Complex networks A. Barrat, LPT, Université Paris-Sud, France I. Alvarez-Hamelin (LPT, Orsay, France) M. Barthélemy (CEA, France) L. Dall’Asta (LPT, Orsay, France) R. Pastor-Satorras (Barcelona, Spain) A. Vespignani (LPT, Orsay, France)

● Complex networks: examples ● Small-world networks ● Scale-free networks: evidences, modeling, tools for characterization ● Consequences of SF structure ● Perspectives: weighted complex networks Plan of the talk

Examples of complex networks ● Internet ● WWW ● Transport networks ● Protein interaction networks ● Food webs ● Social networks ●...

Social networks: Milgram’s experiment Milgram, Psych Today 2, 60 (1967) Dodds et al., Science 301, 827 (2003) “Six degrees of separation”

Small-world properties: also in the Internet Distribution of chemical distances between two nodes Average fraction of nodes within a chemical distance d

Usual random graphs: Erdös-Renyi model (1960) BUT... N points, links with proba p: static random graphs short distances (log N) Poisson distribution (p=O(1/N))

Clustering coefficient C = # of links between 1,2,…n neighbors n(n-1)/ n Higher probability to be connected Clustering: My friends will know each other with high probability! (typical example: social networks)

Asymptotic behavior LatticeRandom graph

In-between: Small-world networks Watts & Strogatz, Nature 393, 440 (1998) N = 1000 Large clustering coeff. Short typical path N nodes forms a regular lattice. With probability p, each edge is rewired randomly =>Shortcuts

Size-dependence Amaral & Barthélemy Phys Rev Lett 83, 3180 (1999) Newman & Watts, Phys Lett A 263, 341 (1999) Barrat & Weigt, Eur Phys J B 13, 547 (2000) p >> 1/N => Small-world structure

Is that all we need ? NO, because... Random graphs, Watts-Strogatz graphs are homogeneous graphs (small fluctuations of the degree k): While.....

Airplane route network

CAIDA AS cross section map

Scale-free properties P(k) = probability that a node has k links P(k) ~ k -  (    3) = const   Diverging fluctuations The Internet and the World-Wide-Web Protein networks Metabolic networks Social networks Food-webs and ecological networks Are Heterogeneous networks Topological characterization

Exp. vs. Scale-Free Poisson distribution Exponential Network Power-law distribution Scale-free Network

Main Features of complex networks Many interacting units Self-organization Small-world Scale-free heterogeneity Dynamical evolution Many interacting units Self-organization Small-world Scale-free heterogeneity Dynamical evolution Standard graph theory Static Ad-hoc topology Random graphs

Two important observations (1) The number of nodes (N) is NOT fixed. Networks continuously expand by the addition of new nodes Examples: WWW : addition of new documents Citation : publication of new papers (2) The attachment is NOT uniform. A node is linked with higher probability to a node that already has a large number of links. Examples : WWW : new documents link to well known sites (CNN, YAHOO, NewYork Times, etc) Citation : well cited papers are more likely to be cited again Origins SF

Scale-free model (1) GROWTH : A t every timestep we add a new node with m edges (connected to the nodes already present in the system). (2) PREFERENTIAL ATTACHMENT : The probability Π that a new node will be connected to node i depends on the connectivity k i of that node A.-L.Barabási, R. Albert, Science 286, 509 (1999) P(k) ~k -3 BA model

BA network Connectivity distribution

More models Generalized BA model (Redner et al. 2000) (Mendes et al. 2000) (Albert et al. 2000) Non-linear preferential attachment :  (k) ~ k  Initial attractiveness :  (k) ~ A+k  Rewiring Highly clustered (Dorogovtsev et al. 2001) (Eguiluz & Klemm 2002) Fitness Model (Bianconi et al. 2001) Multiplicative noise (Huberman & Adamic 1999) (....)

Tools for characterizing the various models ● Connectivity distribution P(k) =>Homogeneous vs. Scale-free ● Clustering ● Assortativity ●... =>Compare with real-world networks

Topological correlations: clustering i k i =5 c i =0. k i =5 c i =0.1 a ij : Adjacency matrix

Topological correlations: assortativity k i =4 k nn,i =( )/4=4.5 i k=3 k=7 k=4

Assortativity ● Assortative behaviour: growing k nn (k) Example: social networks Large sites are connected with large sites ● Disassortative behaviour: decreasing k nn (k) Example: internet Large sites connected with small sites, hierarchical structure

Consequences of the topological heterogeneity ● Robustness and vulnerability ● Propagation of epidemics

Robustness Complex systems maintain their basic functions even under errors and failures (cell  mutations; Internet  router breakdowns) node failure fcfc 01 Fraction of removed nodes, f 1 S Robustness S: fraction of giant component

Case of Scale-free Networks s fcfc 1 Random failure f c =1 (    3) Attack =progressive failure of the most connected nodes f c <1 Internet maps R. Albert, H. Jeong, A.L. Barabasi, Nature (2000)

Failures vs. attacks 1 S 01 f fcfc Attacks   3 : f c =1 (R. Cohen et al PRL, 2000) Failures Topological error tolerance Robust-SF

Other attack strategies ● Most connected nodes ● Nodes with largest betweenness ● Removal of links linked to nodes with large k ● Removal of links with largest betweenness ● Cascades ●...

