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Effects of Social Influence on the Wisdom of CrowdsApr 16 2012Wisdom of crowds refers to the phenomenon that the aggregate prediction or forecast of a group of individuals can be surprisingly more accurate than most individuals in the group, and sometimes - than any of the individuals comprising it. This article ... More

Reproducing scientists' mobility: A data-driven modelNov 17 2018Nov 27 2018This paper makes two important contributions to understand the mobility patterns of scientists. First, by combining two large-scale data sets covering the publications of 3.5 mio scientists over 60 years, we are able to reveal the geographical "career ... More

Modelling Migration and Economic Agglomeration with Active Brownian ParticlesNov 24 1999We propose a stochastic dynamic model of migration and economic aggregation in a system of employed (immobile) and unemployed (mobile) agents which respond to local wage gradients. Dependent on the local economic situation, described by a production function ... More

The Rise and Fall of R&D NetworksApr 12 2013Mar 17 2016Drawing on a large database of publicly announced R&D alliances, we empirically investigate the evolution of R&D networks and the process of alliance formation in several manufacturing sectors over a 24-year period (1986-2009). Our goal is to empirically ... More

Agent-Based Modeling of Intracellular TransportSep 16 2011We develop an agent-based model of the motion and pattern formation of vesicles. These intracellular particles can be found in four different modes of (undirected and directed) motion and can fuse with other vesicles. While the size of vesicles follows ... More

Modelling Collective Opinion Formation by Means of Active Brownian ParticlesNov 25 1999Feb 08 2000The concept of active Brownian particles is used to model a collective opinion formation process. It is assumed that individuals in community create a two-component communication field that influences the change of opinions of other persons and/or can ... More

Optimal migration promotes the outbreak of cooperation in heterogeneous populationsFeb 06 2012Jun 24 2012We consider a population of agents that are heterogeneous with respect to (i) their strategy when interacting $n_{g}$ times with other agents in an iterated prisoners dilemma game, (ii) their spatial location on $K$ different islands. After each generation, ... More

Nonlinear Voter Models: The Transition from Invasion to CoexistenceJul 30 2003Dec 13 2008In nonlinear voter models the transitions between two states depend in a nonlinear manner on the frequencies of these states in the neighborhood. We investigate the role of these nonlinearities on the global outcome of the dynamics for a homogeneous network ... More

Social signals and algorithmic trading of BitcoinJun 04 2015Sep 24 2015The availability of data on digital traces is growing to unprecedented sizes, but inferring actionable knowledge from large-scale data is far from being trivial. This is especially important for computational finance, where digital traces of human behavior ... More

Self-Organization, Active Brownian Dynamics, and Biological ApplicationsNov 26 2002Jan 23 2004After summarizing basic features of self-organization such as entropy export, feedbacks and nonlinear dynamics, we discuss several examples in biology. The main part of the paper is devoted to a model of active Brownian motion that allows a stochastic ... More

A conceptual approach to model co-evolution of urban structuresJun 15 2016Jun 16 2016Urban structures encompass settlements, characterized by the spatial distribution of built-up areas, but also transportation structures, to connect these built-up areas. These two structures are very different in their origin and function, fulfilling ... More

On Spatial Consensus Formation: Is the Sznajd Model Different from a Voter Model?Jun 23 2003In this paper, we investigate the so-called ``Sznajd Model'' (SM) in one dimension, which is a simple cellular automata approach to consensus formation among two opposite opinions (described by spin up or down). To elucidate the SM dynamics, we first ... More

Neighborhood approximations for non-linear voter modelsDec 15 2015Non-linear voter models assume that the opinion of an agent depends on the opinions of its neighbors in a non-linear manner. This allows for voting rules different from majority voting. While the linear voter model is known to reach consensus, non-linear ... More

Self-Assembling of Networks in an Agent-Based ModelDec 11 2002We propose a model to show the self-assembling of network-like structures between a set of nodes without using preexisting positional information or long-range attraction of the nodes. The model is based on Brownian agents that are capable of producing ... More

