# doc. RNDr. Dušan Knop, Ph.D.

- +420224359879
- dusan.knop@fit.cvut.cz
- TH:A-1237

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### Bribery Can Get Harder in Structured Multiwinner Approval Election

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Kusek, B.; Bredereck, R.; Faliszewski, P.; Kaczmarczyk, A.; Knop, D.

Rok

2023

Publikováno

Proceedings of the 22nd International Conference on Autonomous Agents and Multiagent Systems, AAMAS 2023. County of Richland: IFAAMAS, 2023. p. 1725-1733. ISSN 1548-8403. ISBN 978-1-4503-9432-1.

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We study the complexity of constructive bribery in the context of structured multiwinner approval elections. Given such an election, we ask whether a certain candidate can join the winning committee by adding, deleting, or swapping approvals, where each such action comes at a cost and we are limited by a budget. We assume our elections to either have the candidate interval or the voter interval property, and we require the property to hold also after the bribery. While structured elections usually make manipulative attacks significantly easier, our work also shows examples of the opposite behavior. We conclude by presenting preliminary insights regarding the destructive variant of our problem.

### Constant factor approximation for tracking paths and fault tolerant feedback vertex set

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Rok

2023

Publikováno

Discrete Optimization. 2023, 47 ISSN 1572-5286.

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Consider a vertex-weighted graph G with a source s and a target t. TRACKING PATHS requires finding a minimum weight set of vertices (trackers) such that the sequence of trackers in each path from s to t is unique. In this work, we derive a factor 6-approximation algorithm for TRACKING PATHS in weighted graphs and a factor 4-approximation algorithm if the input is unweighted. This is the first constant factor approximation for this problem. While doing so, we also study approximation of the closely related r-FAULT TOLERANT FEEDBACK VERTEX SET problem. There, for a fixed integer r and a given vertex-weighted graph G, the task is to find a minimum weight set of vertices intersecting every cycle of G in at least r+1 vertices. We give a factor O(r) approximation algorithm for r-FAULT TOLERANT FEEDBACK VERTEX SET if r is a constant.

### Establishing Herd Immunity is Hard Even in Simple Geometric Networks

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Dvořák, M.; Knop, D.; Schierreich, Š.

Rok

2023

Publikováno

Proceedings of the 18th Workshop on Algorithms and Models for the Web Graph. Cham: Springer, 2023. p. 68-82. Lecture Notes in Computer Science. vol. 13894. ISSN 0302-9743. ISBN 978-3-031-32295-2.

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We study the following model of disease spread in a social network. In the beginning, all individuals are either ``infected'' or ``healthy''. Next, in discrete rounds, the disease spreads in the network from infected to healthy individuals such that a healthy individual gets infected if and only if a sufficient number of its direct neighbours are already infected.
We represent the social network as a graph. Inspired by the real-world restrictions in the current epidemic, especially by social and physical distancing requirements, we restrict ourselves to networks that can be represented as geometric intersection graphs.
We show that finding a minimal vertex set of initially infected individuals to spread the disease in the whole network is computationally hard, already on unit disk graphs. Hence, to provide some algorithmic results, we focus ourselves on simpler geometric graph families, such as interval graphs and grid graphs.

### Fine-grained view on bribery for group identification

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Boehmer, N.; Bredereck, R.; Knop, D.; Luo, J.

Rok

2023

Publikováno

Autonomous Agents and Multi-Agent Systems. 2023, 37(1), ISSN 1387-2532.

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Given a set of agents qualifying or disqualifying each other, group identification is the task of identifying a socially qualified subgroup of agents. Social qualification depends on the specific rule used to aggregate individual qualifications . The classical bribery problem in this context asks how many agents need to change their qualifications in order to change the outcome in a certain way. Complementing previous results showing polynomial-time solvability or NP-hardness of bribery for various social rules in the constructive (aiming at making specific agents socially qualified) or destructive (aiming at making specific agents socially disqualified) setting, we provide a comprehensive picture of the parameterized computational complexity landscape. Conceptually, we also consider a more fine-grained concept of bribery cost, where we ask how many single qualifications need to be changed, nonunit prices for different bribery actions, and a more general bribery goal that combines the constructive and destructive setting.

### Hedonic Diversity Games: A Complexity Picture with More than Two Colors

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Rok

2023

Publikováno

Artificial Intelligence. 2023, 325 1-20. ISSN 0004-3702.

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Hedonic diversity games are a variant of the classical hedonic games designed to better model a variety of questions concerning diversity and fairness. Previous works mainly targeted the case with two diversity classes (represented as colors in the model) and provided some initial complexity-theoretic and existential results concerning Nash and individually stable outcomes. Here, we design new algorithms accompanied with lower bounds which provide a comprehensive parameterized-complexity picture for computing Nash and individually stable outcomes with respect to the most natural parameterizations of the problem. Crucially, our results hold for general hedonic diversity games where the number of colors is not necessarily restricted to two, and show that---apart from two trivial cases---a necessary condition for tractability in this setting is that the number of colors is bounded by the parameter. Moreover, for the special case of two colors we resolve an open question asked in previous work~(Boehmer and Elkind, AAAI 2020).

### High-Multiplicity Fair Allocation Using Parametric Integer Linear Programming

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Bredereck, R.; Kaczmarczyk, A.; Knop, D.; Niedermeier, R.

Rok

2023

Publikováno

European Conference on Artificial Intelligence 2023. Amsterdam: IOS Press, 2023. p. 303-310. Frontiers in Artificial Intelligence and Applications. vol. 372. ISSN 0922-6389. ISBN 978-1-64368-436-9.

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Using insights from parametric integer linear programming, we improve the work of Bredereck et al. [Proc. ACM EC 2019] on high-multiplicity fair allocation. Answering an open question from their work, we proved that the problem of finding envy-free Pareto-efficient allocations of indivisible items is fixed-parameter tractable with respect to the combined parameter “number of agents” plus “number of item types.” Our central improvement, compared to their result, is to break the condition that the corresponding utility and multiplicity values have to be encoded in unary, which is required there. Concretely, we show that, while preserving fixed-parameter tractability, these values can be encoded in binary. Thus, we substantially expand the range of feasible utility and multiplicity values.

### Host Community Respecting Refugee Housing

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Rok

2023

Publikováno

Proceedings of the 22nd International Conference on Autonomous Agents and Multiagent Systems, AAMAS 2023. County of Richland: IFAAMAS, 2023. p. 966-975. ISSN 1548-8403. ISBN 978-1-4503-9432-1.

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We propose a novel model for refugee housing respecting the preferences of the accepting community and the refugees themselves. In particular, we are given a topology representing the local community, a set of inhabitants occupying some vertices of topology, and a set of refugees that should be placed on the empty vertices of the graph. Both the inhabitants and the refugees have preferences over the structure of their neighbourhood.
We are specifically interested in the problem of finding placements such that the preferences of every individual are met; using game-theoretical words, we are looking for placements that are stable with respect to some well-defined notion of stability. We investigate conditions under which the existence of equilibrium is guaranteed and study the computational complexity of finding such a stable outcome. As the problem is NP-hard even in very simple settings, we employ the parameterised complexity framework to give a finer-grained view on the problem's complexity with respect to natural parameters and structural restrictions of the given topology.

### Maximizing Social Welfare in Score-Based Social Distance Games

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Rok

2023

Publikováno

Proceedings of the 19th Conference on Theoretical Aspects of Rationality and Knowledge, TARK 2023. Sydney: Open Publishing Association, 2023. p. 272-286. Electronic Proceedings in Theoretical Computer Science. vol. 379. ISSN 2075-2180.

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Social distance games have been extensively studied as a coalition formation model where the utilities of agents in each coalition were captured using a utility function u that took into account distances in a given social network. In this paper, we consider a non-normalized score-based definition of social distance games where the utility function u_v depends on a generic scoring vector v, which may be customized to match the specifics of each individual application scenario.
As our main technical contribution, we establish the tractability of computing a welfare-maximizing partitioning of the agents into coalitions on tree-like networks, for every score-based function u_v. We provide more efficient algorithms when dealing with specific choices of u_v or simpler networks, and also extend all of these results to computing coalitions that are Nash stable or individually rational. We view these results as a further strong indication of the usefulness of the proposed score-based utility function: even on very simple networks, the problem of computing a welfare-maximizing partitioning into coalitions remains open for the originally considered canonical function u.

