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Title: Extreme Value k-means Clustering

Paper content:
Clustering is the central task in unsupervised learning and data mining.
k-means is one of the most widely used clustering algorithms.
Unfortunately, it is generally non-trivial to extend k-means to cluster data points beyond Gaussian distribution, particularly, the clusters with non-convex shapes (Beliakov & King, 2006).
To this end, we, for the first time, introduce Extreme Value Theory (EVT) to improve the clustering ability of k-means.
Particularly, the Euclidean space was transformed into a novel probability space denoted as extreme value space by EVT.
We thus propose a novel algorithm called Extreme Value k-means (EV k-means), including GEV k-means and GPD k-means.
In addition, we also introduce the tricks to accelerate Euclidean distance computation in improving the computational efficiency of classical k-means.
Furthermore, our EV k-means is extended to an online version, i.e., online Extreme Value k-means, in utilizing the Mini Batch k-means to cluster streaming data.
Extensive experiments are conducted to validate our EV k-means and online EV k-means on synthetic datasets and real datasets.
Experimental results show that our algorithms significantly outperform competitors in most cases.
Clustering is a fundamental and important task in the unsupervised learning (Jain, 2010; Rui Xu & Wunsch, 2005) .
It aims at clustering data samples of high similarity into the same cluster.
The most well-known clustering algorithm is the k-means, whose objective is to minimize the sum of squared distances to their closest centroids.
k-means has been extensively studied in the literature, and some heuristics have been proposed to approximate it (Jain, 2010; Dubes & Jain, 1988) .
The most famous one is Lloyd's algorithm (Lloyd, 1982) .
The k-means algorithm is widely used due to its simplicity, ease of use, geometric intuition (Bottesch et al., 2016) .
Unfortunately, its bottleneck is that computational complexity reaches O(nkd) (Rui Xu & Wunsch, 2005) , since it requires computing the Euclidean distances between all samples and all centroids.
The data is embedded in the Euclidean space (Stemmer & Kaplan, 2018) , which causes the failure on clustering non-convex clusters (Beliakov & King, 2006) .
Even worse, k-means is highly sensitive to the initial centroids, which usually are randomly initialized.
Thus, it is quite possible that the objective of k-means converges to a local minimum, which causes the instability of k-means, and is less desirable in practice.
Despite a stable version -k-means++ (Arthur & Vassilvitskii, 2007) gives a more stable initialization, fundamentally it is still non-trivial to extend k-means in clustering data samples of non-convex shape.
To solve these problems, this paper improves the clustering ability of k-means by measuring the similarity between samples and centroids by EVT (Coles et al., 2001 ).
In particular, we consider the generalized extreme value (GEV) (Jenkinson, 1955 ) distribution or generalized Pareto distribution (GPD) (Pickands III et al., 1975; DuMouchel, 1975) to transform the Euclidean space into a probability space defined as, extreme value space.
GEV and GPD are employed to model the maximum distance and output the probability that a distance is an extreme value, which indicates the similarity of a sample to a centroid.
Further, we adopt the Block Maxima Method (BMM) (Gumbel, 2012) to choose the maximal distance for helping GEV fit the data.
The Peaks-Over-Thresh (POT) method (Leadbetter, 1991 ) is utilized to model the excess of distance exceeding the threshold, and thus very useful in fitting the data for GPD.
Formally, since both GEV and GPD can measure the similarity of samples and centroids, they can be directly utilized in k-means, i.e., GEV k-means and GPD k-means, which are uniformly called Extreme Value k-means (EV k-means) algorithm.
In contrast to k-means, EV k-means is a probability-based clustering algorithm that clusters samples according to the probability output from GEV or GPD.
Furthermore, to accelerate the computation of Euclidean distance, We expand the samples and the centroids into two tensors of the same shape, and then accelerate with the high performance parallel computing of GPU.
For clustering steaming data, we propose online Extreme Value k-means based on Mini Batch kmeans (Sculley, 2010) .
When fit the GEV distribution, we use mini batch data as a block.
For the fitting of GPD, we dynamically update the threshold.
The parameters of GEV or GPD are learned by stochastic gradient descent (SGD) (LeCun et al., 1998) .
The main contributions are described as follows.
(1) This paper utilizes EVT to improve k-means in addressing the problem of clustering data of non-convex shape.
We thus propose the novel Extreme Value k-means, including GEV k-means and GPD k-means.
A method for accelerating Euclidean distance computation has also been proposed to solve the bottleneck of k-means.
(2) Under the strong theoretical support provided by EVT, we use GEV and GPD to transform Euclidean space into extreme value space, and measure the similarity between samples and centroids.
(3) Based on Mini Batch k-means, We propose online Extreme value k-means for clustering streaming data, which can learn the parameters of GEV and GPD online.
We corroborate the effectiveness of EV k-means and online EV k-means by conducting experiments on synthetic datasets and real datasets.
Experimental results show that EV k-means and online EV k-means significantly outperform compared algorithms consistently across all experimented datasets.

assistant: text: This paper introduces Extreme Value Theory into k-means to measure similarity and proposes a novel algorithm called Extreme Value k-means for clustering.