Jari Saramäki

Associate Professor

Complex Systems research area

Dept. of Computer Science

Postal Address:
Aalto University
Department of Computer Science
P.O. Box 15400
FI-00076 AALTO, FINLAND
Visiting Address:
TUAS building, 3rd floor, room 3156, Otaniemi campus, Espoo
My Aalto People page
Tel.
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 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 worked in close collaboration with social scientists, computer scientists, and complexity scientists to uncover the dynamics of techno-social networks, and similar lines of research are continued in the project "Temporal networks of human interactions", funded by the Academy of Finland. I am also interested in networks of the human brain and network analysis of genetic information (stay tuned, first papers should be out in 2014).

Here is a popular-science-level talk on mobile phones & social networks.


In the news

  • Der Spiegel Jan 7, 2014: Neue Freunde verdrängen alte Freunde
  • The Times Jan 7, 2014: Old pals' act: why we only have so many new best friends
  • Fast Company Jan 6, 2014: Meet Your New Friends, The Same As Your Old Friends
  • Science Jan 6, 2014: The only thing constant about friendship may be the number of your friends
  • Yle Tiede Dec 5, 2013: Ystäväpiirien rajallisuus rajoittaa tiedonkulkua
  • Nature 22.8.2012: Computational social science: Making the links
  • United Nations Global Pulse 10.11.2011: Social Networks and Collective Human Behavior
  • 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. Two betweenness centrality measures based on Randomized Shortest Paths, I. Kivimäki, B. Lebichot, J. Saramäki, and M. Saerens, Scientific Reports 6, 19688 (2016), arXiv:1509.03147
    2. Exploring temporal networks with greedy walks, J. Saramäki and P. Holme, Eur. Phys. J. B 88, 334 (2015), arXiv:1508.00693
    3. From calls to communities: a model for time varying social networks, G. Laurent, J. Saramäki and M. Karsai, Eur. Phys. J. B 88, 301 (2015) arXiv:1506.00393
    4. On the digital daily cycles of individuals, T. Aledavood, J. Saramäki and S. Lehmann, Frontiers in Physics 3, 00073 (2015), arXiv:1507.08199
    5. From seconds to months: multi-scale dynamics of mobile telephone calls, J. Saramäki and E. Moro, Eur. Phys. J. B 88, 164 (2015), arXiv:1504.01479
    6. Reorganization of functionally connected brain subnetworks in high-functioning autism, E. Glerean, R. Kumar Pan, J. Salmi, R. Kujala, J. Lahnakoski, U. Roine, L. Nummenmaa, S. Leppämäki, T. Nieminen-von Wendt, P. Tani, J. Saramäki, M. Sams, and I.P. Jääskeläinen, arXiv:1503.04851 (2015)
    7. Daily rhythms in mobile telephone communication, T. Aledavood, E. López, S.G.B. Roberts, F. Reed-Tsochas, E. Moro, R.I.M. Dunbar, and J. Saramäki, arXiv:1502.06866 (2015)
    8. Spatial patterns of close relationships across the lifespan, H.-H. Jo, J. Saramäki, R.I.M. Dunbar, K. Kaski, Scientific Reports 4, 6988 (2014)
    9. EDENetworks: a user-friendly software to build and analyze networks in biogeography, ecology and population genetics, M. Kivelä, S. Arnaud-Haond, J. Saramäki, Mol. Ecol. Resour. doi: 10.1111/1755-0998.12290 (2014)
    10. Inferring human mobility using communication patterns, V. Palchykov, M. Mitrovic, H.-H. Jo, J. Saramäki, and R. Kumar Pan, Scientific Reports 4, 6174 (2014), arXiv:1404.7675
    11. Effects of temporal correlations on cascades: Threshold models on temporal networks, V.-P. Backlund, J. Saramäki, and R. Kumar Pan, Phys. Rev. E 89, 062815 (2014), arXiv: 1403.1177
    12. Adding network structure onto the map of collective behavior, S. Fortunato, J. Saramäki, J.-P. Onnela, Behav. Brain Sci. 37, 82 (2014)
    13. 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, Proc. Natl. Acad. Sci. (USA) 111, 942 (2014), arXiv:1204.5602
    14. Temporal motifs reveal homophily, gender-specific patterns and group talk in mobile communication networks, L. Kovanen, K. Kaski, J. Kertész, and J. Saramäki, Proc. Natl. Acad. Sci. (USA) 110, 18070 (2013), arXiv:1302.2563
    15. The evolution of interdisciplinarity in physics research, R.K. Pan, S. Sinha, K. Kaski, and J. Saramäki, Scientific Reports 2, 551 (2012)
    16. 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 1:4, arXiv:1202.1145 (2012)
    17. 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
    18. Networks of emotion concepts, R. Toivonen, M. Kivelä, J. Saramäki, M. Viinikainen, M. Vanhatalo, and M. Sams, PLoS One 7(1), e28883 (2012)
    19. Temporal networks, P. Holme and J. Saramäki, Physics Reports 519, 97-125, (2012), arXiv: 1108.1780
    20. The strength of strong ties in scientific collaboration networks, R.K. Pan and J. Saramäki, EPL 97, 18007 (2012), arXiv:1106.5249
    21. 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)
    22. Path lengths, correlations, and centrality in temporal networks, R.K. Pan and J. Saramäki, Phys. Rev. E 84, 016105 arXiv:1101.5913 (2011)
    23. 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
    24. 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
    25. 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
    26. Characterizing the community structure of complex networks, A. Lancichinetti, M. Kivelä, J. Saramäki, and S. Fortunato, PLoS One, 5(8), e11976 (2010)
    27. Reciprocity of mobile phone calls, L. Kovanen, J. Saramäki, and K. Kaski, Dynamics of Socio-Economic Systems, in press (2010), arXiv:1002.0763v1
    28. 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)
    29. Dynamics of latent voters, R. Lambiotte, J. Saramäki, and V.D. Blondel, Phys. Rev. E 79, 046107 (2009), arXiv: 0811.1464
    30. 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)
    31. 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
    32. 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
    33. 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
    34. 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
    35. 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)
    36. 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).
    37. Efficient data structures for sparse network representation, J. Hyvönen, J. Saramäki, and K. Kaski, Int. J. Comp. Math. 85, 1219 (2008)
    38. 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
    39. 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
    40. 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)
    41. 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
    42. 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
    43. 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
    44. 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
    45. 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
    46. 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
    47. 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
    48. A model for social networks, R. Toivonen, J.-P. Onnela, J. Saramäki, J. Hyvönen and K. Kaski, Physica A 371, 851 (2006), physics/0601114
    49. 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
    50. 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)
    51. Scale free networks from self-organisation, T. Evans and J. Saramäki, Phys. Rev. E 72, 026138 (2005), cond-mat/0411390
    52. 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
    53. 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
    54. Modelling the development of epidemics with dynamic small-world networks, J. Saramäki and K. Kaski, J. Theor. Biol. 234/3, 413-421 (2005)
    55. 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
    56. 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).
    57. 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).
    58. 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).
    59. Scale-free networks generated by random walkers , J. Saramäki and K. Kaski, Physica A 341, 80-86 (2004), cond-mat/0404088

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