Source: Albert and Barabasi, “Statistical mechanics of complex networks.” Review of Modern Physics . 74:48-94. (2002)
When I first decided to move to India, I wondered about how I could make new friends.
First, I met Bash at Microsoft, through an NGO called Asha for Eduation.
Bash suggested I contact Asha’s Bangalore chapter. On my second day in India, I ended up going on a field trip with Asha Bangalore, and met Ranjeet. We started to talk, and when I mentioned I had gone to graduate school at Yale, he said, “I know one person from Yale, but I think he was there earlier than you. His name was Sharad.”
Sharad was my roommate at Yale.
(Actually, Amitabh is closer to Kevin, but the minimal path of 4 uses two actors who always play minor parts in movies, so I used this path, which involves better known actors.)
Source: Watts (2003), Six Degrees ; Barabasi (2003), Linked .
Ben and I are good friends from college. Ben wrote a book, Bringing Down the House , for which the movie rights were sold to Kevin Spacey (I’ve seen a photo of Ben and Kevin posing in a picture with Hugh Hefner, of all people). Kevin is two movies away from Kevin Bacon.
Source: http://www.oakland.edu/enp/trivia.html
Milgram was a brilliant psychologist, notorious for his studies of obedience, where subjects continued to deliver what they believed to be painful electric shocks, just because a person in a lab coat told them they should do so as part of an experiment. Note on the small-worlds experiment: – Not many of the letters in the original experiment arrived. -- The experiment presaged some later discoveries, including hubs, tendency to go by geography and profession, etc.
This link to Milgram is my best guess, based on information I could find on the Internet: Allan Wagner was at the Yale Psychology department through 1961-1962, when Milgram did his experiments. Sternberg was chairman of the psych dept in 1992, during which time, both Allan Wagner and Mike Tarr were in the same department. I know Mike.
This is similar to phase transitions in physics, when gases become liquids and liquids become solids. (Even Bose-Einstein condensation is also relevant, it seems.)
The ln(N) expression is consistent with intuition about branching factors for trees. Even with lots of node repetition, if only 10% of each person’s contacts are not duplicates, links would rapidly span the entire population. I know Peter from the computer vision community. Peter was a student of Mumford’s, I think. David Mumford is a Fields-Medal-winning mathematician, who has singlehandedly reduced the Erdos number of most computer vision researchers by doing work in computer vision. Fan Chung is a mathematician who has authored papers with both Erdos and Mumford.
Source: Albert and Barabasi, “Statistical mechanics of complex networks.” Review of Modern Physics . 74:48-94. (2002)
Source: Albert and Barabasi, “Statistical mechanics of complex networks.” Review of Modern Physics . 74:48-94. (2002)
Source: Jon Kleinberg, “Navigation in a small world.” Science, 406:845 (2000).
Source: Jon Kleinberg, “Navigation in a small world.” Science, 406:845 (2000).
Source: Jon Kleinberg, “Navigation in a small world.” Science, 406:845 (2000).
Source: Duncan Watts, Six Degrees (2003). I know Ramin as a computer vision researcher. Ramin is in the CS department at Cornell; I assume he knows Jon Kleinberg.
Milgram was a brilliant psychologist, notorious for his studies of obedience, where subjects continued to deliver what they believed to be painful electric shocks, just because a person in a lab coat told them they should do so as part of an experiment. Note on the small-worlds experiment: – Not many of the letters in the original experiment arrived. -- The experiment presaged some later discoveries, including hubs, tendency to go by geography and profession, etc.
Source: Duncan Watts, Six Degrees (2003).
Source: Duncan Watts, Six Degrees (2003). Small-world phenomena of random graphs emerges even with less random links. Jonathan Donner will be working with us at MSR India. Jonathan is now at the Earth Institute at Columbia, where he knows Nobuyuki Hanaki, who has co-authored papers with Duncan Watts.
Transcript
1.
Social Networks First half based on slides by Kentaro Toyama, Microsoft Research, India And their applications to Web
2.
Networks—Physical & Cyber Typhoid Mary (Mary Mallon) Patient Zero (Gaetan Dugas)
6.
Society as a Graph People are represented as nodes.
7.
People are represented as nodes. Relationships are represented as edges. (Relationships may be acquaintanceship, friendship, co-authorship, etc.) Society as a Graph
8.
People are represented as nodes. Relationships are represented as edges. (Relationships may be acquaintanceship, friendship, co-authorship, etc.) Allows analysis using tools of mathematical graph theory Society as a Graph
21.
Trying to make friends Kentaro Ranjeet Bash Microsoft Asha
22.
Trying to make friends Kentaro Ranjeet Bash Sharad Microsoft Asha New York City Yale Ranjeet and I already had a friend in common!
23.
I didn’t have to worry… Kentaro Bash Karishma Sharad Maithreyi Anandan Venkie Soumya
24.