Betweenness  measures the “centrality” of a node i: for each pair of nodes (l,m) in the graph, there are  lm shortest paths between l and m  i lm shortest paths going through i b i is the sum of  i lm /  lm over all pairs (l,m) i j b i is large b j is small

Other attack strategies ● Most connected nodes ● Nodes with largest betweenness ● Removal of links linked to nodes with large k ● Removal of links with largest betweenness ● Cascades ●... P. Holme et al., P.R.E 65 (2002) A. Motter et al., P.R.E 66 (2002) , D. Watts, PNAS 99 (2002) 5766 Problem of reinforcement ?

Natural computer virus DNS-cache computer viruses Routing tables corruption Data carried viruses ftp, file exchange, etc. Internet topology Computer worms diffusing self-replicating network topology Ebel et al. (2002) Epidemic sprea on SF networks Epidemic spreading on SF networks EpidemiologyAir travel topology

Mathematical models of epidemics Coarse grained description of individuals and their state Individuals exist only in few states: Healthy or Susceptible * Infected * Immune * Dead Particulars on the infection mechanism on each individual are neglected. Topology of the system: the pattern of contacts along which infections spread in population is identified by a network Each node represents an individual Each link is a connection along which the virus can spread

Non-equilibrium phase transition epidemic threshold = critical point prevalence  =order parameter  cc Active phase Absorbing phase Finite prevalence Virus death The epidemic threshold is a general result The question of thresholds in epidemics is central (in particular for immunization strategies) Each node is infected with rate if connected to one or more infected nodes Infected nodes are recovered (cured) with rate  without loss of generality  =1 (sets the time scale) Definition of an effective spreading rate  =   =prevalence SIS model:

What about computer viruses? ● Very long average lifetime (years!) compared to the time scale of the antivirus ● Small prevalence in the endemic case  cc Active phase Absorbing phase Finite prevalence Virus death Computer viruses ??? Long lifetime + low prevalence = computer viruses always tuned infinitesimally close to the epidemic threshold ???

SIS model on SF networks SIS= Susceptible – Infected – Susceptible Mean-Field usual approximation: all nodes are “equivalent” (same connectivity) => existence of an epidemic threshold 1/ for the order parameter   density of infected nodes) Scale-free structure => necessary to take into account the strong heterogeneity of connectivities =>  k =density of infected nodes of connectivity k cc = =>epidemic threshold

Order parameter behavior in an infinite system cc = cc  0   Epidemic threshold in scale-free networks

Rationalization of computer virus data Wide range of spreading rate with low prevalence (no tuning) Lack of healthy phase = standard immunization cannot drive the system below threshold!!!

If    3 we have absence of an epidemic threshold and no critical behavior. If    4 an epidemic threshold appears, but it is approached with vanishing slope (no criticality). If   4 the usual MF behavior is recovered. SF networks are equal to random graph. If    3 we have absence of an epidemic threshold and no critical behavior. If    4 an epidemic threshold appears, but it is approached with vanishing slope (no criticality). If   4 the usual MF behavior is recovered. SF networks are equal to random graph. Results can be generalized to generic scale-free connectivity distributions P(k)~ k - 

Absence of an epidemic/immunization threshold The network is prone to infections (endemic state always possible) Small prevalence for a wide range of spreading rates Progressive random immunization is totally ineffective Infinite propagation velocity (NB: Consequences for immunization strategies) Main results for epidemics spreading on SF networks Pastor-Satorras & Vespignani (2001, 2002), Boguna, Pastor-Satorras, Vespignani (2003), Dezso & Barabasi (2001), Havlin et al. (2002), Barthélemy, Barrat, Pastor-Satorras, Vespignani (2004) Very important consequences of the SF topology!

Perspectives: Weighted networks ● Scientific collaborations ● Internet ● s ● Airports' network ● Finance, economic networks ●... => are weighted networks !!

Weights: examples ● Scientific collaborations: i, j: authors; k: paper; n k : number of authors  : 1 if author i has contributed to paper k (M. Newman, P.R.E. 2001) ● Internet, s: traffic, number of exchanged s ● Airports: number of passengers for the year 2002

Weights ● Weights: heterogeneous (broad distributions)? ● Correlations between topology and traffic ? ● Effects of the weights on the dynamics ?

Weights: recent works and perspectives ● Empirical studies (airport network; collaboration network: PNAS 2004) ● New tools (PNAS 2004) ● strength ● weighted clustering coefficient (vs. clustering coefficient) ● weighted assortativity (vs. assortativity) ● New models (PRL 2004) ● New effects on dynamics (resilience, epidemics...) on networks (work in progress)