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Published byClara Day Modified over 9 years ago
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Generative Models for the Web Graph José Rolim
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Aim Reproduce emergent properties: –Distribution site size –Connectivity of the Web –Power law distriubutions –Small World Properties
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Classical Model Random Graphs Erdos-Renyi Graph G(n,p) – n number of nodes – p probability of connextion pc threshold probability – p < pc -many disconnected components – p=pc - a large connected component – p=1 – a complete graph
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Limitations To model the web graph: –Constant number of nodes –Same probability among sommets – etc, etc
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Web Page Growth Model Sites with short term (daily) size fluctuations proportional to their size Assume an overall growth rate a such that: – S(t+1)=a(1+vb)S(t) – S(t)= # pages of site s at time t – v=+-1 – Bernouilli variable avec prob. 0.5 – b= absolute rate of daily fluctuations
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Web Page Growth Donc: S(T)= a T S(0) π T 0 (1+n i b) ou: logS(T)=Tloga+logS(0)+Σ T 0 log(1+n i b)= l log(1+b)+(T-l)log(1-b) –l= # positive fluctuations Therefore: S(T) has a lognormal distribution or follows a power law:
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Web page growth Probability P(s) of a site to have s pages: –P(S)=Σ i P(s/b i )P(b i )= Σ i c i /S gi = c/S g –Power Law g has been experimentaly evaluated for the web as between 1.6 and 2.0
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Small world models Properties: –Sparse –Cliquishness –Small Diameter Two models –Edge-reassigning small world network –Edge addition small world network
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Edge reassigning model Evolution starts with a ring of n nodes and each node connected to d nearest neighbors Then each edge is randomly reassigned to distant nodes with probability p in a round robin fashion See example page 10 with n=10 and d=4
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Edge addition model At the original ring additional edges are added randomly giving an expected number –p.d.n/2 new edges –p probability of addition of an edge –See example page 13 Criticism to small world: –No newpages neither deletion of pages –No deletion of links
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Rich get richer Preferential attachement model Start with a null graph with no nodes At each time step add a new node and connect it to m nodes selected randomly with probability proportional to their degree See ex. page 16
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Important measures Average diameter Cliquishness ( measure the average density of local connections): –Take a node v sith degree d –Its d neighbors have max=d.(d-1)/2 links –Let c v =real number of links / max –C= Σ v c v /V.
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Remarks on rich get richer Reproduces the power law of number of links. Eg: the probability of a page i to have degree di is A/di c –A is proportional to the square of the network – c is a constant – c was found empirically to be 2.9 and theoretically 3
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Criticism on Rich Get Richer Does not allow reconnection of existing edges Addition of new edges take place only when new nodes are added
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Copy models At each time step a node is added –With prob. p a new edge is created between this node and a randomly chosen node –With prob. 1-p: we choose randomly a node and uniformly one of the out edges and we link the new node to the node that this chosen edge enters.
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Remarks Why is called copy? There are more elaborated models which allow addition of more than a edge each time It is also a sort of « rich get richer »
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Applications Distributed search algorithms Subgraph patterns and communities Robusteness and vulnerability Page rank algorithms
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