It’s a small world after all! Kentaro Ranjeet Bash Karishma Sharad Maithreyi Anandan Prof. Sastry Venkie PM Manmohan Singh Prof. Balki Pres. Kalam Prof. Jhunjhunwala Dr. Montek Singh Ahluwalia Ravi Dr. Isher Judge Ahluwalia Pawan Aishwarya Prof. McDermott Ravi’s Father Amitabh Bachchan Prof. Kannan Prof. Prahalad Soumya Nandana Sen Prof. Amartya Sen Prof. Veni Rao
Aidan Quinn Kevin Spacey Kentaro Toyama Bringing Down the House (2004) Cavedweller (2004) Looking for Richard (1996) Ben Mezrich Roommates in college (1991)
Dimitris Achlioptas Bernard Schoelkopf Kentaro Toyama Andrew Blake Mike Molloy Toyama, K. and A. Blake (2002). Probabilistic tracking with exemplars in a metric space. International Journal of Computer Vision . 48(1):9-19. Romdhani, S., P. Torr, B. Schoelkopf, and A. Blake (2001). Computationally efficient face detection. In Proc. Int’l. Conf. Computer Vision , pp. 695-700. Achlioptas, D., F. McSherry and B. Schoelkopf. Sampling Techniques for Kernel Methods. NIPS 2001 , pages 335-342. Achlioptas, D. and M. Molloy (1999). Almost All Graphs with 2.522 n Edges are not 3-Colourable. Electronic J. Comb. (6), R29. Alon, N., P. Erdos, D. Gunderson and M. Molloy (2002). On a Ramsey-type Problem. J. Graph Th. 40, 120-129. An Example
John Guare wrote a play called Six Degrees of Separation, based on this concept.
Milgram (1967) “ Everybody on this planet is separated by only six other people. Six degrees of separation. Between us and everybody else on this planet. The president of the United States. A gondolier in Venice… It’s not just the big names. It’s anyone. A native in a rain forest. A Tierra del Fuegan. An Eskimo. I am bound to everyone on this planet by a trail of six people…” Robert Sternberg Mike Tarr Kentaro Toyama Allan Wagner ?
A pair of nodes has probability p of being connected.
Average degree, k ≈ pN
What interesting things can be said for different values of p or k ?
(that are true as N ∞ )
Erdős and Renyi (1959) p = 0.0 ; k = 0 N = 12 p = 0.09 ; k = 1 p = 1.0 ; k ≈ ½ N 2
40.
Random Graphs Erdős and Renyi (1959) p = 0.0 ; k = 0 p = 0.09 ; k = 1 p = 1.0 ; k ≈ N p = 0.045 ; k = 0.5 Let’s look at… Size of the largest connected cluster Diameter (maximum path length between nodes) of the largest cluster If Diameter is O(log(N)) then it is a “Small World” network Average path length between nodes (if a path exists)
42.
Random Graphs Erdős and Renyi (1959) p = 0.0 ; k = 0 p = 0.09 ; k = 1 p = 1.0 ; k ≈ N p = 0.045 ; k = 0.5 Size of largest component Diameter of largest component Average path length between (connected) nodes 1 5 11 12 0 4 7 1 0.0 2.0 4.2 1.0
But, many real-world networks exhibit a power-law distribution.
also called “Heavy tailed” distribution
Degree distribution of a random graph, N = 10,000 p = 0.0015 k = 15. (Curve is a Poisson curve, for comparison.) Typically 2<r<3. For web graph r ~ 2.1 for in degree distribution 2.7 for out degree distribution Note that poisson decays exponentially while power law decays polynomially Long tail k -r Sharp drop Rare events are not so rare! Both mean and variance are
Power-law distributions are straight lines in log-log space.
-- slope being r
y=k -r log y = -r log k ly= -r lk
How should random graphs be generated to create a power-law distribution of node degrees?
Hint:
Pareto’s * Law: Wealth distribution follows a power law.
Albert and Barabasi (1999) Power laws in real networks: (a) WWW hyperlinks (b) co-starring in movies (c) co-authorship of physicists (d) co-authorship of neuroscientists * Same Velfredo Pareto, who defined Pareto optimality in game theory.
Paths between nodes can be computed by shortest path algorithms
E.g. All pairs shortest path
..so, small-world ness is trivial to exploit..
This is what ORKUT, Friendster etc are trying to do..
Case 2: Local access to network structure
Each node only knows its own neighborhood
Search without children-generation function
Idea 1: Broadcast method
Obviously crazy as it increases traffic everywhere
Idea 2: Directed search
But which neighbors to select?
Are there conditions under which decentralized search can still be easy?
How does a node in a social network find a path to another node? 6 degrees of separation will lead to n 6 search space (n=num neighbors) Easy if we have global graph.. But hard otherwise Computing one’s Erdos number used to take days in the past! There are very few “fully decentralized” search applications. You normally have hybrid methods between Case 1 and Case 2
Searchability is measured in terms of Expected time to go from a random source to a random destination
We know that in Smallworld networks, the diameter is exponentially smaller than the size of the network.
