Prof. Dr. Tobias Friedrich

Research Seminar (Summer Term 2024)

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A series of talks on current research and interesting topics in algorithm engineering and theoretical computer science. Everyone is welcome to attend talks. The usual timeslot of the seminar for this semester is Tuesday 10:00-10:45 am at K-2.04.

If you want to give a talk about a topic in algorithm engineering or theoretical computer science, please write an e-mail to Dr. Andreas Göbel or Nadym Mallek.


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Talks (click on title to show abstract)

30.04. 11:00K-2.03Nicolas Klodt

Diffusion of information in networks is at the core of many problems in AI. Common examples include the spread of ideas and rumors as well as marketing campaigns. Typically, information diffuses at a non-linear rate, for example, if markets become saturated or if users of social networks reinforce each other's opinions. Despite these characteristics, this area has seen little research, compared to the vast amount of results for linear models, which exhibit less complex dynamics. Especially, when considering the possibility of re-infection, no fully rigorous guarantees exist so far.

We address this shortcoming by studying a very general non-linear diffusion model that captures saturation as well as reinforcement. More precisely, we consider a variant of the SIS model in which vertices get infected at a rate that scales polynomially in the number of their infected neighbors, weighted by an infection coefficient λ. We give the first fully rigorous results for thresholds of λ at which the expected survival time becomes super-polynomial. For cliques we show that when the infection rate scales sub-linearly, the threshold only shifts by a poly-logarithmic factor, compared to the standard SIS model. In contrast, super-linear scaling changes the process considerably and shifts the threshold by a polynomial term. For stars, sub-linear and super-linear scaling behave similar and both shift the threshold by a polynomial factor. Our bounds are almost tight, as they are only apart by at most a poly-logarithmic factor from the lower thresholds, at which the expected survival time is logarithmic.

07.05. 11:00K-2.03Martin Bullinger

Partitioning a large group of employees into teams can prove difficult because unsatisfied employees may want to transfer to other teams. In this case, the team (coalition) formation is unstable and incentivizes deviation from the proposed structure. Such a coalition formation scenario can be modeled in the framework of hedonic games and a significant amount of research has been devoted to the study of stability in such games. Unfortunately, stable coalition structures are not guaranteed to exist in general and their practicality is further hindered by computational hardness barriers. We offer a new perspective on this matter by studying a random model of hedonic games. For three prominent stability concepts based on single-agent deviations, we provide a high probability analysis of stability in the large agent limit.

Our first main result is an efficient algorithm that outputs an individually and contractually Nash-stable partition with high probability. Our second main result is that the probability that a random game admits a Nash-stable partition tends to zero. Our approach resolves the two major downsides associated with individual stability and contractual Nash stability and reveals agents acting single-handedly are usually to blame for instabilities.

This is joined work with Sonja Kraiczy.

14.05. 11:00K-2.03Stefan Neubert

Given an instance of a scheduling problem where we want to start executing jobs as soon as possible, it is advantageous if a scheduling algorithm emits the first parts of its solution early, in particular before the algorithm completes its work. Therefore, in this position paper, we analyze core scheduling problems in regards to their enumeration complexity, i. e. the computation time to the first emitted schedule entry (preprocessing time) and the worst case time between two consecutive parts of the solution (delay).

Specifically, we look at scheduling instances that reduce to ordering problems. We apply a known incremental sorting algorithm for scheduling strategies that are at their core comparison-based sorting algorithms and translate corresponding upper and lower complexity bounds to the scheduling setting. For instances with n jobs and a precedence DAG with maximum degree Δ, we incrementally build a topological ordering with O(n) preprocessing and O(Δ) delay. We prove a matching lower bound and show with an adversary argument that the delay lower bound holds even in case the DAG has constant average degree and the ordering is emitted out-of-order in the form of insert operations.

We complement our theoretical results with experiments that highlight the improved time-to-first-output and discuss research opportunities for similar incremental approaches for other scheduling problems.