An Agent-Based Model of Collective Emotions in Online CommunitiesJun 28 2010Sep 06 2010We develop a agent-based framework to model the emergence of collective emotions, which is applied to online communities. Agents individual emotions are described by their valence and arousal. Using the concept of Brownian agents, these variables change ... More

Correlations between thresholds and degrees: An analytic approach to model attacks and failure cascadesJun 14 2017Jun 16 2017Two node variables determine the evolution of cascades in random networks: a node's degree and threshold. Correlations between both fundamentally change the robustness of a network, yet, they are disregarded in standard analytic methods as local tree ... More

A Network Perspective on Software ModularityJan 18 2012Feb 28 2013Modularity is a desirable characteristic for software systems. In this article we propose to use a quantitative method from complex network sciences to estimate the coherence between the modularity of the dependency network of large open source Java projects ... More

On Zero Divisors with Small Support in Group Rings of Torsion-Free GroupsFeb 29 2012Kaplanski's Zero Divisor Conjecture envisions that for a torsion-free group G and an integral domain R, the group ring R[G] does not contain non-trivial zero divisors. We define the length of an element a in R[G] as the minimal non-negative integer k ... More

A Model of a Trust-based Recommendation System on a Social NetworkNov 28 2006Sep 18 2007In this paper, we present a model of a trust-based recommendation system on a social network. The idea of the model is that agents use their social network to reach information and their trust relationships to filter it. We investigate how the dynamics ... More

Personalised and Dynamic Trust in Social NetworksFeb 09 2009May 09 2009We propose a novel trust metric for social networks which is suitable for application in recommender systems. It is personalised and dynamic and allows to compute the indirect trust between two agents which are not neighbours based on the direct trust ... More

Modeling Evolving Innovation NetworksDec 17 2007We develop a new framework for modeling innovation networks which evolve over time. The nodes in the network represent firms, whereas the directed links represent unilateral interactions between the firms. Both nodes and links evolve according to their ... More

Coordination of Decisions in a Spatial Agent ModelSep 06 2001For a binary choice problem, the spatial coordination of decisions in an agent community is investigated both analytically and by means of stochastic computer simulations. The individual decisions are based on different local information generated by ... More

Redistribution spurs growth by using a portfolio effect on human capitalOct 13 2012We demonstrate by mathematical analysis and systematic computer simulations that redistribution can lead to sustainable growth in a society. The human capital dynamics of each agent is described by a stochastic multiplicative process which, in the long ... More

Modeling the formation of R\&D alliances: An agent-based model with empirical validationDec 05 2017We develop an agent-based model to reproduce the size distribution of R\&D alliances of firms. Agents are uniformly selected to initiate an alliance and to invite collaboration partners. These decide about acceptance based on an individual threshold that ... More

Control contribution identifies top driver nodes in complex networksJun 11 2019We propose a new measure to quantify the impact of a node $i$ in controlling a directed network. This measure, called `control contribution' $\mathcal{C}_{i}$, combines the probability for node $i$ to appear in a set of driver nodes and the probability ... More

The Mobility Network of Scientists: Analyzing Temporal Correlations in Scientific CareersMay 15 2019To understand the mobility patterns of scientists, we combine two large-scale bibliographic data sets to reveal the geographical "career trajectories" of scientists and their temporal properties. Each trajectory contains, on the individual level, information ... More

How groups can foster consensus: The case of local culturesApr 05 2009A local culture denotes a commonly shared behaviour within a cluster of firms. Similar to social norms or conventions, it is an emergent feature resulting from the firms' interaction in an economic network. To model these dynamics, we consider a distributed ... More

Evolution of Cooperation in a Spatial Prisoner's DilemmaNov 26 2002We investigate the spatial distribution and the global frequency of agents who can either cooperate or defect. The agent interaction is described by a deterministic, non-iterated prisoner's dilemma game, further each agent only locally interacts with ... More

A stochastic model of social interaction in wild house miceDec 04 2012We investigate to what extent the interaction dynamics of a population of wild house mouse (Mus musculus domesticus) in their environment can be explained by a simple stochastic model. We use a Markov chain model to describe the transitions of mice in ... More

