Small-world phenomenon Small-world phenomenon Jeroen Keijser - PowerPoint PPT Presentation

Small-world phenomenon Small-world phenomenon Jeroen Keijser Jeroen Keijser March 18, 2003 March 18, 2003

Table of Contents Table of Contents ß Introduction ß Introduction ß Do they really exist? Do they really exist? ß - Colloquial definition - Colloquial definition - Kevin Bacon - Kevin Bacon ß ß Duncan Watts Duncan Watts - Power Grid - Power Grid - motivation - motivation - Worm - Worm - mathematical - mathematical ß Dynamic Systems Dynamic Systems ß definition a,b,f,y definition a,b,f,y - Disease Spreading - Disease Spreading ß ß

Table of Contents Cont. Table of Contents Cont. ß As a Computational As a Computational ß Conclusions/Questions Conclusions/Questions ß ß Model Model - cellular automata - cellular automata - b revisited - b revisited ß Small world after all? Small world after all? ß -Milgram revisited -Milgram revisited

“Colloquial definition Colloquial definition” ” “ ß You meet someone on the train, at a party, You meet someone on the train, at a party, ß at some show, (perhaps even in another at some show, (perhaps even in another country) and you realize that you have a country) and you realize that you have a common acquaintance/friend. In surprise common acquaintance/friend. In surprise you exclaim, “ “It It’ ’s a small world s a small world…” …” you exclaim,

“Six Degrees of Separation Six Degrees of Separation” ” “ ß The common belief that between any one The common belief that between any one ß person(say a Calgary Computer Scientist) person(say a Calgary Computer Scientist) and any other one person (say Saddam and any other one person (say Saddam Hussein) there are at most only 6 links… … ( a ( a Hussein) there are at most only 6 links friend of brother of spouse of … …etc.) etc.) friend of brother of spouse of

Where’ ’d it come from? d it come from? Where ß Stanley Milgram Stanley Milgram “ “Small World Study Small World Study” ” 1967, 1967, ß published in Psychology Today, Today, coined the coined the published in Psychology phrase “ “six degrees of separation six degrees of separation” ” applying applying phrase it to the American population. it to the American population. ß Quickly was adopted into popular culture Quickly was adopted into popular culture ß and applied to the whole world. and applied to the whole world.

Duncan Watts arrives... Duncan Watts arrives... Mathematical Motivation Mathematical Motivation ß Ordered to Random spectrum of Ordered to Random spectrum of ß Networks(graphs). Networks(graphs). ß Ordered graphs and random graphs are Ordered graphs and random graphs are ß generally both well understood and readily generally both well understood and readily defined. defined. ß Region between the two is not well Region between the two is not well ß understood and small-world graphs appear understood and small-world graphs appear to be right in the middle. to be right in the middle.

Connected Cave Man Connected Cave Man

Toward a Mathematical Definition Toward a Mathematical Definition ß To be of interest a small world graph must be : To be of interest a small world graph must be : ß highly clustered (i.e. you have many close friends a) highly clustered a) (i.e. you have many close friends who are all friends of each other), and who are all friends of each other), and sparse ( it is a big world and not everyone knows b) sparse ( it is a big world and not everyone knows b) everyone) everyone) Yet there is only a small degree of separation Yet there is only a small degree of separation between any two people. between any two people.

a -Model R i,j = (5) where R i,j = a measure of vertex i ’s propensity to connect 1 ; m k ≥ i , j m j i , [ ] ( 1 - ) p p k m 0 a + > > i , j k m 0 = p i , j a £ a . £ •

Need for a substrate Need for a substrate ß Without a substrate a caveman effect occurs Without a substrate a caveman effect occurs ß for low a and small disconnected graphs for low a and small disconnected graphs occur (infinite characteristic length). occur (infinite characteristic length). ß Watts makes a restriction on the model that Watts makes a restriction on the model that ß graphs to be considered must be connected. graphs to be considered must be connected. He uses a ring substrate for the a -model. He uses a ring substrate for the a -model.

-Model b -Model b

and y models F and y models F ß Definition 3.1.4. Definition 3.1.4. Given a graph of M = Given a graph of M = ß (k . . n)/2 edges, the fraction of those edges n)/2 edges, the fraction of those edges (k that are shortcuts is denoted by f . that are shortcuts is denoted by f . ß Definition 3.1.6. Definition 3.1.6. Y is the fraction of all pairs ß Y is the fraction of all pairs of vertices that are not connected and have of vertices that are not connected and have one and only one common neighbour. one and only one common neighbour.

Do they really exist? Do they really exist? ß Kevin Bacon game (Rod Steiger) Kevin Bacon game (Rod Steiger) ß ß Power Grid Power Grid ß ß the C. elegans the C. elegans (Caenorhabditis elegans ) worm ß (Caenorhabditis elegans ) worm

Dynamic Systems Dynamic Systems ß Disease spreading (susceptible, infectious, Disease spreading (susceptible, infectious, ß removed) removed) ß Topology is important: Small world graphs Topology is important: Small world graphs ß seem to be in the critical area where the seem to be in the critical area where the disease will begin to take over the whole disease will begin to take over the whole population. population.

As a Computational Model As a Computational Model ß Remember cellular automata Remember cellular automata ß ß They have an ordered lattice structure. They have an ordered lattice structure. ß

The b -Model revisited The b -Model revisited

Density and Synchronization Density and Synchronization Problems Problems ß Majority rules doesn Majority rules doesn’ ’t work for cellular t work for cellular ß automata, but does for small world graphs. automata, but does for small world graphs. Therein can treat each node as a simple Therein can treat each node as a simple CPU. CPU. ß For both problems small world graphs had For both problems small world graphs had ß better results (for density 0.9 versus 0.769) better results (for density 0.9 versus 0.769)

ß For other problems similar to cellular For other problems similar to cellular ß automata it is hard to see an approach to a automata it is hard to see an approach to a solution. solution. ß Insight: Insight: Should probably apply genetic Should probably apply genetic ß algorithms not only to the rule set but also to algorithms not only to the rule set but also to all the possible connectivities. all the possible connectivities.

Is it a small world after all? Is it a small world after all? ß Milgram Experiment revisited. Milgram Experiment revisited. ß ß Send a package from some random person Send a package from some random person ß to some other random destination person in to some other random destination person in America. America. ß Only 30% made it through. Only 30% made it through. ß

Questions? Questions?

Fin. Fin.