### Polynomial kernels for tracking shortest paths

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Rok

2023

Publikováno

Information Processing Letters. 2023, 179(1), ISSN 1872-6119.

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Given an undirected graph G = (V , E), vertices s,t ∈ V , and an integer k, Tracking
Shortest Paths requires deciding whether there exists a set of k vertices T ⊆ V such
that for any two distinct shortest paths between s and t, say P1 and P2, we have
T ∩ V (P1) != T ∩ V (P2). In this paper, we give the first polynomial size kernel for the
problem. Specifically we show the existence of a kernel with O(k2) vertices and edges in
general graphs and a kernel with O(k) vertices and edges in planar graphs for the Tracking Paths in DAG problem. This problem admits a polynomial parameter transformation to Tracking Shortest Paths, and this implies a kernel with O(k4) vertices and edges for Tracking Shortest Paths in general graphs and a kernel with O(k2) vertices and edges in planar graphs. Based on the above we also give a single exponential algorithm for Tracking Shortest Paths in planar graphs.

### The Parameterized Complexity of Network Microaggregation

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Blažej, V.; Ganian, R.; Knop, D.; Pokorný, J.; Schierreich, Š.; Simonov, K.

Rok

2023

Publikováno

Proceedings of the 37th AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2023. p. 6262-6270. ISSN 2159-5399.

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Microaggregation is a classical statistical disclosure control technique which requires the input data to be partitioned into clusters while adhering to specified size constraints. We provide novel exact algorithms and lower bounds for the task of microaggregating a given network while considering both unrestricted and connected clusterings, and analyze these from the perspective of the parameterized complexity paradigm. Altogether, our results assemble a complete complexity-theoretic picture for the network microaggregation problem with respect to the most natural parameterizations of the problem, including input-specified parameters capturing the size and homogeneity of the clusters as well as the treewidth and vertex cover number of the network.

### Balancing the Spread of Two Opinions in Sparse Social Networks (Student Abstract)

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Rok

2022

Publikováno

Proceedings of the 36th AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2022. p. 12987-12988. ISSN 2159-5399. ISBN 978-1-57735-876-3.

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We propose a new discrete model for simultaneously spreading two opinions within a social network inspired by the famous Target Set Selection problem. We are given a social network, a seed-set of agents for each opinion, and two thresholds per agent. The first threshold represents the willingness of an agent to adopt an opinion if she has no opinion at all, while the second threshold states the readiness to acquire a second opinion. The goal is to add as few agents as possible to the initial seed-sets such that, once the process started with these seed-set stabilises, each agent has either both opinions or none. We perform an initial study of its computational complexity. It is not surprising that the problem is NP-hard even in quite restricted settings. Therefore, we investigate the complexity of the problem from the parameterized point-of-view with special focus on sparse networks, which appears often in practice. Among other things, we show that the proposed problem is in the FPT complexity class if we parameterize by the vertex cover number of the underlying graph.

### Controlling the Spread of Two Secrets in Diverse Social Networks (Student Abstract)

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Blažej, V.; Knop, D.; Schierreich, Š.

Rok

2022

Publikováno

Proceedings of the 36th AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2022. p. 12919-12920. ISSN 2159-5399. ISBN 978-1-57735-876-3.

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nformation diffusion in social networks is a well-studied concept in social choice theory. We propose the study of the diffusion of two secrets in a heterogeneous environment from the complexity perspective, that is, there are two different networks with the same set of agents (e.g., the structure of the set of followers might be different in two distinct social networks). Formally, our model combines two group identification processes for which we do have independent desiderata---either constructive, where we would like a given group of agents to be exposed to a secret, or destructive, where a given group of agents should not be exposed to a secret. To be able to reach these targets, we can either delete an agent or introduce a previously latent agent. Our results are mostly negative---all of the problems are NP-hard. Therefore, we propose a parameterized study with respect to the natural parameters, the number of influenced agents, the size of the required/protected agent sets, and the duration of the diffusion process. Most of the studied problems remain W[1]-hard even for a combination of these parameters. We complement these results with nearly optimal XP algorithms.

### Hedonic Diversity Games: A Complexity Picture with More than Two Colors

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Rok

2022

Publikováno

Proceedings of the 36th AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2022. p. 5034-5042. ISSN 2159-5399. ISBN 978-1-57735-876-3.

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Hedonic diversity games are a variant of the classical Hedonic games designed to better model a variety of questions concerning diversity and fairness. Previous works mainly targeted the case with two diversity classes (represented as colors in the model) and provided a set of initial complexity-theoretic and existential results concerning Nash and Individually stable outcomes. Here, we design new algorithms accompanied with lower bounds which provide a full parameterized-complexity picture for computing Nash and Individually stable outcomes with respect to the most natural parameterizations of the problem. Crucially, our results hold for general Hedonic diversity games where the number of colors is not necessarily restricted to two, and show that---apart from two trivial cases---a necessary condition for tractability in this setting is that the number of colors is bounded by the parameter. Moreover, for the special case of two colors we resolve an open question asked in previous work~(Boehmer and Elkind, AAAI 2020).

### Integer programming in parameterized complexity: Five miniatures

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Gavenčiak, T.; Koutecký, M.; Knop, D.

Rok

2022

Publikováno

Discrete Optimization. 2022, 44 ISSN 1572-5286.

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Powerful results from the theory of integer programming have recently led to substantial advances in parameterized complexity. However, our perception is that, except for Lenstra's algorithm for solving integer linear programming in fixed dimension, there is still little understanding in the parameterized complexity community of the strengths and limitations of the available tools. This is understandable: it is often difficult to infer exact runtimes or even the distinction between FPT and XP algorithms, and some knowledge is simply unwritten folklore in a different community. We wish to make a step in remedying this situation. To that end, we first provide an easy to navigate quick reference guide of integer programming algorithms from the perspective of parameterized complexity. Then, we show their applications in three case studies, obtaining FPT algorithms with runtime f(k)poly(n). We focus on: • Modeling: since the algorithmic results follow by applying existing algorithms to new models, we shift the focus from the complexity result to the modeling result, highlighting common patterns and tricks which are used. • Optimality program: after giving an FPT algorithm, we are interested in reducing the dependence on the parameter; we show which algorithms and tricks are often useful for speed-ups. • Minding the poly(n): reducing f(k) often has the unintended consequence of increasing poly(n); so we highlight the common trade-offs and show how to get the best of both worlds. Specifically, we consider graphs of bounded neighborhood diversity which are in a sense the simplest of dense graphs, and we show several FPT algorithms for CAPACITATED DOMINATING SET, SUM COLORING, MAX-q-CUT, and certain other coloring problems by modeling them as convex programs in fixed dimension, n-fold integer programs, bounded dual treewidth programs, indefinite quadratic programs in fixed dimension, parametric integer programs in fixed dimension, and 2-stage stochastic integer programs.

### Length-bounded cuts: Proper interval graphs and structural parameters

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Bentert, M.; Heeger, K.; Knop, D.

Rok

2022

Publikováno

Journal of Computer and System Sciences. 2022, 126 21-43. ISSN 0022-0000.

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We study the LENGTH-BOUNDED CUT problem for special graph classes and from a parameterized complexity viewpoint. Here, we are given a graph G, two vertices s and t, and positive integers β and λ. The task is to find a set F of at most β edges such that each s-t-path of length at most λ in G contains some edge in F. Bazgan et al. [20] conjectured that LENGTH-BOUNDED CUT admits a polynomial-time algorithm if the input graph is a proper interval graph. We confirm this conjecture by providing a dynamic-programming-based polynomial-time algorithm. Moreover, we strengthen the W[1]-hardness result of Dvořák and Knop for LENGTH-BOUNDED CUT parameterized by pathwidth by showing W[1]-hardness for the combined parameter pathwidth and maximum degree of the input graph. Finally, we prove that LENGTH-BOUNDED CUT is W[1]-hard for the feedback vertex number. Both our hardness results complement known XP algorithms.

