Jari Saramäki

Senior Researcher, Dr.Tech.
Group Leader (Complex Networks)

Postal Address:
Aalto University
Department of Biomedical Engineering and Computational Science (BECS)
P.O. Box 12200
FI-00076 AALTO, FINLAND
Visiting Address:
F-building, room F348, Rakentajanaukio 2, Otaniemi campus, Espoo
Tel.
+358 9 451 5328
Fax:
+358 9 451 4830
Email:

Research Interests

Networks are everywhere. We, human beings, are bound by social ties to our friends and colleagues, who in turn are bound to others, and so on - finally, each of us is part of an enormous social network spanning the entire planet. We think, feel, and perform actions as a result of electric signals flowing in the network of neurons which forms our brain. Within these neurons, and within the rest of the cells forming our bodies, proteins and metabolites form complex networks of interactions, giving rise to a living organism. This web page which you are reading has travelled to your computer through a very large network of interconnected computers - a network where no-one has central authority, but which is nevertheless remarkably efficient and robust.

Our research group at BECS studies these fascinating objects: networks which are rich, complex, and almost always self-organizing. The study of complex networks is part of a large-scale undertaking which has emerged during the recent years: attempting to understand the structure and function of complex systems, which consist of large numbers of interacting elements - for example, the above-mentioned people, neurons, or computers. The network approach to complex systems has turned out to be particularly fruitful. In this approach, the interacting elements are represented by vertices of a network, and their interactions by edges connecting these vertices. So far, studies of the characteristics of complex networks have produced unexpected findings, such as the "small-world" property and broad connectivity distributions common to a large number of networks. It has turned out that seemingly very different networks share similar characteristics.

Network theory provides tools for understanding a variety of complex systems, and hence our studies also touch several fields of science. We study large-scale techno-social networks, harnessing electronic communication records which for the first time allow understanding the structure and dynamics of social systems even on the societal scale, where networks of social ties span entire nations. Within the EU-funded ICTeCollective research project, we work in close collaboration with social scientists, computer scientists, and complexity scientists to uncover the dynamics of techno-social networks. We also study various networks in biology and life sciences, participating in the EDEN project, where network methods are used in studies of genetic diversity of endangered Mediterranean marine plants and in targeting of conservation efforts. Network methods will also be utilized in understanding the anatomical connectivity patterns of the human brain, obtained from MRI measurements and tractography analysis.

Those proficient in the Finnish language might be interested in Kompleksisten verkostojen fysiikkaa, published in Arkhimedes 01/07.

Our social network studies in the news

  • Yle Radio 1 - Matematiikan aika 3.8.2009: Radio interview on social networks (in Finnish)
  • New Scientist 24.4.2007: Cellphone study shows the ties that bind nations
  • Spiegel Online 25.4.2007: Flüchtige Bekanntschaften halten die Info-Gesellschaft zusammen
  • Yleisradio (Finnish national radio broadcaster): Interview in Tieteen viikko (Weekly Science News)
  • PhysicsWeb newsflash
  • Science News Focus 10. November 2006, Tracking People's Electronic Footprints

    • Examples of dense mesoscopic structures, communities, in networks of different categories.

    A. Lancichinetti, M. Kivelä, J. Saramäki, and S. Fortunato, PLoS One 5(8), e11976 (2010)


    Visualization of a small sample of a large social network reconstructed from data on one-to-one mobile telephone communications.

    J.-P. Onnela, J. Saramäki, J. Hyvönen, G. Szabó, D. Lazer, K. Kaski, J. Kertész, and A.-L. Barabási, Proc. Natl. Acad. Sci. (USA) 104, 7332 (2007).

    Publications

    Minimum spanning tree of the genetic distance network of Mediterranean P. Oceanica samples.


    Spontaneously emerging spiral waves in excitable media with small-world connectivity.

    Recent papers:

