1041-4347 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TKDE.2018.2832205, IEEE
Transactions on Knowledge and Data Engineering
IEEE TRANSACTIONS ON KNOWLEDGE AND DATA ENGINEERING 3
increasing amount of attention because they can precisely
characterize and model systems in real world [24].
Community detection in networks has become a hot
topic since communities shed light on structure-function
relations, which has been extensively studied in the single-
layer networks. The most straightforward strategy is to ex-
tend the single-layer community detection algorithms to the
multi-layer networks to develop network-compression- and
consensus-based approaches. Network-compression-based
approaches compress multi-layer networks into a single-
layer network in which the single-layer community detec-
tion algorithms are adopted [31]. However, these methods
cannot preserve the community structure in multi-layer
networks, thereby leading to the low accuracy of algorithms
[25]. Thus, it is promising to extract community without
collapsing multi-layer networks. In this case, the single-
layer community detection algorithms are independently
applied to each layer, and then the communities at various
layers are combined to establish a consensus community
structure for multi-layer networks. However, this strategy
is also criticized for ignoring the connection among various
layers.
Thus, there is a critical need to develop effective algo-
rithms for community detection in multi-layer networks,
rather than by simply extending the available single-layer
network algorithms. To identify communities in multi-layer
networks, we must simultaneously take into account multi-
ple layers during the community search procedure. The first
step in multi-layer community detection is to quantify the
community in multi-layer networks. Ma et al. [29] quantified
the connectivity of communities in multi-layer networks
using information entropy and transformed the community
detection in multi-layer networks into an entropy optimiza-
tion problem. They also proposed a greedy-search-based
algorithm (M-Module) for multi-layer community detection.
Didier et al. [40] quantified communities by extending the
modularity function to detect communities from multi-layer
networks and developed the (MolTi) algorithm for multi-
layer communities. Some alternatives based on random
network models are also available for the quantification
of multi-layer communities [41], [42]. The successful ap-
plication of these algorithms highlights the need for the
integrative analysis of multiple layers, which is also one of
the major motivations of this study.
In other words, the layers of networks can be consid-
ered as different views of data, which provide information
complementary to each other. To integrate information from
multiple views, multi-view clustering algorithms cluster
multiple views simultaneously to derive a solution that
uncovers the common latent structure shared by multiple
views. Many multi-view clustering algorithms has been
widely developed [33]–[39]. Existing multi-view clustering
methods can be roughly categorized into two classes: loss-
function and subspace-based approaches. The algorithms in
the first category incorporate multiple views into the cluster-
ing process by optimizing the predefined loss functions [33],
[38], while the subspace-based algorithms project multiple
views into a common lower dimensional subspace where
communities are discovered [35]. For example, to explore
the information in each view, Kumar et al. [38] developed
the multi-view spectral clustering MVspec algorithm, where
a clustering solution is derived from each individual view
and then all the solutions are fused based on consensus.
However, MVspec is criticized for the independence of
features from various views. To solve this problem, Han
et al. [39] proposed MVnmf by formulating a joint matrix
factorization process with the constraint that pushes the
clustering solution of each view toward a common consen-
sus instead of fixing such solution directly.
Even though great efforts have been devoted to commu-
nity detection in multi-layer networks, few attempt has been
made to draw the relation among various algorithms. In the
forthcoming sections, we address the equivalence relation
among algorithms.
TABLE 1
Main Symbols
Symbol Definition and Description
G graph with vertex set V and edge set E
G multi-layer network {G
[1]
, . . . , G
[m]
}
W
[l]
the weighted adjacency matrix for G
[l]
D
[l]
degree diagonal matrix D
[l]
= diag(d
[l]
1
, . . . , d
[l]
n
)
W adjacency matrix = {W
[1]
, . . . , W
[m]
}
w
ij
the element at i-th row jth column in matrix W
w
i.
the i-th row of matrix W
w
.j
the j-th column of matrix W
X the indication matrix for V , where x
ij
=1
if v
i
belongs to cluster C
j
, 0 otherwise
X
′
the transpose of matrix X
˜
X column normalized matrix X, i.e. x
.j
/
√
x
′
.j
x
.j
l the index for the layers of network l ∈ {1, . . . , m}
{C
c
}
k
c=1
the communities for G
Q
G
D
({C
i
}
k
i=1
) multi-layer modularity density for communities
{C
i
}
k
i=1
) in G (Q
G
D
for short)
trace(W ) the trace of matrix W , i.e. trace(W ) =
∑
i
w
ii
3 MULTI-LAYER MODULARITY DENSITY
Before giving a detailed description of multi-layer modular-
ity density, we introduce some terminologies that are widely
used in the forthcoming sections.
Given an undirected and unweighted network G =
(V, E) with vertex set V = {v
1
, v
2
, . . . , v
n
} (n is the number
of vertices in G, i.e., n = |V |) and edge set E = {(v
i
, v
j
)},
the adjacency matrix A = (a
ij
) is constructed whose ele-
ment is a
ij
= 1 if there is an edge between vertex v
i
and
v
j
, 0 otherwise. The degree of vertex v
i
is defined as the
number of neighbors in the network, i.e., d
i
=
j
a
ij
. If
G is weighted, the weighted adjacency matrix is denoted
by W = (w
ij
), where element w
ij
is the weight on edge
(v
i
, v
j
). The weighted degree of vertex v
i
is the sum of
weight on edge connecting to v
i
, i.e., d
i
=
j
w
ij
. For a pair
of subsets V
1
and V
2
of V , let L(V
1
, V
2
) =
i∈V
1
,j∈V
2
w
ij
and V
c
= V \ V
c
.
Let {V
c
}
k
c=1
be a hard partitioning of G (i.e. V
i
∩ V
j
= ∅
if
i
=
j
, and
∪
i
V
i
=
V
), where
V
c
is the set of vertices in the
c-th community and k is the number of communities. The
modularity density Q
D
for {V
c
}
k
c=1
is defined as [10]
Q
D
({V
c
}
k
c=1
) =
k
c=1
L(V
c
, V
c
) − L(V
c
, V
c
)
|V
c
|
. (1)