When the Filter Bubble Bursts: Collective Evaluation Dynamics in Online CommunitiesFeb 18 2016We analyze online collective evaluation processes through positive and negative votes in various social media. We find two modes of collective evaluations that stem from the existence of filter bubbles. Above a threshold of collective attention, negativity ... More

Data-driven modeling of collaboration networks: A cross-domain analysisApr 05 2017We analyze large-scale data sets about collaborations from two different domains: economics, specifically 22.000 R&D alliances between 14.500 firms, and science, specifically 300.000 co-authorship relations between 95.000 scientists. Considering the different ... More

The network structure of city-firm relationsDec 09 2015How are economic activities linked to geographic locations? To answer this question, we use a data-driven approach that builds on the information about location, ownership and economic activities of the world's 3,000 largest firms and their almost one ... More

Testing an agent-based model of bacterial cell motility: How nutrient concentration affects speed distributionSep 16 2011We revisit a recently proposed agent-based model of active biological motion and compare its predictions with own experimental findings for the speed distribution of bacterial cells, \emph{Salmonella typhimurium}. Agents move according to a stochastic ... More

Explicit size distributions of failure cascades redefine systemic risk on finite networksFeb 08 2018How big is the risk that a few initial failures of nodes in a network amplify to large cascades that span a substantial share of all nodes? Predicting the final cascade size is critical to ensure the functioning of a system as a whole. Yet, this task ... More

The Role of Emotions in Contributors Activity: A Case Study on the GENTOO CommunityJun 15 2013Jun 20 2013We analyse the relation between the emotions and the activity of contributors in the Open Source Software project Gentoo. Our case study builds on extensive data sets from the project's bug tracking platform Bugzilla, to quantify the activity of contributors, ... More

Phase transitions in social impact models of opinion formationApr 03 2000We study phase transitions in models of opinion formation which are based on the social impact theory. Two different models are discussed: (i) a cellular--automata based model of a finite group with a strong leader where persons can change their opinions ... More

Selection rules in alliance formation: strategic decisions or abundance of choice?Mar 13 2014Mar 14 2014We study how firms select partners using a large database of publicly announced R&D alliances over a period of 25 years. We identify, for the first time, two distinct behavioral strategies of firms in forming these alliances. By reconstructing and analysing ... More

The Network of Counterparty Risk: Analysing Correlations in OTC DerivativesJun 15 2015Sep 15 2015Counterparty risk denotes the risk that a party defaults in a bilateral contract. This risk not only depends on the two parties involved, but also on the risk from various other contracts each of these parties holds. In rather informal markets, such as ... More

New Power Law Signature of Media Exposure in Human Response Waiting Time DistributionsMar 08 2009May 25 2010We study the humanitarian response to the destruction brought by the tsunami generated by the Sumatra earthquake of December 26, 2004, as measured by donations, and find that it decays in time as a power law ~ 1/t^(alpha) with alpha=2.5 +/- 0.1. This ... More

Positive words carry less information than negative wordsOct 18 2011May 27 2012We show that the frequency of word use is not only determined by the word length \cite{Zipf1935} and the average information content \cite{Piantadosi2011}, but also by its emotional content. We have analyzed three established lexica of affective word ... More

Social Resilience in Online Communities: The Autopsy of FriendsterFeb 25 2013We empirically analyze five online communities: Friendster, Livejournal, Facebook, Orkut, Myspace, to identify causes for the decline of social networks. We define social resilience as the ability of a community to withstand changes. We do not argue about ... More

The optimal trajectory to control complex networksJun 11 2018Controllability, a basic property of various networked systems, has gained profound theoretical applications in complex social, technological, biological, and brain networks. Yet, little attention has been given to the control trajectory (route), along ... More

Statistical Mechanics of Canonical-Dissipative Systems and Applications to Swarm DynamicsMar 16 2001Sep 26 2001We develop the theory of canonical-dissipative systems, based on the assumption that both the conservative and the dissipative elements of the dynamics are determined by invariants of motion. In this case, known solutions for conservative systems can ... More

