nips nips2009 nips2009-9 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Samuel R. Bulò, Marcello Pelillo
Abstract: Hypergraph clustering refers to the process of extracting maximally coherent groups from a set of objects using high-order (rather than pairwise) similarities. Traditional approaches to this problem are based on the idea of partitioning the input data into a user-defined number of classes, thereby obtaining the clusters as a by-product of the partitioning process. In this paper, we provide a radically different perspective to the problem. In contrast to the classical approach, we attempt to provide a meaningful formalization of the very notion of a cluster and we show that game theory offers an attractive and unexplored perspective that serves well our purpose. Specifically, we show that the hypergraph clustering problem can be naturally cast into a non-cooperative multi-player “clustering game”, whereby the notion of a cluster is equivalent to a classical game-theoretic equilibrium concept. From the computational viewpoint, we show that the problem of finding the equilibria of our clustering game is equivalent to locally optimizing a polynomial function over the standard simplex, and we provide a discrete-time dynamics to perform this optimization. Experiments are presented which show the superiority of our approach over state-of-the-art hypergraph clustering techniques.
Reference: text
sentIndex sentText sentNum sentScore
1 it Abstract Hypergraph clustering refers to the process of extracting maximally coherent groups from a set of objects using high-order (rather than pairwise) similarities. [sent-3, score-0.378]
2 Traditional approaches to this problem are based on the idea of partitioning the input data into a user-defined number of classes, thereby obtaining the clusters as a by-product of the partitioning process. [sent-4, score-0.21]
3 In contrast to the classical approach, we attempt to provide a meaningful formalization of the very notion of a cluster and we show that game theory offers an attractive and unexplored perspective that serves well our purpose. [sent-6, score-0.517]
4 Specifically, we show that the hypergraph clustering problem can be naturally cast into a non-cooperative multi-player “clustering game”, whereby the notion of a cluster is equivalent to a classical game-theoretic equilibrium concept. [sent-7, score-1.053]
5 From the computational viewpoint, we show that the problem of finding the equilibria of our clustering game is equivalent to locally optimizing a polynomial function over the standard simplex, and we provide a discrete-time dynamics to perform this optimization. [sent-8, score-0.623]
6 Experiments are presented which show the superiority of our approach over state-of-the-art hypergraph clustering techniques. [sent-9, score-0.727]
7 1 Introduction Clustering is the problem of organizing a set of objects into groups, or clusters, in a way as to have similar objects grouped together and dissimilar ones assigned to different groups, according to some similarity measure. [sent-10, score-0.214]
8 Objects similarities are typically expressed as pairwise relations, but in some applications higherorder relations are more appropriate, and approximating them in terms of pairwise interactions can lead to substantial loss of information. [sent-12, score-0.201]
9 Consider for instance the problem of clustering a given set of d-dimensional Euclidean points into lines. [sent-13, score-0.299]
10 The problem of clustering objects using high-order similarities is usually referred to as the hypergraph clustering problem. [sent-17, score-1.073]
11 Both approaches transform the similarity hypergraph into an edge-weighted graph, whose edge-weights are a function of the hypergraph’s original weights. [sent-20, score-0.504]
12 This way they are able to tackle 1 the problem with standard pairwise clustering algorithms. [sent-21, score-0.343]
13 Bolla [6] defines a Laplacian matrix for an unweighted hypergraph and establishes a link between the spectral properties of this matrix and the hypergraph’s minimum cut. [sent-22, score-0.448]
14 Zhou and co-authors [23] generalize their earlier work on regularization on graphs and define a hypergraph normalized cut criterion for a k-partition of the vertices, which can be achieved by finding the second smallest eigenvector of a normalized Laplacian. [sent-24, score-0.421]
15 This approach generalizes the well-known “Normalized cut” pairwise clustering algorithm [19]. [sent-25, score-0.343]
16 It is worth noting that the approaches mentioned above are devised for dealing with higher-order relations, but can all be reduced to standard pairwise clustering approaches [1]. [sent-27, score-0.447]
