Networks and sciences - VUB Artificial Intelligence Lab · PDF fileKenny Burrel Mon...

Networks and sciences:

The story of the “small-world” Hugues Bersini

IRIDIA – ULB

2013 Networks and sciences 1

The story begins with Stanley

Milgram (1933-1984)

In 1960, the famous experience of the submission to

authority

En 1967, the as famous experience of the six degrees

of separation

Stanley Milgram 2013 Networks and sciences 2

2013 Networks and sciences 3

The story moves on with Watts and

Strogatz (1998)

2013 Networks and sciences 4

But networks are far from homogeneous!!

Random

Scale-Free

With aggregates

2013 Networks and sciences 5

Poisson distribution

Exponential Network

Power-law distribution

Scale-free Network 2013 Networks and sciences 6

The story ends up with Babarasi (in the years 2000)

2013 Networks and sciences 7

Networks are « scale-free »:

P(k) ~k-

connecteurs

2013 Networks and sciences 8

The kingdom of hubs

Ron Carter

Terry Clark

Kenny Burrel

Mon réseau de musiciens

de Jazz

2013 Networks and sciences 9

Trophic Network

2013 Networks and sciences 10

How topology and hubs influence the global

behaviour of networks

• The small-world effect very small

distance among nodes

• Epidemic propagation (epidemiology or

viral marketting)

• Study of robustness: target (on hubs) or

random attacks

2013 Networks and sciences 11

Why scale-free ?

The rule of preferential attachment

Rich get richer

2013 Networks and sciences 12

Research at IRIDIA on networks

2013 Networks and sciences 13

Network Dynamics

• Homogeneous units ai (t) (the same temporal evolution – the same differential equations)

– dai/dt = F(aj, Wij,I)

• A given topology in the connectivity matrix: Wij

• Entries I which perturb the dynamics and to which the network gives meaning ( attractors)

• A very large family of concerned biological networks

– Idiotypic immune network

– Hopfield network

– Coupled Map Lattice

– Boolean network

– Ecological network (Lokta-Volterra)

– Genetic network

2013 Networks and sciences 14

Dynamics

Chemistry Ecosystem:

prey/predator Brain, heart

2013 Networks and sciences 15

Topology influences the dynamics of

immune network

2013 Networks and sciences 16

Frustrated chaos in biological networks

0

50

100

150

200

250A

b co

nc

0 50 100 150 200 250

time [d]

300 350 400

2-clone case 0 1

1 0

0

50

100

150

200

250

Ab

conc

entr

atio

ns

0 50 100 150 200 250

time [d]

300 350 400

0

100

200

300

400

500

0

Ab

conc

entr

atio

ns

100 200 300 400 500 600time [d]

700 800

3-clone open chain 0 1 0

1 0 1

0 1 0

3-clone closed chain

0 1 1

1 0 1

1 1 0

2013 Networks and sciences 17

Network of chemical reactions

Dynamics = kinetics

Metadynamics = appearance

and disappearance of molecules

2013 Networks and sciences 18

Immune Networks

2013 Networks and sciences 19

Neural Networks

2013 Networks and sciences 20

2013 Networks and sciences 21

Elementary dynamics:propagation

Epidemic propagation

2013 Networks and sciences 22

A new route to cooperation

2013 Networks and sciences 23

The prisoner's dilemma

P1/P2 Cooperate Compete

Cooperate (1,1) (-2,3)

Compete (3,-2) (-1,-1)

The winning strategy for both players is to compete. But

doing so, they miss the cooperating one which is collectively

better. The common good is subverted by individual rationality

and self-interest.

2013 Networks and sciences 24

But is competitive behaviour and

collective distress avoidable ?

• So far the prisoner's dilemma is lacking some crucial quality that real

world situations have.

• 1) Iterated version: play several moves and cumulate your reward over

these moves.

• 2) Distribute spatially the players (CA): each cell just cooperates with

its immediate neighbours and adapts the local best strategy. Cluster of

nice individuals emerge and can prosper in hostile environments ->

EVOLUTIONARY GAME THEORY

2013 Networks and sciences 25

The spatial cellular automata

simulation

• Largely inspired by Nowak’s work on spatial prisoner dilemma

• A cellular automata in which every cell contains one agent (specialist or generalist)

• In all cells, asynchronously, an agent will subsequently:

– interact with its neighbors (Moore neighborhood) to “consume” them.

• Sum the payoff according to the payoff matrix

– replicate

• Adopt the identity of the fittest neighbor

• For a given number of iteration steps

2013 Networks and sciences 26

2013 Networks and sciences 27

Nowak’s cooperators vs defectors

2013 Networks and sciences 28

Setting the stage

• Stochastic replicator dynamics:

– Vertex x plays kx times per

generation and accumulates payoff

fx.

– Choose a random neighbor y

with payoff fy.

– Replace strategy mx by my with

probability:

k4=3

k1=3

k2=4 k

2=4

k3=3 k

5=2

k6=5

p max 0,fx fy

k(T S)

2013 Networks and sciences 29

Games on graphs

• Conclusions:

– The more heterogeneous, the more cooperative.

– Cs benefit most from heterogeneity.

2013 Networks and sciences 30

Plastic networks: parametrically and

structurally: Network Metadynamics

• Various dynamical changes, that Varela called: dynamics and metadynamics

• This is the case for neuro, immuno, chemical, sociological networks, PC networks

- modification of connexions

- addition of connexions

- addition of new nodes

- suppression of existing nodes

The organisation is maintained

independently of the constituants

2013 Networks and sciences 31

A key interdependency

2013 Networks and sciences 32

Springer Link : With networks ?

33

• Crawler

• How ? – Python script

– HTTP Requests

• What ? – DOI

– References

– Date

2013 Networks and sciences

Results

34 2013 Networks and sciences

Book analysis

35 2013 Networks and sciences

Book analysis

36

• Clustering coefficient and Degree

2013 Networks and sciences

Systemic contagion in financial

systems

2013 Networks and sciences 37

Liasons Dangereuses:

Increasing connectivity,

risk sharing and systemic risk

(by Stefano Battiston,

Domenico Delli Gatti,

Mauro Gallegati,

Bruce C. Greenwald and Joseph E. Stiglitz)

Intraday network structure

of payment activity in

the Canadian

Large Value Transfer System (LVTS)

Financial Network

2013 Networks and sciences 38

2013 Networks and sciences 39

2013 Networks and sciences 40

2013 Networks and sciences 41

Conclusions: networks are everywhere

2013 Networks and sciences

42