- [1]
- Bence Ferdinandy, Kunal
Bhattacharya, Dániel Ábel, and Tamás Vicsek.
Landing together: How flocks arrive at a coherent action in time and space in
the presence of perturbations.
Physica A, 391(4):1207–1215, 01 2012.
(doi:10.1016/j.physa.2011.10.010)
Collective motion is abundant in nature, producing a vast amount of phenomena which have been studied in recent years, including the landing of flocks of birds. We investigate the collective decision making scenario where a flock of birds decides the optimal time of landing in the absence of a global leader. We introduce a simple phenomenological model in the spirit of the statistical mechanics-based self-propelled particles (SPPs) approach to interpret this process. We expect that our model is applicable to a larger class of spatiotemporal decision making situations than just the landing of flocks (which process is used as a paradigmatic case). In the model birds are only influenced by observable variables, like position and velocity. Heterogeneity is introduced in the flock in terms of a depletion time after which a bird feels increasing bias to move towards the ground. Our model demonstrates a possible mechanism by which animals in a large group can arrive at an egalitarian decision about the time of switching from one activity to another in the absence of a leader. In particular, we show the existence of a paradoxical effect where noise enhances the coherence of the landing process.

- [2]
- Norbert Tarcai, Csaba Virágh,
Dániel Ábel, Máté Nagy, Péter L Várkonyi, Gábor Vásárhelyi, and
Tamás Vicsek.
Patterns,
transitions and the role of leaders in the collective dynamics of a simple
robotic flock.
Journal of Statistical Mechanics: Theory and Experiment,
2011(04):P04010, 2011.
We have developed an experimental setup of very simple self-propelled robots to observe collective motion emerging as a result of inelastic collisions only. A circular pool and commercial RC boats were the basis of our first setup, where we demonstrated that jamming, clustering, disordered and ordered motion are all present in such a simple experiment and showed that the noise level has a fundamental role in the generation of collective dynamics. Critical noise ranges and the transition characteristics between the different collective patterns were also examined. In our second experiment we used a real-time tracking system and a few steerable model boats to introduce intelligent leaders into the flock. We demonstrated that even a very small portion of guiding members can determine group direction and enhance ordering through inelastic collisions. We also showed that noise can facilitate and speed up ordering with leaders. Our work was extended with an agent-based simulation model, too, and close similarity between real and simulation results was observed. The simulation results show clear statistical evidence of three states and negative correlation between density and ordered motion due to the onset of jamming. Our experiments confirm the different theoretical studies and simulation results in the literature on the subject of collision-based, noise-dependent and leader-driven self-propelled particle systems.

- [3]
- Anna Lázár, Dániel Ábel, and Tamás
Vicsek.
Modularity
measure of networks with overlapping communities.
Europhysics Letters, 90:18001, 2010.
(Awarded 2nd prize for "best paper" award at ECCS 2010).
In this paper we introduce a non-fuzzy measure which has been designed to rank the partitions of a network’s nodes into overlapping communities. Such a measure an be useful for both quantifying clusters detected by various methods and during finding the overlapping community-structure by optimization methods. The theoretical problem referring to the separation of overlapping modules is discussed, and an example for possible applications is given as well.

- [4]
- Dániel Ábel.
Tracking the
movement of hundreds of bacteria.
2009.
paper presented at KÉPAF 2009.
Methods and results are presented for tracking the movements of hundreds of rodlike bacteria on a black-and-white video. The bacteria are identified with a modified watershed segmenting algorithm and their trajectories are reconstructed. Measurement of such trajectories is a fundamental step towards studying the collective motion of bacteria.

- [5]
- Gergely Palla, Dániel Ábel, Illés J. Farkas, Péter Pollner, Imre Derényi, and Tamás Vicsek. k-clique Percolation and Clustering, chapter 9, pages 369–408. Springer, 2009. ISBN: 978-3-540-69394-9.
- [6]
- Gergely Palla, Péter Pollner,
Illés J. Farkas, Imre Derényi, Dániel Ábel, András Vicsek, and Tamás
Vicsek.
Centrality
properties of directed module members in social networks.
Physica A, 387(19-20):4959–4966, 2008.
(doi:10.1016/j.physa.2008.04.025)
Several recent studies of complex networks have suggested algorithms for locating network communities, also called modules or clusters, which are mostly defined as groups of nodes with dense internal connections. Along with the rapid development of these clustering techniques, the ability of revealing overlaps between communities has become very important as well. An efficient search technique for locating overlapping modules is provided by the Clique Percolation Method (CPM) and its extension to directed graphs, the CPMd algorithm. Here we investigate the centrality properties of directed module members in social networks obtained from e-mail exchanges and from sociometric questionnaires. Our results indicate that nodes in the overlaps between modules play a central role in the studied systems. Furthermore, the two different types of networks show interesting differences in the relation between the centrality measures and the role of the nodes in the directed modules.

- [7]
- Dániel Ábel, Péter Pollner, András Vicsek, Gergely Palla, and Tamás Vicsek. A real-world hierarchical social network: a case-study of a large european company. 2008. poster presented at NetSci 2008.
- [8]
- Dániel Ábel, Gergely Palla, Illés J. Farkas, Imre Derényi, Péter Pollner, and Tamás Vicsek. Cfinder: efficient module finder in directed, undirected and weighted networks. 2007. poster presented at ECCS 2007.
- [9]
- Dániel Ábel. Komplex hálózatok átfedő csoportosulásainak vizsgálata, 2007. Master's thesis, in hungarian; title in english: Investigating overlapping communities of complex networks.
- [10]
- Illés J. Farkas, Gergely Palla, Dániel
Ábel, and Tamás Vicsek.
Weighted network modules.
New Journal of Physics, 9(6):180, 2007.
(doi:10.1088/1367-2630/9/6/180)
The inclusion of link weights into the analysis of network properties allows a deeper insight into the (often overlapping) modular structure of real-world webs. We introduce a clustering algorithm clique percolation method with weights (CPMw) for weighted networks based on the concept of percolating k-cliques with high enough intensity. The algorithm allows overlaps between the modules. First, we give detailed analytical and numerical results about the critical point of weighted k-clique percolation on (weighted) Erdős-Rényi graphs. Then, for a scientist collaboration web and a stock correlation graph we compute three-link weight correlations and with the CPMw the weighted modules. After reshuffling link weights in both networks and computing the same quantities for the randomized control graphs as well, we show that groups of three or more strong links prefer to cluster together in both original graphs.