### Multivariate Algorithmics for Eliminating Envy by Donating Goods

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Boehmer, N.; Bredereck, R.; Heeger, K.; Knop, D.; Luo, J.

Rok

2022

Publikováno

21st International Conference on Autonomous Agents and Multiagent Systems, AAMAS 2022. County of Richland: IFAAMAS, 2022. p. 127-135. ISSN 1548-8403. ISBN 978-1-4503-9213-6.

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Fairly dividing a set of indivisible resources to a set of agents is
of utmost importance in some applications. However, after an allocation has been implemented the preferences of agents might
change and envy might arise. We study the following problem
to cope with such situations: Given an allocation of indivisible
resources to agents with additive utility-based preferences, is it
possible to socially donate some of the resources (which means
removing these resources from the allocation instance) such that
the resulting modified allocation is envy-free (up to one good). We
require that the number of deleted resources and/or the caused
utilitarian welfare loss of the allocation are bounded. We conduct a
thorough study of the (parameterized) computational complexity
of this problem considering various natural and problem-specific
parameters (e.g., the number of agents, the number of deleted resources, or the maximum number of resources assigned to an agent
in the initial allocation) and different preference models, including
unary and 0/1-valuations. In our studies, we obtain a rich set of
(parameterized) tractability and intractability results and discover
several surprising contrasts, for instance, between the two closely
related fairness concepts envy-freeness and envy-freeness up to
one good and between the influence of the parameters maximum
number and welfare of the deleted resources.

### On Polynomial Kernels for Traveling Salesperson Problem and Its Generalizations

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Rok

2022

Publikováno

30th Annual European Symposium on Algorithms (ESA 2022). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2022. p. 22:1-22:16. Leibniz International Proceedings in Informatics (LIPIcs). vol. 244. ISSN 1868-8969. ISBN 978-3-95977-247-1.

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For many problems, the important instances from practice possess certain structure that one should reflect in the design of specific algorithms. As data reduction is an important and inextricable part of today’s computation, we employ one of the most successful models of such precomputation - the kernelization. Within this framework, we focus on Traveling Salesperson Problem (TSP) and some of its generalizations.
We provide a kernel for TSP with size polynomial in either the feedback edge set number or the size of a modulator to constant-sized components. For its generalizations, we also consider other structural parameters such as the vertex cover number and the size of a modulator to constant-sized paths. We complement our results from the negative side by showing that the existence of a polynomial-sized kernel with respect to the fractioning number, the combined parameter maximum degree and treewidth, and, in the case of {Subset TSP}, modulator to disjoint cycles (i.e., the treewidth two graphs) is unlikely.

### Parameterized complexity of stable roommates with ties and incomplete lists through the lens of graph parameters

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Bredereck, R.; Heeger, K.; Knop, D.; Niedermeier, R.

Rok

2022

Publikováno

Information and Computation. 2022, 289 ISSN 0890-5401.

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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.

### Scheduling Kernels via Configuration LP

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Knop, D.; Koutecký, M.

Rok

2022

Publikováno

30th Annual European Symposium on Algorithms (ESA 2022). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2022. p. 73:1-73:15. Leibniz International Proceedings in Informatics (LIPIcs). vol. 244. ISSN 1868-8969. ISBN 978-3-95977-247-1.

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Makespan minimization (on parallel identical or unrelated machines) is arguably the most natural and studied scheduling problem. A common approach in practical algorithm design is to reduce the size of a given instance by a fast preprocessing step while being able to recover key information even after this reduction. This notion is formally studied as kernelization (or simply, kernel) - a polynomial time procedure which yields an equivalent instance whose size is bounded in terms of some given parameter. It follows from known results that makespan minimization parameterized by the longest job processing time p_max has a kernelization yielding a reduced instance whose size is exponential in p_max. Can this be reduced to polynomial in p_max?
We answer this affirmatively not only for makespan minimization, but also for the (more complicated) objective of minimizing the weighted sum of completion times, also in the setting of unrelated machines when the number of machine kinds is a parameter.
Our algorithm first solves the Configuration LP and based on its solution constructs a solution of an intermediate problem, called huge N-fold integer programming. This solution is further reduced in size by a series of steps, until its encoding length is polynomial in the parameters. Then, we show that huge N-fold IP is in NP, which implies that there is a polynomial reduction back to our scheduling problem, yielding a kernel.
Our technique is highly novel in the context of kernelization, and our structural theorem about the Configuration LP is of independent interest. Moreover, we show a polynomial kernel for huge N-fold IP conditional on whether the so-called separation subproblem can be solved in polynomial time. Considering that integer programming does not admit polynomial kernels except for quite restricted cases, our "conditional kernel" provides new insight.

### TARGET SET SELECTION IN DENSE GRAPH CLASSES

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Dvorak, P.; Knop, D.; Toufar, T.

Rok

2022

Publikováno

SIAM Journal on Discrete Mathematics. 2022, 36(1), 536-572. ISSN 0895-4801.

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In this paper, we study the TARGET SET SELECTION problem from a parameterized complexity perspective. Here for a given graph and a threshold for each vertex, the task is to find a set of vertices (called a target set) that activates the whole graph during the following iterative process. A vertex outside the active set becomes active if the number of so far activated vertices in its neighborhood is at least its threshold. We give two parameterized algorithms for a special case where each vertex has the threshold set to half of its neighbors (the so-called MAJORITY TARGET SET SELECTION problem) for parameterizations by the neighborhood diversity and the twin cover number of the input graph. We complement these results from the negative side. We give a hardness proof for the MAJORITY TARGET SET SELECTION problem when parameterized by (a restriction of) the modular-width -a natural generalization of both previous structural parameters. We also show the TARGET SET SELECTION problem parameterized by the neighborhood diversity or by the twin cover number is W[1]-hard when there is no restriction on the thresholds.

### Constant Factor Approximation for Tracking Paths and Fault Tolerant Feedback Vertex Set

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Rok

2021

Publikováno

Approximation and Online Algorithms - 19th International Workshop, WAOA 2021, Lisbon, Portugal, September 6-10, 2021, Revised Selected Papers. Springer, Cham, 2021. p. 23-38. Lecture Notes in Computer Science (LNCS). vol. 12982. ISSN 0302-9743. ISBN 978-3-030-92701-1.

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Consider a vertex-weighted graph G with a source s and a target t. Tracking Paths requires finding a minimum weight set of vertices (trackers) such that the sequence of trackers in each path from s to t is unique. In this work, we derive a factor 66-approximation algorithm for Tracking Paths in weighted graphs and a factor 4-approximation algorithm if the input is unweighted. This is the first constant factor approximation for this problem. While doing so, we also study approximation of the closely related r -Fault Tolerant Feedback Vertex Set problem. There, for a fixed integer r and a given vertex-weighted graph G, the task is to find a minimum weight set of vertices intersecting every cycle of G in at least r+1 vertices. We give a factor O(r^2) approximation algorithm for r -Fault Tolerant Feedback Vertex Set if r is a constant.

### Graph isomorphism restricted by lists

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Klavík, P.; Knop, D.; Zeman, P.

Rok

2021

Publikováno

Theoretical Computer Science. 2021, 860 51-71. ISSN 0304-3975.

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The complexity of graph isomorphism (GRAPHISO) is a famous problem in computer science. For graphs G and H, it asks whether they are the same up to a relabeling of vertices. In 1981, Lubiw proved that list restricted graph isomorphism (LISTISO ) is NP -complete: for each u∈V(G), we are given a list L(u)⊆V(H) of possible images of u. After 35 years, we revive the study of this problem and consider which results for GRAPHISO can be modified to solve LISTISO. We prove: 1) Under certain conditions, GI -completeness of a class of graphs implies NP -completeness of LISTISO. 2) Several combinatorial algorithms for GRAPHISO can be modified to solve LISTISO: for trees, planar graphs, interval graphs, circle graphs, permutation graphs, and bounded treewidth graphs. 3) LISTISO is NP -complete for cubic colored graphs with sizes of color classes bounded by 8 with all lists of size at most 3.