17 A different formulation is introduced in [18], where the clustering problem with higher-order (super-symmetric) similarities is cast into a nonnegative factorization of the closest hyper-stochastic version of the input affinity tensor. [sent-28, score-0.338]
18 All the afore-mentioned approaches to hypergraph clustering are partition-based. [sent-29, score-0.716]
19 This renders these approaches vulnerable to applications where the number of classes is not known in advance, or where data is affected by clutter elements which do not belong to any cluster (as in figure/ground separation problems). [sent-31, score-0.192]
20 In this paper, following [14, 20] we offer a radically different perspective to the hypergraph clustering problem. [sent-35, score-0.74]
21 Instead of insisting on the idea of determining a partition of the input data, and hence obtaining the clusters as a by-product of the partitioning process, we reverse the terms of the problem and attempt instead to derive a rigorous formulation of the very notion of a cluster. [sent-36, score-0.172]
22 This allows one, in principle, to deal with more general problems where clusters may overlap and/or outliers may get unassigned. [sent-37, score-0.223]
23 We found that game theory offers a very elegant and general mathematical framework that serves well our purposes. [sent-38, score-0.228]
24 The basic idea behind our approach is that the hypergraph clustering problem can be considered as a multi-player non-cooperative “clustering game”. [sent-39, score-0.693]
25 Within this context, the notion of a cluster turns out to be equivalent to a classical equilibrium concept from (evolutionary) game theory, as the latter reflects both the internal and external cluster conditions alluded to before. [sent-40, score-0.88]
26 Experiments on two standard hypergraph clustering problems show the superiority of the proposed approach over state-of-the-art hypergraph clustering techniques. [sent-42, score-1.42]
27 2 Basic notions from evolutionary game theory Evolutionary game theory studies models of strategic interactions (called games) among large numbers of anonymous agents. [sent-43, score-0.654]
28 A game can be formalized as a triplet Γ = (P, S, π), where P = {1, . [sent-44, score-0.251]
29 , n} is the set of pure strategies (in the terminology of game-theory) available to each player and π : S k → R is the payoff function, which assigns a payoff to each strategy profile, i. [sent-50, score-0.484]
30 The payoff function π is assumed to be invariant to permutations of the strategy profile. [sent-53, score-0.204]
31 It is worth noting that in general games, each player may have its own set of strategies and own payoff function. [sent-54, score-0.315]
32 For a comprehensive introduction to evolutionary game theory we refer to [22]. [sent-55, score-0.398]
33 By undertaking an evolutionary setting we assume to have a large population of non-rational agents, which are randomly matched to play a game Γ = (P, S, π). [sent-56, score-0.571]
34 Evolution in the population takes place, because we assume that there exists a selection mechanism, which, by analogy with a Darwinian process, spreads the fittest strategies in the population to the detriment of the weakest one, which will in turn be driven to extinction. [sent-59, score-0.403]
35 2 The state of the population at a given time t can be represented as a n-dimensional vector x(t), where xi (t) represents the fraction of i-strategists in the population at time t. [sent-61, score-0.373]
36 The set of all possible states describing a population is given by ∆= x ∈ Rn : xi = 1 and xi ≥ 0 for all i ∈ S , i∈S which is called standard simplex. [sent-62, score-0.227]
37 In the sequel we will drop the time reference from the population state, where not necessary. [sent-63, score-0.198]
38 , the set of strategies still alive in population x ∈ ∆: σ(x) = {i ∈ S : xi > 0}. [sent-66, score-0.257]
39 If y(i) ∈ ∆ is the probability distribution identifying which strategy the ith player will adopt if drawn to play the game Γ, then the average payoff obtained by the agents can be computed as k u(y(1) , . [sent-67, score-0.625]
40 Assuming that the agents are randomly and independently drawn from a population x ∈ ∆ to play the game Γ, the population average payoff is given by u(xk ), where xk is a shortcut for x, . [sent-78, score-1.019]
41 Furthermore, the average payoff that an i-strategist obtains in a population x ∈ ∆ is given by u(ei , xk−1 ), where ei ∈ ∆ is a vector with xi = 1 and zero elsewhere. [sent-82, score-0.492]
42 An important notion in game theory is that of equilibrium [22]. [sent-83, score-0.361]
43 A population x ∈ ∆ is in equilibrium when the distribution of strategies will not change anymore, which intuitively happens when every individual in the population obtains the same average payoff and no strategy can thus prevail on the other ones. [sent-84, score-0.767]
