390 H. Shi et al.
The k-means and spectral clustering method are hard clustering [5], which
assign each object to exactly one cluster. Hard clustering methods do not address
satisfactorily the relationships between an element and a cluster. It is obviously
that there may be three types of relationships between an element and a cluster,
namely, belong-to fully, belong-to partially (i.e., also not belong-to partially),
and not belong-to fully. To represent the three types of relationship, it is nec-
essary to introduce a notion of three-way clustering [9–11], as an alternative to
conventional hard clustering. In hard clustering, cluster is represented by a set
that divides the space into two regions. Objects belong to the cluster if they
are in the set, otherwise they do not belong to the cluster. In three-way clus-
tering, a cluster is represented by pair of sets called the core region and fringe
region, respectively. The core region, the fringe region, the complement of the
core region and the fringe region give rise to a trisection of the space. A trisection
captures the three types of relationships between a cluster and an object. With
this understanding of clusters, Yu et al. [9–11] initiated studies on three-way
clustering by drawing ideas from a theory of three-way decision [13–15].
In this paper, we propose the three-way spectral clustering by combining
three-way decision and spectral clustering. Different from the classical spectral
clustering, the result of three-way spectral clustering uses core region and fringe
region to represent a cluster. The rest of this paper is organized as follows.
Section 2 reviews some basic concepts of spectral clustering, three-way deci-
sion and three-way clustering. Section 3 presents the process of three-way spec-
tral clustering (TWSC for short). Section 4 reviews several evaluation functions.
Experimental results are reported in Sect. 5.
2 Preliminaries
In this section, we briefly introduce the background of the proposed method,
which consists of spectral clustering, three-way decision and three-way clustering.
2.1 Spectral Clustering
Spectral clustering was first suggested by Donath and Hoffman [16] in 1973 to
construct graph partitions based on eigenvectors of the adjacency matrix. A
comprehensive review of spectral clustering can be found in [17]. We introduce
some related concepts of spectral clustering and one popular spectral clustering
algorithm: NJW algorithm, which was presented by Ng, Jordan and Weiss [18].
Let G =(V,E) be an undirected weighted graph with vertex set
V ={v
1
, ··· ,v
n
}. Each vertex v
i
in this graph represents a data point and non-
negative weight w
ij
represents the similarity of two vertices v
i
and v
j
.The
weighted adjacency matrix of the graph is the matrix W =(w
ij
)(i, j =1, ··· ,n).
As G is undirected grape, it means W is symmetric, i.e., w
ij
= w
ji
. The basic
idea of spectral clustering is to find a partition of the graph such that the edges
between different groups have a low similarity and the edges within a group have
a high similarity.