### High-Multiplicity Fair Allocation Made More Practical

Autoři

Bredereck, R.; Figiel, A.; Kaczmarczyk, A.; Knop, D.; Niedermeier, R.

Rok

2021

Publikováno

AAMAS '21: 20th International Conference on Autonomous Agents and Multiagent Systems, Virtual Event, United Kingdom, May 3-7, 2021. New York: ACM, 2021. p. 260-268. ISSN 1548-8403. ISBN 978-1-7138-3262-1.

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The envy-free, Pareto-efficient allocation of indivisible goods leads to computationally hard problems. There is a big variety of modeling issues, such as agent-specific utility functions or (high numbers of) different types of goods. In recent work, Bredereck et al. [ACM EC~2019] addressed this topic by showing (theoretical) fixed-parameter tractability results for "high-multiplicity fair allocation", exploiting parameters such as the number of agents or maximum absolute utility values. To this end, they used a number of tools from (theoretical) integer linear programming. We "engineer" their work towards practical usefulness, thereby being able to solve all real-world instances from the state-of-art online platform "spliddit.org for provably fair solutions". Besides providing the foundations for a fast tool for fair allocations, we also offer a flexible framework with the possibility to relax fairness or efficiency demands so to, e.g., allow tradeoffs between fairness and social welfare. Moreover, our framework provides ways to interpret and explain "solution paths" which makes it possible to perform further explorations in cases when no envy-free and efficient allocations exist.

### Kernelization of graph hamiltonicity: Proper H-graphs

Autoři

Chaplick, S.; Fomin, F.V.; Golovach, P.A.; Knop, D.; Zeman, P.

Rok

2021

Publikováno

SIAM Journal on Discrete Mathematics. 2021, 35(2), 840-892. ISSN 0895-4801.

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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 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 >= 1, in time polynomial in n and ||H||, outputs a graph G' of size O(||H||^8) and k' <= |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 O(||H||^8). Cycle Cover admits a polynomial kernel. We prove it by providing a compression of size O(||H||^10) into another 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 O(||H||^10). In all our algorithms we assume that a proper H-decomposition is given as a part of the input.

### Local linear set on graphs with bounded twin cover number

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Inspired by recent results for the so-called fair and local linear problems, we define two problems, LOCAL LINEAR SET and LOCAL LINEAR VERTEX COVER. Here the task is to find a set of vertices X subset of V (which is a vertex cover) of the input graph (V, E) such that the local linear constraints l(v) <= vertical bar X boolean AND N(v)vertical bar <= u(v) hold for all v is an element of V; here, l(v) and u( v) are on the input. We prove that both problems are in FPT for the parameter twin cover number (using integer linear programming).

### Parameterized Approximation Schemes for Steiner Trees with Small Number of Steiner Vertices

Autoři

Dvorak, P.; Feldmann, Andreas E.; Knop, D.; Masarik, T.; Toufar, T.; Veselý, P.

Rok

2021

Publikováno

SIAM Journal on Discrete Mathematics. 2021, 35(1), 546-574. ISSN 0895-4801.

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We study the Steiner Tree problem, in which a set of terminal vertices needs to be connected in the cheapest possible way in an edge-weighted graph. This problem has been extensively studied from the viewpoint of approximation and also parameterization. In particular, on one hand Steiner Tree is known to be APX-hard, and W[2]-hard on the other, if parameterized by the number of nonterminals (Steiner vertices) in the optimum solution. In contrast to this, we give an efficient parameterized approximation scheme (EPAS), which circumvents both hardness results. Moreover, our methods imply the existence of a polynomial size approximate kernelization scheme (PSAKS) for the considered parameter. We further study the parameterized approximability of other variants of Steiner Tree, such as Directed Steiner Tree and Steiner Forest. For none of these is an EPAS likely to exist for the studied parameter. For Steiner Forest an easy observation shows that the problem is APX-hard, even if the input graph contains no Steiner vertices. For Directed Steiner Tree we prove that approximating within any function of the studied parameter is W[1]-hard. Nevertheless, we show that an EPAS exists for Unweighted Directed Steiner Tree, but a PSAKS does not. We also prove that there is an EPAS and a PSAKS for Steiner Forest if in addition to the number of Steiner vertices, the number of connected components of an optimal solution is considered to be a parameter.

### Parameterized complexity of configuration integer programs

Autoři

Knop, D.; Koutecky, M.; Levin, A.; Mnich, M.; Onn, S.

Rok

2021

Publikováno

Operations Research Letters. 2021, 49(6), 908-913. ISSN 0167-6377.

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Configuration integer programs (IP) have been key in the design of algorithms for NP-hard high-multiplicity problems. First, we develop fast exact (exponential-time) algorithms for Configuration IP and matching hardness results. Second, we showcase the implications of these results to bin-packing and facility-location-like problems.

### The complexity landscape of decompositional parameters for ILP: Programs with few global variables and constraints

Autoři

Dvorak, P.; Eiben, E.; Ganian, R.; Knop, D.; Ordyniak, S.

Rok

2021

Publikováno

Artificial Intelligence. 2021, 300 ISSN 0004-3702.

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Integer Linear Programming (ILP) has a broad range of applications in various areas of artificial intelligence. Yet in spite of recent advances, we still lack a thorough understanding of which structural restrictions make ILP tractable. Here we study ILP instances consisting of a small number of "global" variables and/or constraints such that the remaining part of the instance consists of small and otherwise independent components; this is captured in terms of a structural measure we call fracture backdoors which generalizes, for instance, the well-studied class of N-fold ILP instances.

### Adapting Stable Matchings to Evolving Preferences

Autoři

Bredereck, R.; Chen, J.; Knop, D.; Luo, J.; Niedermeier, R.

Rok

2020

Publikováno

Proceedings of the AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2020. p. 1830-1837. ISSN 2159-5399. ISBN 978-1-57735-835-0.

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Adaptivity to changing environments and constraints is key to success in modern society. We address this by proposing “incrementalized versions” of Stable Marriage and Stable Roommates. That is, we try to answer the following question: for both problems, what is the computational cost of adapting an existing stable matching after some of the preferences of the agents have changed. While doing so, we also model the constraint that the new stable matching shall be not too different from the old one. After formalizing these incremental versions, we provide a fairly comprehensive picture of the computational complexity landscape of Incremental Stable Marriage and Incremental Stable Roommates. To this end, we exploit the parameters “degree of change” both in the input (difference between old and new preference profile) and in the output (difference between old and new stable matching). We obtain both hardness and tractability results, in particular showing a fixed-parameter tractability result with respect to the parameter “distance between old and new stable matching”.

### Approximation Algorithms for Steiner Tree Based on Star Contractions: A Unified View

Autoři

Hušek, R.; Knop, D.; Masařík, T.

Rok

2020

Publikováno

15th International Symposium on Parameterized and Exact Computation (IPEC 2020). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2020. p. 16:1-16:18. ISSN 1868-8969. ISBN 978-3-95977-172-6.

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In the Steiner Tree problem, we are given an edge-weighted undirected graph G = (V,E) and a set of terminals R ⊆ V. The task is to find a connected subgraph of G containing R and minimizing the sum of weights of its edges. Steiner Tree is well known to be NP-complete and is undoubtedly one of the most studied problems in (applied) computer science. We observe that many approximation algorithms for Steiner Tree follow a similar scheme (meta-algorithm) and perform (exhaustively) a similar routine which we call star contraction. Here, by a star contraction, we mean finding a star-like subgraph in (the metric closure of) the input graph minimizing the ratio of its weight to the number of contained terminals minus one; and contract. It is not hard to see that the well-known MST-approximation seeks the best star to contract among those containing two terminals only. Zelikovsky’s approximation algorithm follows a similar workflow, finding the best star among those containing three terminals. We perform an empirical study of star contractions with the relaxed condition on the number of terminals in each star contraction motivated by a recent result of Dvořák et al. [Parameterized Approximation Schemes for Steiner Trees with Small Number of Steiner Vertices, STACS 2018]. Furthermore, we propose two improvements of Zelikovsky’s 11/6-approximation algorithm and we empirically confirm that the quality of the solution returned by any of these is better than the one returned by the former algorithm. However, such an improvement is exchanged for a slower running time (up to a multiplicative factor of the number of terminals).