44 (2) In other words, every agent in the population performs at most as well as the population average payoff. [sent-86, score-0.449]
45 , all the agents that survived the evolution obtain the same average payoff, which coincides with the population average payoff. [sent-89, score-0.429]
46 A key concept pertaining to evolutionary game theory is that of an evolutionary stable strategy [7, 22]. [sent-90, score-0.608]
47 Such a strategy is robust to evolutionary pressure in an exact sense. [sent-91, score-0.21]
48 Assume that in a population x ∈ ∆, a small share ǫ of mutant agents appears, whose distribution of strategies is y ∈ ∆. [sent-92, score-0.386]
49 The resulting postentry population is given by wǫ = (1 − ǫ)x + ǫy. [sent-93, score-0.21]
50 Biological intuition suggests that evolutionary forces select against mutant individuals if and only if the average payoff of a mutant agent in the postentry population is lower than that of an individual from the original population, i. [sent-94, score-0.818]
51 (4) A population x ∈ ∆ is evolutionary stable (or an ESS) if inequality (4) holds for any distribution of mutant agents y ∈ ∆ \ {x}, granted the population share of mutants ǫ is sufficiently small (see, [22] for pairwise contests and [7] for n-wise contests). [sent-97, score-0.78]
52 An alternative, but equivalent, characterization of ESSs involves a leveled notion of evolutionary stable strategies [7]. [sent-98, score-0.266]
53 3 3 The hypergraph clustering game The hypergraph clustering problem can be described by an edge-weighted hypergraph. [sent-111, score-1.614]
54 Formally, an edge-weighted hypergraph is a triplet H = (V, E, s), where V = {1, . [sent-112, score-0.444]
55 In this paper, we cast the hypergraph clustering problem into a game, called (hypergraph) clustering game, which will be played in an evolutionary setting. [sent-118, score-1.214]
56 Clusters are then derived from the analysis of the ESSs of the clustering game. [sent-119, score-0.272]
57 Specifically, given a k-graph H = (V, E, s) modeling a hypergraph clustering problem, where V = {1, . [sent-120, score-0.693]
58 , n} is the set of objects to cluster and s is the similarity function over the set of objects in E, we can build a game involving k players, each of them having the same set of (pure) strategies, namely the set of objects to cluster V . [sent-123, score-0.857]
59 Under this setting, a population x ∈ ∆ of agents playing a clustering game represents in fact a cluster, where xi is the probability for object i to be part of it. [sent-124, score-0.854]
60 Indeed, any cluster can be modeled as a probability distribution over the set of objects to cluster. [sent-125, score-0.246]
61 The payoff function of the clustering game is defined in a way as to favour the evolution of agents supporting highly coherent objects. [sent-126, score-0.817]
62 Intuitively, this is accomplished by rewarding the k players in proportion to the similarity that the k played objects have. [sent-127, score-0.228]
63 , vk ) ∈ V k to be the tuple of objects selected by k players, the payoff function can be simply defined as 1 s ({v1 , . [sent-131, score-0.282]
64 An ESS of a clustering game incorporates the properties of internal coherency and external incoherency of a cluster: internal coherency: since ESSs are Nash equilibria, from (3), it follows that every object contributing to the cluster, i. [sent-144, score-0.932]
65 , every object in σ(x), has the same average similarity with respect to the cluster, which in turn corresponds to the cluster’s overall average similarity. [sent-146, score-0.232]
66 Hence, the cluster is internally coherent; external incoherency: from (2), every object external to the cluster, i. [sent-147, score-0.393]
67 , every object in V \ σ(x), has an average similarity which does not exceed the cluster’s overall average similarity. [sent-149, score-0.232]
68 There may still be cases where the average similarity of an external object is the same as that of an internal object, mining the cluster’s external incoherency. [sent-150, score-0.416]
69 However, since x is an ESS, from (7) we see that whenever we try to extend a cluster with small shares of external objects, the cluster’s overall average similarity drops. [sent-151, score-0.387]
70 This guarantees the external incoherency property of a cluster to be also satisfied. [sent-152, score-0.336]
71 Finally, it is worth noting that this theory generalizes the dominant-sets clustering framework which has recently been introduced in [14]. [sent-153, score-0.33]
72 clustering games defined on graphs, correspond to the dominant-set clusters [20, 17]. [sent-156, score-0.384]
73 4 Evolution towards a cluster In this section we will show that the ESSs of a clustering game are in one-to-one correspondence with (strict) local solution of a non-linear optimization program. [sent-158, score-0.669]