    1. The persistence of social signatures in human communication, J. Saramäki, E.A. Leicht, E. Lopez, S.G.B. Roberts, F. Reed-Tsochas, and R.I.M. Dunbar, arXiv:1204.5602 (2012)
    2. Effects of time window size and placement on the structure of aggregated networks, G. Krings, M. Karsai, S. Bernardsson, V.D. Blondel, and J. Saramäki, EPJ Data Science (in press), arXiv:1202.1145 (2012)
    3. Multi-scale analysis of spreading in a large communication network, M. Kivelä, R.K. Pan, K. Kaski, J. Kertész, J. Saramäki, and M. Karsai, J. Stat. Mech. P03005 (2012), arXiv:1112.4312
    4. Networks of emotion concepts, R. Toivonen, M. Kivelä, J. Saramäki, M. Viinikainen, M. Vanhatalo, and M. Sams, PLoS One 7(1), e28883 (2012)
    5. Temporal networks, P. Holme and J. Saramäki, Physics Reports (2012) in press, arXiv: 1108.1780
    6. The strength of strong ties in scientific collaboration networks, R.K. Pan and J. Saramäki, EPL 97, 18007 (2012), arXiv:1106.5249
    7. Temporal motifs in time-dependent networks, L. Kovanen, M. Karsai, K. Kaski, J. Kertész, and J. Saramäki, J. Stat. Mech. P11005 (2011), arXiv:1107.5646 (2011)
    8. Path lengths, correlations, and centrality in temporal networks, R.K. Pan and J. Saramäki, Phys. Rev. E 84, 016105 arXiv:1101.5913 (2011)
    9. Using explosive percolation in analysis of real-world networks, R.K. Pan, M. Kivelä, J. Saramäki, K. Kaski, J. Kertész, Phys. Rev. E 83, 046112 (2011), arXiv:1010.3171
    10. Small but slow world: how network topology and burstiness slow down spreading, M. Karsai, M. Kivelä, R. K. Pan, K. Kaski, J. Kertész, A.-L. Barabási, and J. Saramäki, Phys. Rev. E 83, 025102(R) (2011), arXiv:1006.2125
    11. Communities and beyond: mesoscopic analysis of a large social network with complementary methods, G. Tibély, L. Kovanen, M. Karsai, K. Kaski, J. Kertész, and J. Saramäki, Phys. Rev. E 83, 056125 (2011), arXiv:1006.0418
    12. Characterizing the community structure of complex networks, A. Lancichinetti, M. Kivelä, J. Saramäki, and S. Fortunato, PLoS One, 5(8), e11976 (2010)
    13. Reciprocity of mobile phone calls, L. Kovanen, J. Saramäki, and K. Kaski, Dynamics of Socio-Economic Systems, in press (2010), arXiv:1002.0763v1
    14. A comparative study of stochastic algorithmic models for social networks, R. Toivonen, L. Kovanen, M. Kivelä, J.-P. Onnela, J. Saramäki, and K. Kaski, Social Networks 31, 240 (2009)
    15. Dynamics of latent voters, R. Lambiotte, J. Saramäki, and V.D. Blondel, Phys. Rev. E 79, 046107 (2009), arXiv: 0811.1464
    16. Model of community emergence in weighted networks, J.M. Kumpula, J.-P. Onnela, J. Saramäki, J. Kertész, and K. Kaski, Computer Physics Communications 180, 517 (2009)
    17. Maximal spanning trees, asset graphs and random matrix denoising in the analysis of dynamics of financial networks, T. Heimo, K. Kaski, and J. Saramäki, Physica A 388, 145 (2009), arXiv: 0806.4714
    18. Broad lifetime distributions for ordering dynamics in complex networks, R. Toivonen, X. Castelló, V.M. Eguíluz, J. Saramäki, K. Kaski, and M. San Miguel, Phys. Rev. E 79, 016109 (2009), arXiv:0808.3318
    19. A sequential algorithm for fast clique percolation, J.M. Kumpula, M. Kivelä, K. Kaski, and J. Saramäki, Phys. Rev. E 78, 026109 (2008), arXiv:0805.1449
    20. Detecting modules in dense weighted networks with the Potts method, T. Heimo, J. Kumpula, K. Kaski, and J. Saramäki, J. Stat. Mech. P08007 (2008), arXiv:0804.3457
    21. The International Trade Network: weighted network analysis and modelling, K. Bhattacharya, G. Mukherjee, J. Saramäki, K. Kaski and S.S. Manna, J. Stat. Mech. P02002 (2008)
    22. Modelling language competition: bilingualism and complex social networks, X. Castelló, V.M. Eguíluz, M. San Miguel, L. Loureiro-Porto, R. Toivonen, J. Saramäki, and K. Kaski, in The evolution of language: Proceedings of the 7th International Conference (EVOLANG7), Barcelona 2008, eds. A.D.M. Smith, K. Smith, R. Ferrer-Cancho. World Scientific Publishing (2008).
    23. Efficient data structures for sparse network representation, J. Hyvönen, J. Saramäki, and K. Kaski, Int. J. Comp. Math. 85, 1219 (2008)
    24. Spectral methods and cluster structure in correlation-based networks, T. Heimo, G. Tibély, J. Saramäki, K. Kaski, and J. Kertész, Physica A 386, 5930 (2008), arXiv:0708.1862