### Combinatorial n-fold integer programming and applications

Autoři

Knop, D.; Koutecký, M.; Mnich, M.

Rok

2020

Publikováno

Mathematical Programming. 2020, 2020(184), 1-34. ISSN 0025-5610.

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Many fundamental NP-hard problems can be formulated as integer linear programs (ILPs). A famous algorithm by Lenstra solves ILPs in time that is exponential only in the dimension of the program, and polynomial in the size of the ILP. That algorithm became a ubiquitous tool in the design of fixed-parameter algorithms for NP-hard problems, where one wishes to isolate the hardness of a problem by some parameter. However, in many cases using Lenstra’s algorithm has two drawbacks: First, the run time of the resulting algorithms is often double-exponential in the parameter, and second, an ILP formulation in small dimension cannot easily express problems involving many different costs. Inspired by the work of Hemmecke et al. (Math Program 137(1–2, Ser. A):325–341, 2013), we develop a single-exponential algorithm for so-called combinatorial n-fold integer programs, which are remarkably similar to prior ILP formulations for various problems, but unlike them, also allow variable dimension. We then apply our algorithm to many relevant problems like Closest String, Swap Bribery, Weighted Set Multicover, and several others, and obtain exponential speedups in the dependence on the respective parameters, the input size, or both. Unlike Lenstra’s algorithm, which is essentially a bounded search tree algorithm, our result uses the technique of augmenting steps. At its heart is a deep result stating that in combinatorial n-fold IPs, existence of an augmenting step implies existence of a “local” augmenting step, which can be found using dynamic programming. Our results provide an important insight into many problems by showing that they exhibit this phenomenon, and highlights the importance of augmentation techniques.

### Fine-Grained View on Bribery for Group Identification

Autoři

Boehmer, N.; Bredereck, R.; Knop, D.; Luo, J.

Rok

2020

Publikováno

Proceedings of the Twenty-Ninth International Joint Conference on Artificial Intelligence. International Joint Conferences on Artificial Intelligence Organization, 2020. p. 67-73. ISBN 978-0-9992411-6-5.

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Given a set of individuals qualifying or disqualifying each other, group identification is the task of identifying a socially qualified subgroup of individuals. Social qualification depends on the specific rule used to aggregate individual qualifications. The bribery problem in this context asks how many agents need to change their qualifications in order to change the outcome. Complementing previous results showing polynomial-time solvability or NP-hardness of bribery for various social rules in the constructive (aiming at making specific individuals socially qualified) or destructive (aiming at making specific individuals socially disqualified) setting, we provide a comprehensive picture of the parameterized computational complexity landscape. Conceptually, we also consider a more fine-grained concept of bribery cost, where we ask how many single qualifications need to be changed, and a more general bribery goal that combines the constructive and destructive setting.

### Graph Isomorphism Restricted by Lists

Autoři

Klavík, P.; Knop, D.; Zeman, P.

Rok

2020

Publikováno

Graph-Theoretic Concepts in Computer Science - 46th International Workshop, WG 2020, Leeds, UK, June 24-26, 2020, Revised Selected Papers. Berlin: Springer-Verlag, 2020. p. 106-118. Lecture Notes in Computer Science. vol. 12301. ISSN 0302-9743. ISBN 978-3-030-60439-4.

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The complexity of graph isomorphism (GraphIso) is a famous problem in computer science. For graphs G and H, it asks whether they are the same up to a relabeling of vertices. In 1981, Lubiw proved that list restricted graph isomorphism (ListIso) is NP-complete: for each u∈ V(G), we are given a list L(u) ⊆ V(H) of possible images of u. After 35 years, we revive the study of this problem and consider which results for GraphIso can be modified to solve ListIso. We prove: 1) Under certain conditions, GI-completeness of a class of graphs implies NP-completeness of ListIso. 2) Several combinatorial algorithms for GraphIso can be modified to solve ListIso: for trees, planar graphs, interval graphs, circle graphs, permutation graphs, and bounded treewidth graphs. 3) ListIso is NP-complete for cubic colored graphs with sizes of color classes bounded by 8.

### Length-Bounded Cuts: Proper Interval Graphs and Structural Parameters

Autoři

Bentert, M.; Heeger, K.; Knop, D.

Rok

2020

Publikováno

31st International Symposium on Algorithms and Computation (ISAAC 2020). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2020. p. 36:1-36:14. ISSN 1868-8969. ISBN 978-3-95977-173-3.

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In the presented paper, we study the Length-Bounded Cut problem for special graph classes as well as from a parameterized-complexity viewpoint. Here, we are given a graph G, two vertices s and t, and positive integers β and λ. The task is to find a set F of edges of size at most β such that every s-t-path of length at most λ in G contains some edge in F. Bazgan et al. [Networks, 2019] conjectured that Length-Bounded Cut admits a polynomial-time algorithm if the input graph G is a proper interval graph. We confirm this conjecture by providing a dynamic-programming based polynomial-time algorithm. Moreover, we strengthen the W[1]-hardness result of Dvořák and Knop [Algorithmica, 2018] for Length-Bounded Cut parameterized by pathwidth. Our reduction is shorter, and the target of the reduction has stronger structural properties. Consequently, we give W[1]-hardness for the combined parameter pathwidth and maximum degree of the input graph. Finally, we prove that Length-Bounded Cut is W[1]-hard for the feedback vertex number. Both our hardness results complement known XP algorithms.

### Multidimensional Stable Roommates with Master List

Autoři

Bredereck, R.; Heeger, K.; Knop, D.; Niedermeier, R.

Rok

2020

Publikováno

Web and Internet Economics - 16th International Conference, WINE 2020, Beijing, China, December 7-11, 2020, Proceedings. Cham: Springer, 2020. p. 59-73. ISBN 978-3-030-64945-6.

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Since the early days of research in algorithms and complexity, the computation of stable matchings is a core topic. While in the classic setting the goal is to match up two agents (either from different “gender” (this is Stable Marriage) or “unrestricted” (this is Stable Roommates)), Knuth [1976] triggered the study of three- or multidimensional cases. Here, we focus on the study of Multidimensional Stable Roommates, known to be 𝖭𝖯 -hard since the early 1990’s. Many 𝖭𝖯 -hardness results, however, rely on very general input instances that do not occur in at least some of the specific application scenarios. With the quest for identifying islands of tractability, we look at the case of master lists. Here, as natural in applications where agents express their preferences based on “objective” scores, one roughly speaking assumes that all agent preferences are “derived from” a central master list, implying that the individual agent preferences shall be similar. Master lists have been frequently studied in the two-dimensional (classic) stable matching case, but seemingly almost never for the multidimensional case. This work, also relying on methods from parameterized algorithm design and complexity analysis, performs a first systematic study of Multidimensional Stable Roommates under the assumption of master lists.

### Parameterized Algorithms for Finding a Collective Set of Items

Autoři

Bredereck, R.; Faliszewski, P.; Kaczmarczyk, A.; Knop, D.; Niedermeier, R.

Rok

2020

Publikováno

Proceedings of the AAAI Conference on Artificial Intelligence. Menlo Park: AAAI Press, 2020. p. 1838-1845. ISSN 2159-5399. ISBN 978-1-57735-835-0.

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We extend the work of Skowron et al. (AIJ, 2016) by considering the parameterized complexity of the following problem. We are given a set of items and a set of agents, where each agent assigns an integer utility value to each item. The goal is to find a set of k items that these agents would collectively use. For each such collective set of items, each agent provides a score that can be described using an OWA (ordered weighted average) operator and we seek a set with the highest total score. We focus on the parameterization by the number of agents and we find numerous fixed-parameter tractability results (however, we also find some W[1]-hardness results). It turns out that most of our algorithms even apply to the setting where each agent has an integer weight.