21.05. 11:00K-2.03Andreas Göbel

The Weisfeiler-Leman (WL) dimension of a graph parameter f is the minimum k such that, if G1 and G2 are indistinguishable by the k-dimensional WL-algorithm then f(G1)=f(G2). The WL-dimension of f is ∞ if no such k exists. We study the WL-dimension of graph parameters characterised by the number of answers from a fixed conjunctive query to the graph. Given a conjunctive query φ, we quantify the WL-dimension of the function that maps every graph G to the number of answers of φ in G. The works of Dvorák (J. Graph Theory 2010), Dell, Grohe, and Rattan (ICALP 2018), and Neuen (ArXiv 2023) have answered this question for full conjunctive queries, which are conjunctive queries without existentially quantified variables. For such queries φ, the WL-dimension is equal to the treewidth of the Gaifman graph of φ.

In this work, we give a characterisation that applies to all conjunctive qureies. Given any conjunctive query φ, we prove that its WL-dimension is equal to the semantic extension width sew(φ), a novel width measure that can be thought of as a combination of the treewidth of φ and its quantified star size, an invariant introduced by Durand and Mengel (ICDT 2013) describing how the existentially quantified variables of φ are connected with the free variables. Using the recently established equivalence between the WL-algorithm and higher-order Graph Neural Networks (GNNs) due to Morris et al. (AAAI 2019), we obtain as a consequence that the function counting answers to a conjunctive query φ cannot be computed by GNNs of order smaller than sew(φ).

28.05. 11:00K-2.03Michelle Döring
Andreas Göbel
Xiaoyue Sherry Li

In this session, our three young and ambitious scientists will each present us briefly a paper they felt like showing us. Each talk will be 5 minutes long and invite for discussion.

04.06. 11:00K-2.03Previous MP

Many real-world networks, like transportation networks and social networks, are dynamic in the sense that the edge set may change over time, but these changes are known in advance. This behavior is captured by the temporal graphs model, which has recently become a trending topic in theoretical computer science. A core open problem in the field is to prove the existence of linear-size temporal spanners in temporal cliques, i.e., sparse subgraphs of complete temporal graphs that ensure all-pairs reachability via temporal paths. So far, the best known result is the existence of temporal spanners with O(n log n) many edges. We present significant progress towards proving that linear-size temporal spanners exist in all temporal cliques.

We adapt techniques used in previous works and heavily expand and generalize them. This allows to show that the existence of a linear spanner in cliques and bi-cliques is equivalent and also provide a simpler and more intuitive proof of the O(n log n) bound. Moreover, we use our novel approach to show that a large class of temporal cliques, called edge-pivotable graphs, admit linear-size temporal spanners. To contrast this, we investigate other classes of temporal cliques that do not belong to the class of edge-pivotable graphs. We introduce two such graph classes and we develop novel techniques for establishing the existence of linear temporal spanners in these graph classes as well. This work was done in last year’s Master’s project.

11.06. 11:00K-2.03Malin Rau


25.06. 11:00K-2.03Aishwarya Radhakrishnan

While most theoretical run time analyses of discrete randomized search heuristics focus on finite search spaces, we consider the search space $\Z^n$. Understanding this search space is especially relevant for developing better algorithms for mixed-integer black box optimization (MI-BBO) problems.

We consider as a fitness functions the distance to the (unique) non-zero optimum a (based on the l1-metric) and study the (1+1) EA which mutates by applying a step-operator on each component that is determined to be varied. For changing by ±, we show that the expected optimization time is Θ(n . (|a|_{∞} + \log(|a|_H))). In particular, the time is linear in |a|_{∞}, a measure of distance between starting point and target $a$. Employing a different step operator which chooses a step size from a distribution so heavy-tailed that the expectation is infinite, we get an optimization time of O(n . \log^2 (|a|_1) . [(\log (\log (|a|_1))]^{1 + \varepsilon})$.

Furthermore, we show that RLS with step size adaptation achieves an optimization time of Θ(n . \log(|a|_1)) and that l1-symmetric operators (as suggested by Rudolph'94) require a time at least linear in |a|_1.