# Why graph clustering is useful?

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1 Graph Clustering

2 Why graph clustering is useful? Distance matrices are graphs as useful as any other clustering Identification of communities in social networks Webpage clustering for better data management of web data

3 Outline Min s-t cut problem Min cut problem Multiway cut Minimum k-cut Other normalized cuts and spectral graph partitionings

4 Min s-t cut Weighted graph G(V,E) An s-t cut C = (S,T) of a graph G = (V, E) is a cut partition of V into S and T such that s S and t T Cost of a cut: Cost(C) = Σ e(u,v) uєs, v ЄT w(e) Problem: Given G, s and t find the minimum cost s-t cut

5 Max flow problem Flow network Abstraction for material flowing through the edges G = (V,E) directed graph with no parallel edges Two distinguished nodes: s = source, t= sink c(e) = capacity of edge e

6 Cuts An s-t cut is a partition (S,T) of V with sєs and tєt capacity of a cut (S,T) is cap(s,t) = Σ e out of S c(e) Find s-t cut with the minimum capacity: this problem can be solved optimally in polynomial time by using flow techniques

7 Flows An s-t flow is a function that satisfies For each eєe 0 f(e) c(e) [capacity] For each vєv-{s,t}: Σ e in to v f(e) = Σ e out of v f(e) [conservation] The value of a flow f is: v(f) = Σ e out of s f(e)

8 Max flow problem Find s-t flow of maximum value

9 Flows and cuts Flow value lemma: Let f be any flow and let (S,T) be any s-t cut. Then, the net flow sent across the cut is equal to the amount leaving s Σ e out of S f(e) Σ e in to S f(e) = v(f)

10 Flows and cuts Weak duality: Let f be any flow and let (S,T) be any s-t cut. Then the value of the flow is at most the capacity of the cut defined by (S,T): v(f) cap(s,t)

11 Certificate of optimality Let f be any flow and let (S,T) be any cut. If v(f) = cap(s,t) then f is a max flow and (S,T) is a min cut. The min-cut max-flow problems can be solved optimally in polynomial time!

12 Setting Connected, undirected graph G=(V,E) Assignment of weights to edges: w: E R + Cut: Partition of V into two sets: V, V-V. The set of edges with one end point in V and the other in V define the cut The removal of the cut disconnects G Cost of a cut: sum of the weights of the edges that have one of their end point in V and the other in V-V

13 Min cut problem Can we solve the min-cut problem using an algorithm for s-t cut?

14 Randomized min-cut algorithm Repeat : pick an edge uniformly at random and merge the two vertices at its end-points If as a result there are several edges between some pairs of (newly-formed) vertices retain them all Edges between vertices that are merged are removed (no selfloops) Until only two vertices remain The set of edges between these two vertices is a cut in G and is output as a candidate min-cut

15 Example of contraction e

16 Observations on the algorithm Every cut in the graph at any intermediate stage is a cut in the original graph

17 Analysis of the algorithm C the min-cut of size k G has at least kn/2 edges Why? E i : the event of not picking an edge of C at the i-th step for 1 i n-2 Step 1: Probability that the edge randomly chosen is in C is at most 2k/(kn)=2/n Pr(E 1 ) 1-2/n Step 2: If E 1 occurs, then there are at least n(n-1)/2 edges remaining The probability of picking one from C is at most 2/(n-1) Pr(E 2 E 1 ) = 1 2/(n-1) Step i: Number of remaining vertices: n-i+1 Number of remaining edges: k(n-i+1)/2 (since we never picked an edge from the cut) Pr(Ei Π j=1 i-1 E j ) 1 2/(n-i+1) Probability that no edge in C is ever picked: Pr(Π i=1 n-2 E i ) Π i=1 n-2 (1-2/(n-i+1))=2/(n 2 -n) The probability of discovering a particular min-cut is larger than 2/n 2 Repeat the above algorithm n 2 /2 times. The probability that a min-cut is not found is (1-2/n 2 ) n^2/2 < 1/e

18 Multiway cut (analogue of s-t cut) Problem: Given a set of terminals S = {s 1,,s k } subset of V, a multiway cut is a set of edges whose removal disconnects the terminals from each other. The multiway cut problem asks for the minimum weight such set. The multiway cut problem is NP-hard (for k>2)

19 Algorithm for multiway cut For each i=1,,k, compute the minimum weight isolating cut for s i, say C i Discard the heaviest of these cuts and output the union of the rest, say C Isolating cut for s i : The set of edges whose removal disconnects s i from the rest of the terminals How can we find a minimum-weight isolating cut? Can we do it with a single s-t cut computation?