### Partitioning graphs into induced subgraphs

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We study the Partition into H problem from the parameterized complexity point of view. In the Partition into H problem the task is to partition the vertices of a graph G into sets V_1,...,V_r such that the graph H is isomorphic to the subgraph of G induced by each set V_i for i =1,...,r. The pattern graph H is fixed.
For the parameterization we consider three distinct structural parameters of the graph G - namely the tree-width, the neighborhood diversity, and the modular-width. For the parameterization by the neighborhood diversity we obtain an FPT algorithm for every graph H. For the parameterization by the tree-width we obtain an FPT algorithm for every connected graph H, thus resolving an open question of Gajarský et al. (2013). Finally, for the parameterization by the modular-width we derive an FPT algorithm for every prime graph H.

### Recognizing Proper Tree-Graphs

Autoři

Chaplick, S.; Golovach, P.A.; Hartmann, T.A.; Knop, D.

Rok

2020

Publikováno

15th International Symposium on Parameterized and Exact Computation (IPEC 2020). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2020. p. 8:1-8:15. ISSN 1868-8969. ISBN 978-3-95977-172-6.

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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.

### The Clever Shopper Problem

Autoři

Bulteau, L.; Hermelin, D.; Knop, D.; Labarre, A.; Vialette, S.

Rok

2020

Publikováno

Theory of Computing Systems. 2020, 64(1), 17-34. ISSN 1432-4350.

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We investigate a variant of the so-called Internet Shopping problem introduced by Blazewicz et al. (Appl. Math. Comput. Sci. 20, 385–390, 2010), where a customer wants to buy a list of products at the lowest possible total cost from shops which offer discounts when purchases exceed a certain threshold. Although the problem is NP-hard, we provide exact algorithms for several cases, e.g. when each shop sells only two items, and an FPT algorithm for the number of items, or for the number of shops when all prices are equal. We complement each result with hardness proofs in order to draw a tight boundary between tractable and intractable cases. Finally, we give an approximation algorithm and hardness results for the problem of maximising the sum of discounts.

### Tight Complexity Lower Bounds for Integer Linear Programming with Few Constraints

Autoři

Knop, D.; Pilipczuk, M.; Wrochna, M.

Rok

2020

Publikováno

ACM Transactions on Computation Theory. 2020, 12(3), 19:1-19:19. ISSN 1942-3454.

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We consider the standard ILP Feasibility problem: given an integer linear program of the form {Ax = b, x ≥ 0}, where A is an integer matrix with k rows and ℓ columns, x is a vector of ℓ variables, and b is a vector of k integers, we ask whether there exists x ∈ N ℓ that satisfies Ax = b. Each row of A specifies one linear constraint on x; our goal is to study the complexity of ILP Feasibility when both k, the number of constraints, and A∞, the largest absolute value of an entry in A, are small. Papadimitriou was the first to give a fixed-parameter algorithm for ILP Feasibility under parameterization by the number of constraints that runs in time ((A∞ + b∞) ⋅ k)O(k2). This was very recently improved by Eisenbrand and Weismantel, who used the Steinitz lemma to design an algorithm with running time (kA∞)O(k) ⋅ log b∞, which was subsequently refined by Jansen and Rohwedder to O( kA∞)k ⋅ log ( A∞ + b∞) ⋅ log A∞. We prove that for {0, 1}-matrices A, the running time of the algorithm of Eisenbrand and Weismantel is probably optimal: an algorithm with running time 2o(k log k) ⋅ (ℓ + b∞)o(k) would contradict the exponential time hypothesis. This improves previous non-tight lower bounds of Fomin et al. We then consider integer linear programs that may have many constraints, but they need to be structured in a "shallow" way. Precisely, we consider the parameter dual treedepth of the matrix A, denoted tdD(A), which is the treedepth of the graph over the rows of A, where two rows are adjacent if in some column they simultaneously contain a non-zero entry. It was recently shown by Koutecký et al. that ILP Feasibility can be solved in time A∞2O(tdD(A)) ⋅ (k + ℓ + log b∞)O(1). We present a streamlined proof of this fact and prove that, again, this running time is probably optimal: even assuming that all entries of A and b are in {-1, 0, 1}, the existence of an algorithm with running time 22o(tdD(A)) ⋅ (k + ℓ)O(1) would contradict the exponential time hypothesis.

### Voting and Bribing in Single-Exponential Time

Autoři

Knop, D.; Koutecký, M.; Mnich, M.

Rok

2020

Publikováno

ACM TRANSACTIONS ON ECONOMICS AND COMPUTATION. 2020, 8(3), 12:1-12:28. ISSN 2167-8375.

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We introduce a general problem about bribery in voting systems. In the R-Multi-Bribery problem, the goal is to bribe a set of voters at minimum cost such that a desired candidate is a winner in the perturbed election under the voting rule R. Voters assign prices for withdrawing their vote, for swapping the positions of two consecutive candidates in their preference order, and for perturbing their approval count to favour candidates. As our main result, we show that R-Multi-Bribery is fixed-parameter tractable parameterized by the number of candidates |C| with only a single-exponential dependence on |C|, for many natural voting rules R, including all natural scoring protocols, maximin rule, Bucklin rule, Fallback rule, SP-AV, and any C1 rule. The vast majority of previous work done in the setting of few candidates proceeds by grouping voters into at most |C|! types by their preference, constructing an integer linear program with |C|!2 variables, and solving it by Lenstra's algorithm in time |C|!|C|!2, hence double-exponential in |C|. Note that it is not possible to encode a large number of different voter costs in this way and still obtain a fixed-parameter algorithm, as that would increase the number of voter types and hence the dimension. These two obstacles of double-exponential complexity and restricted costs have been formulated as "Challenges #1 and #2"of the "Nine Research Challenges in Computational Social Choice"by Bredereck et al. Hence, our result resolves the parameterized complexity of R-Swap-Bribery for the aforementioned voting rules plus Kemeny's rule, and for all rules except Kemeny brings the dependence on |C| down to single-exponential. The engine behind our progress is the use of a new integer linear programming formulation, using so-called "n-fold integer programming."Since its format is quite rigid, we introduce "extended n-fold IP,"which allows many useful modeling tricks. Then, we model R-Multi-Bribery as an extended n-fold IP and ...

### Complexity of the Steiner Network Problem with Respect to the Number of Terminals

Rok

2019

Publikováno

36th International Symposium on Theoretical Aspects of Computer Science, STACS 2019, March 13-16, 2019, Berlin, Germany. Wadern: Schloss Dagstuhl - Leibniz Center for Informatics, 2019. p. 25:1-25:17. LIPIcs. vol. 126. ISSN 1868-8969. ISBN 978-3-95977-100-9.

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In the Directed Steiner Network problem we are given an arc-weighted digraph G, a set of terminals $T\subseteq V(G)$ with |T|=q, and an (unweighted) directed request graph R with V(R)=T.
Our task is to output a subgraph $H \subseteq G$ of the minimum cost such that there is a directed path from s to t in H for all st \in A(R).
It is known that the problem can be solved in time $|V(G)|^{O(|A(R)|)}$ [Feldman\&Ruhl, SIAM J. Comput. 2006] and cannot be solved in time $|V(G)|^{o(|A(R)|)}$ even if G is planar, unless the Exponential-Time Hypothesis (ETH) fails [Chitnis et al., SODA 2014]. However, the reduction (and other reductions showing hardness of the problem) only shows that the problem cannot be solved in time $|V(G)|^{o(q)}$, unless ETH fails. Therefore, there is a significant gap in the complexity with respect to q in the exponent.
We show that \textsc{Directed Steiner Network} is solvable in time $f(q)\cdot |V(G)|^{O(c_g \cdot q)}$, where $c_g$ is a constant depending solely on the genus of G and f is a computable function.
We complement this result by showing that there is no $f(q)\cdot |V(G)|^{o(q^2/ \log q)}$ algorithm for any function f for the problem on general graphs, unless ETH fails.