Uphill Motion of Active Brownian Particles in Piecewise Linear PotentialsDec 17 1999We consider Brownian particles with the ability to take up energy from the environment, to store it in an internal depot, and to convert internal energy into kinetic energy of motion. Provided a supercritical supply of energy, these particles are able ... More

A k-shell decomposition method for weighted networksMay 16 2012Aug 27 2012We present a generalized method for calculating the k-shell structure of weighted networks. The method takes into account both the weight and the degree of a network, in such a way that in the absence of weights we resume the shell structure obtained ... More

How can social herding enhance cooperation?Nov 06 2012We study a system in which N agents have to decide between two strategies \theta_i (i \in 1... N), for defection or cooperation, when interacting with other n agents (either spatial neighbors or randomly chosen ones). After each round, they update their ... More

Quantifying the Impact of Leveraging and Diversification on Systemic RiskMar 22 2013Excessive leverage, i.e. the abuse of debt financing, is considered one of the primary factors in the default of financial institutions. Systemic risk results from correlations between individual default probabilities that cannot be considered independent. ... More

Online Privacy as a Collective PhenomenonSep 22 2014The problem of online privacy is often reduced to individual decisions to hide or reveal personal information in online social networks (OSNs). However, with the increasing use of OSNs, it becomes more important to understand the role of the social network ... More

Diversity-induced resonance in the response to social normsSep 25 2012In this paper we focus on diversity-induced resonance, which was recently found in bistable, excitable and other physical systems. We study the appearance of this phenomenon in a purely economic model of cooperating and defecting agents. Agent's contribution ... More

git2net - Mining Time-Stamped Co-Editing Networks from Large git RepositoriesMar 25 2019Data from software repositories have become an important foundation for the empirical study of software engineering processes. A recurring theme in the repository mining literature is the inference of developer networks capturing e.g. collaboration, coordination, ... More

Quantifying the effects of social influenceFeb 11 2013How do humans respond to indirect social influence when making decisions? We analysed an experiment where subjects had to repeatedly guess the correct answer to factual questions, while having only aggregated information about the answers of others. While ... More

Systemic Risk in a Unifying Framework for Cascading Processes on NetworksJul 30 2009Jan 04 2010We introduce a general framework for models of cascade and contagion processes on networks, to identify their commonalities and differences. In particular, models of social and financial cascades, as well as the fiber bundle model, the voter model, and ... More

Directed motion of Brownian particles with internal energy depotDec 17 1999A model of Brownian particles with the ability to take up energy from the environment, to store it in an internal depot, and to convert internal energy into kinetic energy of motion, is discussed. The general dynamics outlined in Sect. 2 is investigated ... More

The spatial component of R&D networksSep 28 2015We study the role of geography in R&D networks by means of a quantitative, micro-geographic approach. Using a large database that covers international R&D collaborations from 1984 to 2009, we localize each actor precisely in space through its latitude ... More

Dependence of critical level statistics on the sample shapeApr 29 1998The level-spacing distribution of consecutive energy eigenvalues is calculated numerically at the metal insulator transition for 3d systems with different cuboid shapes. It is found that the scale independent critical $P_c(s)$ changes as a function of ... More

Relation between the Correlation Dimensions of Multifractal Wavefunctions and Spectral Measures in Integer Quantum Hall SystemsJan 26 1994We study the time evolution of wavepackets of non-interacting electrons in a two-dimensional disordered system in strong magnetic field. For wavepackets built from states near the metal-insulator transition in the center of the lowest Landau band we find ... More

Critical conductance of the chiral 2d random flux modelAug 27 2007Aug 29 2007The two-terminal conductance of a random flux model defined on a square lattice is investigated numerically at the band center using a transfer matrix method. Due to the chiral symmetry, there exists a critical point where the ensemble averaged mean conductance ... More

${\mathbb Z}_n$-equivariant $K$-theoryMay 08 2015We construct a sequence of $n-1$ cyclic exact sequences that can be used to compute the $K$-theory of the $C^\star$-algebra crossed product $A \ltimes {\mathbb Z}_n$.