20 Approximation result The previous algorithm achieves an approximation guarantee of 2-2/k Proof

21 Minimum k-cut A set of edges whose removal leaves k connected components is called a k-cut. The minimum k-cut problem asks for a minimum-weight k-cut Recursively compute cuts in G (and the resulting connected components) until there are k components left This is a (2-2/k)-approximation algorithm

22 Minimum k-cut algorithm Compute the Gomory-Hu tree T for G Output the union of the lightest k-1 cuts of the n-1 cuts associated with edges of T in G; let C be this union The above algorithm is a (2-2/k)- approximation algorithm

23 Gomory-Hu Tree T is a tree with vertex set V The edges of T need not be in E Let e be an edge in T; its removal from T creates two connected components with vertex sets (S,S ) The cut in G defined by partition (S,S ) is the cut associated with e in G

24 Gomory-Hu tree Tree T is said to be the Gomory-Hu tree for G if For each pair of vertices u,v in V, the weight of a minimum u-v cut in G is the same as that in T For each edge e in T, w (e) is the weight of the cut associated with e in G

25 Min-cuts again What does it mean that a set of nodes are well or sparsely interconnected? min-cut: the min number of edges such that when removed cause the graph to become disconnected small min-cut implies sparse connectivity min E U,V U U i U j V A i, j U U V-U

26 Measuring connectivity What does it mean that a set of nodes are well interconnected? min-cut: the min number of edges such that when removed cause the graph to become disconnected not always a good idea! U V-U U V-U

27 Graph expansion Normalize the cut by the size of the smallest component Cut ratio: Graph expansion: α G α E U,V -U min U, V U E U,V -U min U min U, V U We will now see how the graph expansion relates to the eigenvalue of the adjacency matrix A

28 Spectral analysis The Laplacian matrix L = D A where A = the adjacency matrix D = diag(d 1,d 2,,d n ) d i = degree of node i Therefore L(i,i) = d i L(i,j) = -1, if there is an edge (i,j)

29 Laplacian Matrix properties The matrix L is symmetric and positive semidefinite all eigenvalues of L are positive The matrix L has 0 as an eigenvalue, and corresponding eigenvector w 1 = (1,1,,1) λ 1 = 0 is the smallest eigenvalue

30 The second smallest eigenvalue The second smallest eigenvalue (also known as Fielder value) λ 2 satisfies λ 2 x min w 1, x 1 x T Lx The vector that minimizes λ 2 is called the Fielder vector. It minimizes 2 xi x j (i, j) E λ2 min where x x 0 2 i i 0 xi i

31 Spectral ordering The values of x minimize min For weighted matrices x 0 min x 0 (i, j) (i, j) E x i i A i, j x 2 i x x x i 2 i i The ordering according to the x i values will group similar (connected) nodes together j 2 x Physical interpretation: The stable state of springs placed on the edges of the graph j 2 x i i 0 x i i 0

32 Spectral partition Partition the nodes according to the ordering induced by the Fielder vector If u = (u 1,u 2,,u n ) is the Fielder vector, then split nodes according to a value s bisection: s is the median value in u ratio cut: s is the value that minimizes α sign: separate positive and negative values (s=0) gap: separate according to the largest gap in the values of u This works well (provably for special cases)

33 Fielder Value The value λ 2 is a good approximation of the graph expansion α(g) 2d λ 2 2 λ 2 2α(G) 2 α(g) λ λ 2 2d 2 d = maximum degree For the minimum ratio cut of the Fielder vector we have that 2 α 2d λ 2 2α(G) If the max degree d is bounded we obtain a good approximation of the minimum expansion cut

34 Conductance The expansion does not capture the intercluster similarity well The nodes with high degree are more important Graph Conductance G E U,V -U min U min d U,d V U weighted degrees of nodes in U d(u) A i, j i U j U

35 Conductance and random walks Consider the normalized stochastic matrix M = D -1 A The conductance of the Markov Chain M is M min U i min π(i)m[i, π U,π the probability that the random walk escapes set U The conductance of the graph is the same as that of the Markov Chain, φ(a) = φ(m) Conductance φ is related to the second eigenvalue of the matrix M 2 8 U j U 1 V μ 2 j] U

36 Interpretation of conductance Low conductance means that there is some bottleneck in the graph a subset of nodes not well connected with the rest of the graph. High conductance means that the graph is well connected

37 Clustering Conductance The conductance of a clustering is defined as the minimum conductance over all clusters in the clustering. Maximizing conductance of clustering seems like a natural choice

38 A spectral algorithm Create matrix M = D -1 A Find the second largest eigenvector v Find the best ratio-cut (minimum conductance cut) with respect to v Recurse on the pieces induced by the cut. The algorithm has provable guarantees

39 A divide and merge methodology Divide phase: Recursively partition the input into two pieces until singletons are produced output: a tree hierarchy Merge phase: use dynamic programming to merge the leafs in order to produce a tree-respecting flat clustering

40 Merge phase or dynamic-progamming on trees The merge phase finds the optimal clustering in the tree T produced by the divide phase k-means objective with cluster centers c 1,,c k : F({ C 2 1,..., Ck}) d( u, ci ) i u C i

41 Dynamic programming on trees OPT(C,i): optimal clustering for C using i clusters C l, C r the left and the right children of node C Dynamic-programming recurrence OPT( C, i) C, when i argmin1 j i F( OPT( Cl, j) OPT( Cr, i j)),otherwise 1

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