Networks - Bonato 1 Modelling, Mining, and Searching Networks Anthony Bonato Ryerson University Master’s Seminar November 2012
Dec 25, 2015
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Modelling, Mining, and Searching Networks
Anthony BonatoRyerson University
Master’s SeminarNovember 2012
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21st Century Graph Theory:Complex Networks
• web graph, social networks, biological networks, internet networks, …
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• a graph G = (V(G),E(G)) consists of a nonempty set of vertices or nodes V, and a set of edges E
nodesedges
• directed graphs (digraphs)
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Degrees• the degree of a node x, written
deg(x)
is the number of edges incident with x
First Theorem of Graph Theory:
V(G)x
|E(G)|2deg(x)
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The web graph
• nodes: web pages
• edges: links
• over 1 trillion nodes, with billions of nodes added each day
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Ryerson
GreenlandTourism
Frommer’s
Four SeasonsHotel
City of Toronto
Nuit Blanche
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Small World Property
• small world networks introduced by social scientists Watts & Strogatz in 1998– low distances
between nodes
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Power laws in the web graph• power law degree distribution
(Broder et al, 01)
2 some ,, bniN bni
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Geometric models• we introduced a
stochastic network model which simulates power law degree distributions and other properties– Spatially Preferred
Attachment (SPA) Model
• nodes have a region of influence whose volume is a function of their degree
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SPA model (Aiello,Bonato,Cooper,Janssen,Prałat, 09)
• as nodes are born, they are more likely to enter a region of influence with larger volume (degree)
• over time, a power law degree distribution results
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Biological networks: proteomics
nodes: proteins
edges:
biochemical interactions
Yeast: 2401 nodes11000 edges
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Protein networks• proteins are essential
macromolecules of life• understanding their
function and role in disease is of importance
• protein-protein interaction networks (PPI)– nodes: proteins– edges:
biochemical interaction
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Domination sets in PPI (Milenkovic, Memisevic, Bonato, Przulj, 2011)
• dominating sets in graphs
• we found that dominating sets in
PPI networks are vital for normal
cellular functioning and signalling– dominating sets capture biologically
vital proteins and drug targets– might eventually lead to new drug
therapies
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On-line Social Networks (OSNs)Facebook, Twitter, LinkedIn, Google+…
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Lady Gaga is the centre of Twitterverse
Dalai Lama
Lady Gaga
Anderson Cooper
Queen Rania of Jordan
Arnold Schwarzenegger
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6 degrees of separation
• Stanley Milgram: famous chain letter experiment in 1967
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6 Degrees in Facebook?• 1 billion users, > 70
billion friendship links• (Backstrom et al., 2012)
– 4 degrees of separation in Facebook
– when considering another person in the world, a friend of your friend knows a friend of their friend, on average
• similar results for Twitter and other OSNs
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Dimension of an OSN
• dimension of OSN: minimum number of attributes needed to classify nodes
• like game of “20 Questions”: each question narrows range of possibilities
• what is a credible mathematical formula for the dimension of an OSN?
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GEO-P model (Bonato, Janssen, Prałat, 2012)
• reverse engineering approach– given network data GEO-P model predicts dimension
of an OSN; i.e. the smallest number of attributes needed to identify users
• that is, given the graph structure, we can (theoretically) recover the social space
Cops and Robbers
• played on reflexive undirected graphs G• two players Cops C and robber R play at alternate
time-steps (cops first) with perfect information• players move to vertices along edges; allowed to
moved to neighbors or pass • cops try to capture (i.e. land on) the robber, while
robber tries to evade capture• minimum number of cops needed to capture the
robber is the cop number c(G)– well-defined as c(G) ≤ |V(G)|
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Applications of Cops and Robbers
• moving target search– missile-defense– gaming
• counter-terrorism– intercepting messages
or agents
How big can the cop number be?
• if the graph G with order n is disconnected, then the cop number can be as n
• if G is connected, then no one knows how big the cop number can be!
• Meyniel’s Conjecture: c(G) = O(n1/2).
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Example of a variantThe robber fights back!
• robber can attack neighbouring cop
• one more cop needed in this graph (check)• Conjecture: For any graph with this modified game, one
more cop needed than for usual cop number.
C
C
C
R
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Thesis topics• what precisely is a community in a complex
network? • biological network models
– more exploration of dominating sets in PPI
• fit GEO-P model to OSN data– machine learning techniques
• new models for complex networks• Cops and Robbers games
– Meyniel’s conjecture, random graphs, variations: good vs bad guy games in graphs
Good guys vs bad guys games in graphs
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slow medium fast helicopter
slow traps, tandem-win
medium robot vacuum Cops and Robbers edge searching eternal security
fast cleaning distance k Cops and Robbers
Cops and Robbers on disjoint edge sets
The Angel and Devil
helicopter seepage Helicopter Cops and Robbers, Marshals, The Angel and Devil,Firefighter
Hex
badgood
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Brief biography
• over 80 papers, two books, two edited proceedings, with 40 collaborators (many of which are my students)
• over 250K in research funding in past 6 years– grants from NSERC, Mprime, and Ryerson
• supervised 8 masters students, 2 doctoral, and 7 post-docs
• over 30 invited addresses world-wide (India, China, Europe, North America)
• won 2011 and 2009 Ryerson Research awards• editor-in-Chief of journal Internet Mathematics; editor of
Contributions to Discrete Mathematics
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AM8204 – Topics in Discrete Mathematics
• Winter 2012• 6 weeks each: complex networks, graph
searching• project based• Prequisite: AM8002 (or permission from
me)