    25. Emergence of communities in weighted networks, J.M. Kumpula, J.-P. Onnela, J. Saramäki, K. Kaski, and J. Kertész, Phys. Rev. Lett. 99, 228701 (2007), arXiv:0708.0925
    26. Limited resolution and multiresolution methods in complex network community detection, J.M. Kumpula, J. Saramäki, K. Kaski, and J. Kertész, Fluctuation and Noise Letters 7, L209 (2007)
    27. Anomalous lifetime distributions and topological traps in ordering dynamics, X. Castelló, R. Toivonen, V.M. Eguíluz, J. Saramäki, K. Kaski, and M. San Miguel, Europhys. Lett. 79, 66006 (2007), arxiv: physics/0705.2560
    28. Emergence of self-sustained patterns in small-world excitable media, S. Sinha, J. Saramäki and K. Kaski, Phys. Rev. E 76, 015101(R) (2007), cond-mat/0701121. Selected for the July 15, 2007 issue of Virtual Journal of Biological Physics Research
    29. Analysis of a large-scale weighted network of one-to-one human communication, J.-P. Onnela, J. Saramäki, J. Hyvönen, G. Szabó, M. Argollo de Menezes, K. Kaski, A.-L. Barabási, and J. Kertész, New Journal of Physics 9, 179 (2007), physics/0702158
    30. Spectral and network methods in the analysis of correlation matrices of stock returns, T. Heimo, J. Saramäki, J.-P. Onnela, and K. Kaski, Physica A 383, 147 (2007), physics/0703061
    31. Structure and tie strengths in mobile communication networks, J.-P. Onnela, J. Saramäki, J. Hyvönen, G. Szabó, D. Lazer, K. Kaski, J. Kertész, and A.-L. Barabási, Proc. Natl. Acad. Sci. (USA) 104, 7332 (2007), physics/0610104
    32. Limited resolution in complex network community detection with Potts model approach, J. Kumpula, J. Saramäki, K. Kaski and J. Kertész, Eur. Phys. J. B 56, 41 (2007), cond-mat/0610370
    33. Generalizations of the clustering coefficient to weighted complex networks, J. Saramäki, M. Kivelä, J.-P. Onnela, K. Kaski and J. Kertész, Phys. Rev. E 75, 027105 (2007), cond-mat/0608670. Selected for the March 1, 2007 issue of Virtual Journal of Biological Physics Research
    34. A model for social networks, R. Toivonen, J.-P. Onnela, J. Saramäki, J. Hyvönen and K. Kaski, Physica A 371, 851 (2006), physics/0601114
    35. Spectrum, intensity and coherence in weighted networks of a financial market, G. Tibély, J.-P. Onnela, J. Saramäki, K. Kaski and J. Kertész, Physica A 370, 145 (2006), physics/0603196
    36. Financial market - a network perspective, J.-P. Onnela, J. Saramäki, K. Kaski and J. Kertész, in Practical Fruits of Econophysics: Nikkei Econophysics III Proceedings, H. Takayasu (Ed.), Springer, Tokyo, 302-306 (2006)
    37. Scale free networks from self-organisation, T. Evans and J. Saramäki, Phys. Rev. E 72, 026138 (2005), cond-mat/0411390
    38. Intensity and coherence of motifs in weighted complex networks, J.-P. Onnela, J. Saramäki, J. Kertész, and K. Kaski, Phys. Rev. E 71, 065103(R) (2005), cond-mat/0408629
    39. Spatial snowdrift game with myopic agents, M. Sysi-Aho, J. Saramäki, J. Kertész, and K. Kaski, Eur. Phys. J. B 44, 129-135 (2005), physics/0411091
    40. Modelling the development of epidemics with dynamic small-world networks, J. Saramäki and K. Kaski, J. Theor. Biol. 234/3, 413-421 (2005)
    41. Evidence of the invisible hand effect in an evolutionary minority game model, M. Sysi-Aho, J. Saramäki, and K. Kaski, Physica A 347 639-652 (2005), cond-mat/0403499
    42. A method for decentralized optimisation in networks, J. Saramäki, in Science of Complex Networks - AIP Conference Proceedings 776, J.F.F. Mendes et al. (eds.), 215 (2005).
    43. Characterizing motifs in weighted complex networks, J. Saramäki, J.-P. Onnela, J. Kertész, and K. Kaski, in Science of Complex Networks - AIP Conference Proceedings 776, J.F.F. Mendes et al. (eds.), 108 (2005).
    44. Modeling epidemics with dynamic small-world networks, K. Kaski and J. Saramäki, in Science of Complex Networks - AIP Conference Proceedings 776, J.F.F. Mendes et al. (eds.), 252 (2005).
    45. Scale-free networks generated by random walkers , J. Saramäki and K. Kaski, Physica A 341, 80-86 (2004), cond-mat/0404088
    Full list of publications (PDF)

    Teaching

    Past courses:

    Other Info

    Page updated 4.2.2010   URL: http://www.lce.hut.fi/~jsaramak/