If the expected time is proportional to some small power of log N, we are doing well
Qn: Is this always the case in small world networks?
To begin to answer this we need to look generative models that take a notion of absolute (lattice or coordinate-based) neighborhood into account
Kleinberg experimented with Lattice networks (where the network is embedded in a lattice—with most connections to the lattice neighbors, but a few shortcuts to distant neighbors)
A network is considered to exhibit small world phenomenon, if its diameter is approximately logarithm of its size (in terms of number of nodes)
Most uniform random networks exhibit small world phenomena
Most real world networks are not uniform random
Their in degree distribution exhibits power law behavior
However, most power law random networks also exhibit small world phenomena
But they are brittle against attack
The fact that a network exhibits small world phenomenon doesn’t mean that an agent with strictly local knowledge can efficiently navigate it (i.e, find paths that are O(log(n)) length
It is always possible to find the short paths if we have global knowledge
This is the case in the FOAF (friend of a friend) networks on the web
59.
Other Power Laws of Interest to CSE494 If this is the power-law curve about in degree distribution, where is Google page on this curve? Homework 2 will be due next week Mid-term is most likely to be on 10/16 Project 2 will be given by the end of next week
60.
Zipf’s Law: Power law distriubtion between rank and frequency
In a given language corpus, what is the approximate relation between the frequency of a k th most frequent word and (k+1) th most frequent word?
For s>1 Most popular word is twice as frequent as the second most popular word! Law of categories in Marketing… Digression Word freq in wikipedia f=1/r
What is the relation between the size of a corpus (in terms of words) and the size of the lexicon (vocabulary)?
V = K n b
K ~ 10—100
b ~ 0.4 – 0.6
So vocabulary grows as a square root of the corpus size..
Explanation? --Assume that the corpus is generated by randomly picking words from a zipfian distribution.. Notice the impact of Zipf on generating random text corpuses!
How often does the digit 1 appear in numerical data describing natural phenomenon?
You would expect 1/9 or 11%
Digression begets its own digression This law holds so well in practice that it is used to catch forged data!! http://mathworld.wolfram.com/BenfordsLaw.html WHY? Iff there exists a universal distribution, it must be scale invariant (i.e., should work in any units) starting from there we can show that the distribution must satisfy the differential eqn x P’(x) = -P(x) For which, the solution is P(x)=1/x ! 0.0791812 5 0.0457575 9 0.09691 4 0.0511525 8 0.124939 3 0.0579919 7 0.176091 2 0.0669468 6 0.30103 1
64.
2/15 Review power laws Small-world phenomena in scale-free networks Link analysis for Web Applications
Watts, Dodds, Newman (2002) show that for d = 2 or 3, real networks are quite searchable.
the dimensions are things like “geography”, “profession”, “hobbies”
Killworth and Bernard (1978) found that people tended to search their networks by d = 2: geography and profession.
Kleinberg (2000) The Watts-Dodds-Newman model closely fitting a real-world experiment Ramin Zabih Kentaro Toyama
69.
..but didn’t Milgram’s letter experiment show that navigation is easy?
… may be not
A large fraction of his test subjects were stockbrokers
So are likely to know how to reach the “goal” stockbroker
A large fraction of his test subjects were in boston
As was the “goal” stockbroker
A large fraction of letters never reached
Only 20% reached
So how about (re)doing Milgram experiment with emails?
People are even more burned out with (e)mails now
Success rate for chain completion < 1% !
70.
Credits Albert, Reka and A.-L. Barabasi. “Statistical mechanics of complex networks.” Reviews of Modern Physics , 74(1):47-94. (2002) Barabasi, Albert-Laszlo. Linked . Plume Publishing. (2003) Kleinberg, Jon M. “Navigation in a small world.” Science, 406:845. (2000) Watts, Duncan. Six Degrees: The Science of a Connected Age . W. W. Norton & Co. (2003)
model: Add edges to nodes, as in random graphs, but makes links more likely when two nodes have a common friend.
For a range of values:
The world is small (average path length is short), and
Groups tend to form (high clustering coefficient).
Watts (1999) Clustering coefficient / Normalized path length Clustering coefficient ( C ) and average path length ( L ) plotted against
77.
The Beta Model Watts and Strogatz (1998) = 0 = 0.125 = 1 People know others at random. Not clustered, but “small world” People know their neighbors, and a few distant people. Clustered and “ small world” People know their neighbors. Clustered, but not a “small world”
First five random links reduce the average path length of the network by half, regardless of N !
Both and models reproduce short-path results of random graphs, but also allow for clustering.
Small-world phenomena occur at threshold between order and chaos.
Watts and Strogatz (1998) Nobuyuki Hanaki Jonathan Donner Kentaro Toyama Clustering coefficient / Normalized path length Clustering coefficient ( C ) and average path length ( L ) plotted against
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