Canonizing Graphs of Bounded Tree Width in LogspaceJun 25 2015Graph canonization is the problem of computing a unique representative, a canon, from the isomorphism class of a given graph. This implies that two graphs are isomorphic exactly if their canons are equal. We show that graphs of bounded tree width can ... More

Risk-Seeking versus Risk-Avoiding Investments in Noisy Periodic EnvironmentsJan 28 2008Sep 07 2008We study the performance of various agent strategies in an artificial investment scenario. Agents are equipped with a budget, $x(t)$, and at each time step invest a particular fraction, $q(t)$, of their budget. The return on investment (RoI), $r(t)$, ... More

Reduction Techniques for Graph Isomorphism in the Context of Width ParametersMar 27 2014We study the parameterized complexity of the graph isomorphism problem when parameterized by width parameters related to tree decompositions. We apply the following technique to obtain fixed-parameter tractability for such parameters. We first compute ... More

Riemannian manifolds not quasi-isometric to leaves in codimension one foliationsNov 24 2009Every open manifold L of dimension greater than one has complete Riemannian metrics g with bounded geometry such that (L,g) is not quasi-isometric to a leaf of a codimension one foliation of a closed manifold. Hence no conditions on the local geometry ... More

Active Walker Model for the Formation of Human and Animal Trail SystemsJun 08 1998Active walker models have recently proved their great value for describing the formation of clusters, periodic patterns, and spiral waves as well as the development of rivers, dielectric breakdown patterns, and many other structures. It is shown that ... More

Quantifying knowledge exchange in R&D networks: a data-driven modelJul 25 2015We develop an agent-based model to reproduce the process of link formation and to understand the effect of knowledge exchange in collaborative inter-firm networks of Research and Development (R&D) alliances. In our model, agents form links based on their ... More

Agent-based simulations of emotion spreading in online social networksMay 29 2012Quantitative analysis of empirical data from online social networks reveals group dynamics in which emotions are involved (\v{S}uvakov et al). Full understanding of the underlying mechanisms, however, remains a challenging task. Using agent-based computer ... More

Generalized Hypergeometric Ensembles: Statistical Hypothesis Testing in Complex NetworksJul 08 2016Aug 08 2016Statistical ensembles of networks, i.e., probability spaces of all networks that are consistent with given aggregate statistics, have become instrumental in the analysis of complex networks. Their numerical and analytical study provides the foundation ... More

Decelerating microdynamics can accelerate macrodynamics in the voter modelNov 07 2007Sep 07 2008For the voter model, we study the effect of a memory-dependent transition rate. We assume that the transition of a spin into the opposite state decreases with the time it has been in its current state. Counter-intuitively, we find that the time to reach ... More

A Tunable Mechanism for Identifying Trusted Nodes in Large Scale Distributed NetworksApr 24 2012In this paper, we propose a simple randomized protocol for identifying trusted nodes based on personalized trust in large scale distributed networks. The problem of identifying trusted nodes, based on personalized trust, in a large network setting stems ... More

Systemic risk in multiplex networks with asymmetric coupling and threshold feedbackJun 22 2015We study cascades on a two-layer multiplex network, with asymmetric feedback that depends on the coupling strength between the layers. Based on an analytical branching process approximation, we calculate the systemic risk measured by the final fraction ... More

Sentiment cascades in the 15M movementMay 14 2015Recent grassroots movements have suggested that online social networks might play a key role in their organization, as adherents have a fast, many-to-many, communication channel to help coordinate their mobilization. The structure and dynamics of the ... More

Categorizing Bugs with Social Networks: A Case Study on Four Open Source Software CommunitiesFeb 27 2013Feb 28 2013Efficient bug triaging procedures are an important precondition for successful collaborative software engineering projects. Triaging bugs can become a laborious task particularly in open source software (OSS) projects with a large base of comparably inexperienced ... More

The Dynamics of Emotions in Online InteractionMay 12 2016We study the changes in emotional states induced by reading and participating in online discussions, empirically testing a computational model of online emotional interaction. Using principles of dynamical systems, we quantify changes in valence and arousal ... More