### Evaluating and Tuning n-fold Integer Programming

Autoři

Altmanová, K.; Knop, D.; Koutecký, M.

Rok

2019

Publikováno

Journal of Experimental Algorithmics. 2019, 24(1), 2.2:1-2.2:22. ISSN 1084-6654.

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DOI

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In recent years, algorithmic breakthroughs in stringology, computational social choice, scheduling, and so on, were achieved by applying the theory of so-called n-fold integer programming. An n-fold integer program (IP) has a highly uniform block structured constraint matrix. Hemmecke, Onn, and Romanchuk [Math. Program., 2013] showed an algorithm with runtime ΔO(rst + r2s) n3, where Δ is the largest coefficient, r,s, and t are dimensions of blocks of the constraint matrix and n is the total dimension of the IP; thus, an algorithm efficient if the blocks are of small size and with small coefficients. The algorithm works by iteratively improving a feasible solution with augmenting steps, and n-fold IPs have the special property that augmenting steps are guaranteed to exist in a not-too-large neighborhood. However, this algorithm has never been implemented and evaluated.
We have implemented the algorithm and learned the following along the way. The original algorithm is practically unusable, but we discover a series of improvements that make its evaluation possible. Crucially, we observe that a certain constant in the algorithm can be treated as a tuning parameter, which yields an efficient heuristic (essentially searching in a smaller-than-guaranteed neighborhood). Furthermore, the algorithm uses an overly expensive strategy to find a “best” step, while finding only an “approximately best” step is much cheaper, yet sufficient for quick convergence. Using this insight, we improve the asymptotic dependence on n from n3 to n2 log n.

### High-Multiplicity Fair Allocation: Lenstra Empowered by N-fold Integer Programming

Autoři

Bredereck, R.; Kaczmarczyk, A.; Knop, D.; Niedermeier, R.

Rok

2019

Publikováno

EC '19 Proceedings of the 2019 ACM Conference on Economics and Computation. New York: ACM, 2019. p. 505-523. ISBN 9781450367929.

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We study the (parameterized) computational complexity of problems in the context of fair allocations of indivisible goods. More specifically, we show fixed-parameter tractability results for a broad set of problems concerned with envy-free, Pareto-efficient allocations of items (with agent-specific utility functions) to agents. In principle, this implies efficient exact algorithms for these in general computationally intractable problems whenever we face instances with few agents and low maximum (absolute) utility values. This holds true also in high-multiplicity settings where we may have high numbers of identical items. On the technical side, our approach provides algorithmic meta-theorems covering a rich set of fair allocation problems in the additive preferences model. To achieve this, our main technical contribution is to make an elaborate use of tools from integer linear programming. More specifically, we exploit results originally going back to a famous theorem of Lenstra [Math. Oper. Res. 1983] concerning (the fixed-parameter tractability of) Integer Linear Programs (ILPs) with bounded dimension (that is, the dimension shall be considered as a (small) parameter) and the more recent framework of (combinatorial) N-fold ILPs. We reveal and exploit a fruitful interaction between these two cornerstones in the theory of integer linear programming, which may be of independent interest in applications going beyond fair allocations.

### Integer Programming and Incidence Treedepth

Autoři

Eiben, E.; Ganian, R.; Knop, D.; Ordyniak, S.; Pilipczuk, Mi.; Wrochna, M.

Rok

2019

Publikováno

Integer Programming and Combinatorial Optimization. Basel: Springer, 2019. p. 194-204. ISSN 0302-9743. ISBN 978-3-030-17952-6.

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Recently a strong connection has been shown between the tractability of integer programming (IP) with bounded coefficients on the one side and the structure of its constraint matrix on the other side. To that end, integer linear programming is fixed-parameter tractable with respect to the primal (or dual) treedepth of the Gaifman graph of its constraint matrix and the largest coefficient (in absolute value). Motivated by this, Koutecký, Levin, and Onn [ICALP 2018] asked whether it is possible to extend these result to a more broader class of integer linear programs. More formally, is integer linear programming fixed-parameter tractable with respect to the incidence treedepth of its constraint matrix and the largest coefficient (in absolute value)?
We answer this question in negative. We prove that deciding the feasibility of a system in the standard form, 𝐴𝐱=𝐛,𝐥≤𝐱≤𝐮, is NP-hard even when the absolute value of any coefficient in A is 1 and the incidence treedepth of A is 5. Consequently, it is not possible to decide feasibility in polynomial time even if both the assumed parameters are constant, unless 𝖯=𝖭𝖯.

### Integer Programming in Parameterized Complexity: Three Miniatures

Autoři

Gavenčiak, T.; Knop, D.; Koutacký, M.

Rok

2019

Publikováno

13th International Symposium on Parameterized and Exact Computation (IPEC 2018). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2019. p. 21:1-21:16. vol. 115. ISSN 1868-8969. ISBN 978-3-95977-084-2.

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Powerful results from the theory of integer programming have recently led to substantial advances in parameterized complexity. However, our perception is that, except for Lenstra's algorithm for solving integer linear programming in fixed dimension, there is still little understanding in the parameterized complexity community of the strengths and limitations of the available tools. This is understandable: it is often difficult to infer exact runtimes or even the distinction between FPT and XP algorithms, and some knowledge is simply unwritten folklore in a different community. We wish to make a step in remedying this situation. To that end, we first provide an easy to navigate quick reference guide of integer programming algorithms from the perspective of parameterized complexity. Then, we show their applications in three case studies, obtaining FPT algorithms with runtime f(k) poly(n). We focus on: - Modeling: since the algorithmic results follow by applying existing algorithms to new models, we shift the focus from the complexity result to the modeling result, highlighting common patterns and tricks which are used. - Optimality program: after giving an FPT algorithm, we are interested in reducing the dependence on the parameter; we show which algorithms and tricks are often useful for speed-ups. - Minding the poly(n): reducing f(k) often has the unintended consequence of increasing poly(n); so we highlight the common trade-offs and show how to get the best of both worlds. Specifically, we consider graphs of bounded neighborhood diversity which are in a sense the simplest of dense graphs, and we show several FPT algorithms for Capacitated Dominating Set, Sum Coloring, and Max-q-Cut by modeling them as convex programs in fixed dimension, n-fold integer programs, bounded dual treewidth programs, and indefinite quadratic programs in fixed dimension.

### Kernelization of graph hamiltonicity: Proper H-graphs

Autoři

Chaplick, S.; Fomin, F.V.; Golovach, P.A.; Knop, D.; Zeman, P.

Rok

2019

Publikováno

16th International Symposium on Algorithms and Data Structures (WADS 2019). Cham: Springer, 2019. p. 296-310. vol. 11646. ISSN 0302-9743. ISBN 978-3-030-24765-2.

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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 O(formula presented), that is, we design an algorithm that for an n-vertex graph G and an integer k≥ 1, in time polynomial in n and (formula presented), outputs a graph G′ of size (formula presented) and k′≤ | 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.Cycle Cover admits a compression of size (formula presented) into another problem, called Prize Collecting Cycle Cover, that is, we design an algorithm that, in time polynomial in n and (formula presented), outputs an equivalent instance of Prize Collecting Cycle Cover of size (formula presented). In all our algorithms we assume that a proper H-decomposition is given as a part of the input.

### Parameterized complexity of fair vertex evaluation problems

Autoři

Knop, D.; Masařík, T.; Toufar, T.

Rok

2019

Publikováno

44th International Symposium on Mathematical Foundations of Computer Science (MFCS 2019). Wadern: Schloss Dagstuhl - Leibniz Center for Informatics, 2019. p. 33:1-33:16. Leibniz International Proceedings in Informatics (LIPIcs). vol. 138. ISSN 1868-8969. ISBN 978-3-95977-117-7.

