? As parameters we take the chromatic number χ, clique number ω and... Read More about Contraction and Deletion Blockers for Perfect Graphs and H -free Graphs.

Independent Feedback Vertex Set for P5-free Graphs (2018)
Journal Article
Bonamy, M., Dabrowski, K., Feghali, C., Johnson, M., & Paulusma, D. (2018). Independent Feedback Vertex Set for P5-free Graphs. Algorithmica, 81(4), 1416-1449. https://doi.org/10.1007/s00453-018-0474-x

The NP-complete problem Feedback Vertex Set is that of deciding whether or not it is possible, for a given integer k≥0 , to delete at most k vertices from a given graph so that what remains is a forest. The variant in which the deleted vertices must... Read More about Independent Feedback Vertex Set for P5-free Graphs.

Surjective H-colouring: New hardness results (2018)
Journal Article
Golovach, P., Johnson, M., Martin, B., Paulusma, D., & Stewart, A. (2019). Surjective H-colouring: New hardness results. Computability, 8(1), 27-42. https://doi.org/10.3233/com-180084

A homomorphism from a graph G to a graph H is a vertex mapping f from the vertex set of G to the vertex set of H such that there is an edge between vertices f(u) and f(v) of H whenever there is an edge between vertices u and v of G. The H-Colouring p... Read More about Surjective H-colouring: New hardness results.

On colouring (2P2,H)-free and (P5,H)-free graphs (2018)
Journal Article
Dabrowski, K., & Paulusma, D. (2018). On colouring (2P2,H)-free and (P5,H)-free graphs. Information Processing Letters, 134, 35-41. https://doi.org/10.1016/j.ipl.2018.02.003

The Colouring problem asks whether the vertices of a graph can be coloured with at most k colours for a given integer k in such a way that no two adjacent vertices receive the same colour. A graph is (H1,H2)-free if it has no induced subgraph isomorp... Read More about On colouring (2P2,H)-free and (P5,H)-free graphs.

Colouring Square-Free Graphs without Long Induced Paths (2018)
Conference Proceeding
Gaspers, S., Huang, S., & Paulusma, D. (2018). Colouring Square-Free Graphs without Long Induced Paths. In R. Niedermeier, & B. Vallée (Eds.), 35th Symposium on Theoretical Aspects of Computer Science (STACS 2018) : February 28–March 3, 2018, Caen, France (35:1-35:15). https://doi.org/10.4230/lipics.stacs.2018.35

The Colouring problem is to decide if the vertices of a graph can be coloured with at most k colours for a given integer k such that no two adjacent vertices are coloured alike. The complexity of Colouring is fully understood for graph classes charac... Read More about Colouring Square-Free Graphs without Long Induced Paths.

Surjective H-Colouring over Reflexive Digraphs (2018)
Conference Proceeding
Larose, B., Martin, B., & Paulusma, D. (2018). Surjective H-Colouring over Reflexive Digraphs. In R. Niedermeier, & B. Vallée (Eds.), 35th Symposium on Theoretical Aspects of Computer Science (STACS 2018) : February 28–March 3, 2018, Caen, France (49:1-49:14). https://doi.org/10.4230/lipics.stacs.2018.49

The Surjective H-Colouring problem is to test if a given graph allows a vertex-surjective homomorphism to a fixed graph H. The complexity of this problem has been well studied for undirected (partially) reflexive graphs. We introduce endo-triviality,... Read More about Surjective H-Colouring over Reflexive Digraphs.

Disconnected Cuts in Claw-free Graphs (2018)
Conference Proceeding
Martin, B., Paulusma, D., & van Leeuwen, E. J. (2018). Disconnected Cuts in Claw-free Graphs. In Y. Azar, H. Bast, & G. Herman (Eds.), 26th Annual European Symposium on Algorithms (ESA 2018) (61:1-61:14). https://doi.org/10.4230/lipics.esa.2018.61

A disconnected cut of a connected graph is a vertex cut that itself also induces a disconnected subgraph. The corresponding decision problem is called Disconnected Cut. It is known that Disconnected Cut is NP-hard on general graphs, while polynomial-... Read More about Disconnected Cuts in Claw-free Graphs.