An ensemble perspective on multi-layer networksJul 01 2015We study properties of multi-layered, interconnected networks from an ensemble perspective, i.e. we analyze ensembles of multi-layer networks that share similar aggregate characteristics. Using a diffusive process that evolves on a multi-layer network, ... More

Quantifying knowledge exchange in R&D networks: A data-driven modelJul 25 2015May 05 2017We propose a model that reflects two important processes in R&D activities of firms, the formation of R&D alliances and the exchange of knowledge as a result of these collaborations. In a data-driven approach, we analyze two large-scale data sets extracting ... More

Emotional persistence in online chatting communitiesMay 11 2012How do users behave in online chatrooms, where they instantaneously read and write posts? We analyzed about 2.5 million posts covering various topics in Internet relay channels, and found that user activity patterns follow known power-law and stretched ... More

Brownian Particles far from EquilibriumDec 28 1999We study a model of Brownian particles which are pumped with energy by means of a non-linear friction function, for which different types are discussed. A suitable expression for a non-linear, velocity-dependent friction function is derived by considering ... More

Slower is faster: Fostering consensus formation by heterogeneous intertiaApr 05 2009We investigate an extension of the voter model in which voters are equipped with an individual inertia to change their opinion. This inertia depends on the persistence time of a voter's current opinion (ageing). We focus on the case of only two different ... More

On Algebraic Graph Theory and the Dynamics of Innovation NetworksDec 17 2007We investigate some of the properties and extensions of a dynamic innovation network model recently introduced in \citep{koenig07:_effic_stabil_dynam_innov_networ}. In the model, the set of efficient graphs ranges, depending on the cost for maintaining ... More

How big is too big? Critical Shocks for Systemic Failure CascadesSep 05 2012Apr 15 2013External or internal shocks may lead to the collapse of a system consisting of many agents. If the shock hits only one agent initially and causes it to fail, this can induce a cascade of failures among neighoring agents. Several critical constellations ... More

The Rise and Fall of R&D NetworksApr 12 2013Aug 05 2017Drawing on a large database of publicly announced R&D alliances, we empirically investigate the evolution of R&D networks and the process of alliance formation in several manufacturing sectors over a 24-year period (1986-2009). Our goal is to empirically ... More

A model of dynamic rewiring and knowledge exchange in R&D networksMay 07 2015Jun 29 2016This paper investigates the process of knowledge exchange in inter-firm Research and Development (R&D) alliances by means of an agent-based model. Extant research has pointed out that firms select alliance partners considering both network-related and ... More

How do OSS projects change in number and size? A large-scale analysis to test a model of project growthJun 23 2015Established Open Source Software (OSS) projects can grow in size if new developers join, but also the number of OSS projects can grow if developers choose to found new projects. We discuss to what extent an established model for firm growth can be applied ... More

The Rise and Fall of a Central Contributor: Dynamics of Social Organization and Performance in the Gentoo CommunityFeb 28 2013Social organization and division of labor crucially influence the performance of collaborative software engineering efforts. In this paper, we provide a quantitative analysis of the relation between social organization and performance in Gentoo, an Open ... More

Ideological and Temporal Components of Network Polarization in Online Political Participatory MediaMar 26 2015Political polarization is traditionally analyzed through the ideological stances of groups and parties, but it also has a behavioral component that manifests in the interactions between individuals. We present an empirical analysis of the digital traces ... More

Predicting Scientific Success Based on Coauthorship NetworksFeb 28 2014We address the question to what extent the success of scientific articles is due to social influence. Analyzing a data set of over 100000 publications from the field of Computer Science, we study how centrality in the coauthorship network differs between ... More

Investments in Random EnvironmentsSep 23 2007Sep 07 2008We present analytical investigations of a multiplicative stochastic process that models a simple investor dynamics in a random environment. The dynamics of the investor's budget, $x(t)$, depends on the stochasticity of the return on investment, $r(t)$, ... More

The Rise and Fall of R&D NetworksApr 12 2013Oct 30 2016Drawing on a large database of publicly announced R&D alliances, we empirically investigate the evolution of R&D networks and the process of alliance formation in several manufacturing sectors over a 24-year period (1986-2009). Our goal is to empirically ... More