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A prototypical graph problem is centered around a graph-theoretic property for a set of vertices and a solution to it is a set of vertices for which the desired property holds. The task is to decide whether, in the given graph, there exists a solution of a certain quality, where we use size as a quality measure. In this work, we are changing the measure to the fair measure (cf. Lin and Sahni [27]). The fair measure of a set of vertices S is (at most) k if the number of neighbors in the set S of any vertex (in the input graph) does not exceed k. One possible way to study graph problems is by defining the property in a certain logic. For a given objective, an evaluation problem is to find a set (of vertices) that simultaneously minimizes the assumed measure and satisfies an appropriate formula. More formally, we study the MSO Fair Vertex Evaluation, where the graph-theoretic property is described by an MSO formula. In the presented paper we show that there is an FPT algorithm for the MSO Fair Vertex Evaluation problem for formulas with one free variable parameterized by the twin cover number of the input graph and the size of the formula. One may define an extended variant of MSO Fair Vertex Evaluation for formulas with ℓ free variables; here we measure a maximum number of neighbors in each of the ℓ sets. However, such variant is W[1]-hard for parameter ℓ even on graphs with twin cover one. Furthermore, we study the Fair Vertex Cover (Fair VC) problem. Fair VC is among the simplest problems with respect to the demanded property (i.e., the rest forms an edgeless graph). On the negative side, Fair VC is W[1]-hard when parameterized by both treedepth and feedback vertex set of the input graph. On the positive side, we provide an FPT algorithm for the parameter modular width.

Autoři

Bredereck, R.; Heeger, K.; Knop, D.; Niedermeier, R.

Rok

2019

Publikováno

30th International Symposium on Algorithms and Computation (ISAAC 2019). Dagstuhl: Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik, 2019. p. 44:1-44:14. vol. 149. ISSN 1868-8969. ISBN 978-3-95977-130-6.

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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 feedback vertex number (these are each time the respective parameters). However, if we `only' ask for perfect stable matchings or the mere existence of a stable matching, then we obtain fixed-parameter tractability 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.

### SIMPLIFIED ALGORITHMIC METATHEOREMS BEYOND MSO: TREEWIDTH AND NEIGHBORHOOD DIVERSITY

Autoři

Knop, D.; Koutecky, M.; Masarik, T.; Toufar, T.

Rok

2019

Publikováno

Logical Methods in Computer Science. 2019, 15(4), 12:1-12:32. ISSN 1860-5974.

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This paper settles the computational complexity of model checking of several extensions of the monadic second order (MSO) logic on two classes of graphs: graphs of bounded treewidth and graphs of bounded neighborhood diversity. A classical theorem of Courcelle states that any graph property definable in MSO is decidable in linear time on graphs of bounded treewidth. Algorithmic metatheorems like Courcelle's serve to generalize known positive results on various graph classes. We explore and extend three previously studied MSO extensions: global and local cardinality constraints (CardMSO and MSO-LCC) and optimizing the fair objective function (fairMSO). First, we show how these extensions of MSO relate to each other in their expressive power. Furthermore, we highlight a certain "linearity" of some of the newly introduced extensions which turns out to play an important role. Second, we provide parameterized algorithm for the aforementioned structural parameters. On the side of neighborhood diversity, we show that combining the linear variants of local and global cardinality constraints is possible while keeping the linear (FPT) runtime but removing linearity of either makes this impossible. Moreover, we provide a polynomial time (XP) algorithm for the most powerful of studied extensions, i.e. the combination of global and local constraints. Furthermore, we show a polynomial time (XP) algorithm on graphs of bounded treewidth for the same extension. In addition, we propose a general procedure of deriving XP algorithms on graphs on bounded treewidth via formulation as Constraint Satisfaction Problems (CSP). This shows an alternate approach as compared to standard dynamic programming formulations.

### Solving Integer Quadratic Programming via Explicit and Structural Restrictions

Autoři

Eiben, E.; Ganian, R.; Knop, D.; Ordyniak, S.

Rok

2019

Publikováno

Proceedings of the Thirty-Third AAAI Conference on Artificial Intelligence. Menlo Park, California: AAAI Press, 2019. p. 1477-1484. ISSN 2159-5399. ISBN 978-1-57735-809-1.

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We study the parameterized complexity of Integer Quadratic Programming under two kinds of restrictions: explicit restrictions on the domain or coefficients, and structural restrictions on variable interactions. We argue that both kinds of restrictions are necessary to achieve tractability for Integer Quadratic Programming, and obtain four new algorithms for the problem that are tuned to possible explicit restrictions of instances that we may wish to solve. The presented algorithms are exact, deterministic, and complemented by appropriate lower bounds.

### Tight Complexity Lower Bounds for Integer Linear Programming with Few Constraints

Autoři

Knop, D.; Pilipczuk, M.; Wrochna, M.

Rok

2019

Publikováno

36th International Symposium on Theoretical Aspects of Computer Science, STACS 2019, March 13-16, 2019, Berlin, Germany. Wadern: Schloss Dagstuhl - Leibniz Center for Informatics, 2019. p. 44:1-44:15. LIPIcs. vol. 126. ISSN 1868-8969. ISBN 978-3-95977-100-9.

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We consider the standard ILP FEASIBILITY problem: given an integer linear program of the form {Ax = b, x 0}, where A is an integer matrix with k rows and Ji columns, x is a vector of Ji variables, and b is a vector of k integers, we ask whether there exists x E Ilk that satisfies Ax = b. Each row of A specifies one linear constraint on x; our goal is to study the complexity of ILP FEASIBILITY when both k, the number of constraints, and MAILD.0, the largest absolute value of an entry in A, are small. Papadimitriou [29] was the first to give a fixed-parameter algorithm for ILP FEASIBILITY under parameterization by the number of constraints that runs in time ((MAIL, +1113110.0) " k) lk2). This was very recently improved by Eisenbrand and Weismantel [9], who used the Steinitz lemma to design an algorithm with running time (142410.) (k) "11b112, which was subsequently improved by Jansen and Rohwedder [17] to 0(14241o)k " log MbIfx,. We prove that for {0, 1}-matrices A, the running time of the algorithm of Eisenbrand and Weismantel is probably optimal: an algorithm with running time 2 (k log k) " 14 0Y(k) would contradict the Exponential Time Hypothesis (ETH). This improves previous non-tight lower bounds of Fomin et al. [10]. We then consider integer linear programs that may have many constraints, but they need to be structured in a "shallow" way. Precisely, we consider the parameter dual treedepth of the matrix A, denoted tdD (A), which is the treedepth of the graph over the rows of A, where two rows are adjacent if in some column they simultaneously contain a non-zero entry. It was recently shown by KouteckST 0 (td (A)) et al. [24] that ILP FEASIBILITY can be solved in time 112,1112 D (k loglIblfx,)(9(1). We present a streamlined proof of this fact and prove that, again, this running time is probably optimal: even assuming that all entries of A and b are in {-1, 0, 1}, the existence of an algorithm with running time 2' tdp (A)) " (k + f)(9(1) would contradict the ETH.

### Target Set Selection in Dense Graph Classes

Autoři

Dvořák, P.; Knop, D.; Toufar, T.

Rok

2018

Publikováno

29th International Symposium on Algorithms and Computation (ISAAC 2018). Saarbrücken: Dagstuhl Publishing,, 2018. p. 18:1-18:13. Leibniz International Proceedings in Informatics (LIPIcs). vol. 123. ISSN 1868-8969. ISBN 978-3-95977-094-1.

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In this paper we study the Target Set Selection problem from a parameterized complexity perspective. Here for a given graph and a threshold for each vertex the task is to find a set of vertices (called a target set) to activate at the beginning which activates the whole graph during the following iterative process. A vertex outside the active set becomes active if the number of so far activated vertices in its neighborhood is at least its threshold. We give two parameterized algorithms for a special case where each vertex has the threshold set to the half of its neighbors (the so called Majority Target Set Selection problem) for parameterizations by the neighborhood diversity and the twin cover number of the input graph. We complement these results from the negative side. We give a hardness proof for the Majority Target Set Selection problem when parameterized by (a restriction of) the modular-width - a natural generalization of both previous structural parameters. We show that the Target Set Selection problem parameterized by the neighborhood diversity when there is no restriction on the thresholds is W[1]-hard.