74 Let H = (V, E, s) be a hypergraph clustering problem and Γ = (P, V, π) be the corresponding clustering game. [sent-160, score-0.965]
75 The problem of extracting ESSs of our hypergraph clustering game can thus be cast into the problem of finding strict local solutions of (9). [sent-167, score-0.994]
76 Since f (x) is a homogeneous polynomial in the variables xi , we can use the transformation of Theorem 1 in order to find a local solution x ∈ ∆ of (9), which in turn provides us with an ESS of the hypergraph clustering game. [sent-179, score-0.759]
77 The complexity of finding a cluster is thus O(ρ|E|), where |E| is the number of edges of the hypergraph describing the clustering problem and ρ is the average number of iteration needed to converge. [sent-181, score-0.928]
78 In order to obtain the clustering, in principle, we have to find the ESSs of the clustering game. [sent-183, score-0.272]
79 By now, we adopt a naive peeling-off strategy for our cluster extraction procedure. [sent-185, score-0.209]
80 Namely, we iteratively find a cluster and remove it from the set of objects, and we repeat this process on the remaining objects until a desired number of clusters have been extracted. [sent-186, score-0.325]
81 We also compare against Super-symmetric Non-negative Tensor Factorization (S NTF) [18], because it is the only approach, other than ours, which does not approximate the hypergraph to a graph. [sent-193, score-0.421]
82 Figure 1: Results on clustering 3, 4 and 5 lines perturbed with increasing levels of Gaussian noise (σ = 0, 0. [sent-245, score-0.395]
83 Moreover, we evaluated the quality of a clustering by computing the average F-measure of each cluster in the ground-truth with the most compatible one in the obtained solution (according to a one-to-one correspondence). [sent-251, score-0.507]
84 1 Line clustering We consider the problem of clustering lines in spaces of dimension greater than two, i. [sent-253, score-0.596]
85 A first experiment consists in clustering 3, 4 and 5 lines generated in the 5-dimensional space [−2, 2]5 . [sent-261, score-0.324]
86 With this setting there are no outliers and every point should be assigned to a line (e. [sent-267, score-0.179]
87 This is due to the fact that points deviating too much from the overall cluster average collinearity will be excluded as they undermine the cluster’s internal coherency. [sent-275, score-0.328]
88 Figure 2: Results on clustering 2, 3 and 4 lines with an increasing number of outliers (0, 10, 20, 40). [sent-313, score-0.468]
89 The second experiment consists in clustering 2, 3 and 4 slightly perturbed lines (with fixed local noise σ = 0. [sent-316, score-0.395]
90 Indeed, our method is not influenced by outliers and therefore it performs almost perfectly, whereas C AVERAGE and S NTF perform well only without outliers and with the optimal K. [sent-326, score-0.288]
91 However, since outliers are not mutually similar and intuitively they do not form a cluster, we have that the performance of C AVERAGE and S NTF drop drastically as the number of outliers increases. [sent-329, score-0.313]
92 2 Illuminant-invariant face clustering In [5] it has been shown that images of a Lambertian object illuminated by a point light source lie in a three dimensional subspace. [sent-332, score-0.365]
93 We use this dissimilarity measure for the face clustering problem and we consider as dataset the Yale Face Database B and its extended version [8, 12]. [sent-334, score-0.369]
94 The case with outliers consists in 10 additional faces each from a different individual. [sent-338, score-0.171]
95 The results are consistent with those obtained in the case of line clustering with the exception of S NTF, which performs worse than the other approaches on this real-world application. [sent-401, score-0.33]
96 C AVERAGE and our algorithm perform comparably well when clustering 4 individuals without outliers. [sent-402, score-0.313]
97 In both the experiments of line and face clustering the execution times of our approach were higher than those of C AVERAGE, but considerably lower than S NTF. [sent-407, score-0.36]
98 The main reason why C AVERAGE run faster is that our approach and S NTF work directly on the hypergraph without resorting to pairwise relations, which is indeed what C AVERAGE does. [sent-408, score-0.523]
99 6 Discussion In this paper, we offered a game-theoretic perspective to the hypergraph clustering problem. [sent-410, score-0.717]
100 Within our framework the clustering problem is viewed as a multi-player non-cooperative game, and classical equilibrium notions from evolutionary game theory turn out to provide a natural formalization of the notion of a cluster. [sent-411, score-0.888]
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