Quantifying Triadic Closure in Multi-Edge Social NetworksMay 08 2019Multi-edge networks capture repeated interactions between individuals. In social networks, such edges often form closed triangles, or triads. Standard approaches to measure this triadic closure, however, fail for multi-edge networks, because they do not ... More

What is the Entropy of a Social Organization?May 23 2019We quantify a social organization's potentiality, that is its ability to attain different configurations. The organization is represented as a network in which nodes correspond to individuals and (multi-)edges to their multiple interactions. Attainable ... More

From Relational Data to Graphs: Inferring Significant Links using Generalized Hypergeometric EnsemblesJun 14 2017Jul 07 2017The inference of network topologies from relational data is an important problem in data analysis. Exemplary applications include the reconstruction of social ties from data on human interactions, the inference of gene co-expression networks from DNA ... More

Exotic open $4$-manifolds which are non-leavesOct 29 2014Sep 11 2017We study the possibility of realizing exotic smooth structures on punctured simply connected $4$-manifolds as leaves of a codimension one foliation on a compact manifold. In particular, we show the existence of uncountably many smooth open $4$-manifolds ... More

Spherically symmetric model atmospheres for low mass pre-Main Sequence stars with effective temperatures between 2000 and 6800 KAug 29 2000We present a grid of spherically symmetric model atmospheres for young pre-MS stars. This grid spans the parameter range $2000\K \leq \Teff \leq 6800\K$ and $2.0 \leq \logg \leq 3.5$ for $M=0.1\Msun$, appropriate for low mass stars and brown dwarfs. A ... More

Graphs Identified by Logics with CountingMar 30 2015We classify graphs and, more generally, finite relational structures that are identified by C2, that is, two-variable first-order logic with counting. Using this classification, we show that it can be decided in almost linear time whether a structure ... More

The role of endogenous and exogenous mechanisms in the formation of R&D networksMar 17 2014Jul 15 2014We develop an agent-based model of strategic link formation in Research and Development (R&D) networks. Empirical evidence has shown that the growth of these networks is driven by mechanisms which are both endogenous to the system (that is, depending ... More

Induced Minor Free Graphs: Isomorphism and Clique-widthMay 27 2016Given two graphs $G$ and $H$, we say that $G$ contains $H$ as an induced minor if a graph isomorphic to $H$ can be obtained from $G$ by a sequence of vertex deletions and edge contractions. We study the complexity of Graph Isomorphism on graphs that exclude ... More

Causality-Driven Slow-Down and Speed-Up of Diffusion in Non-Markovian Temporal NetworksJul 15 2013Aug 26 2015Recent research has highlighted limitations of studying complex systems with time-varying topologies from the perspective of static, time-aggregated networks. Non-Markovian characteristics resulting from the ordering of interactions in temporal networks ... More

Minimal Asymmetric GraphsMay 04 2016Confirming a conjecture of Ne\v{s}et\v{r}il, we show that up to isomorphism there is only a finite number of finite minimal asymmetric undirected graphs. In fact, there are exactly 18 such graphs. We also show that these graphs are exactly the finite ... More

A Quantitative Study of Social Organisation in Open Source Software CommunitiesAug 21 2012Mar 04 2013The success of open source projects crucially depends on the voluntary contributions of a sufficiently large community of users. Apart from the mere size of the community, interesting questions arise when looking at the evolution of structural features ... More

Betweenness Preference: Quantifying Correlations in the Topological Dynamics of Temporal NetworksAug 02 2012May 02 2013We study correlations in temporal networks and introduce the notion of betweenness preference. It allows to quantify to what extent paths, existing in time-aggregated representations of temporal networks, are actually realizable based on the sequence ... More

Hierarchical Consensus Formation Reduces the Influence of Opinion BiasMar 30 2012Jul 21 2012We study the role of hierarchical structures in a simple model of collective consensus formation based on the bounded confidence model with continuous individual opinions. For the particular variation of this model considered in this paper, we assume ... More