topology was presented. The mean-square consensus problem of multi-agent systems was converted into the mean-square
stability problem of an equivalent system by a system transformation.
Very recently, increasing interest has turned to group consensus problems for multi-agent systems. The main idea of
group consensus is that the interaction topology includes several subgroups, and each subgroup has a spanning tree. The
agents belong to different subgroups will reach different consensus states [15].In[15], the definition of group consensus
was presented and a novel consensus protocol was designed to solve the group consensus problem. In [16], the group con-
sensus problem of multi-agent systems with switching topologies was studied. The group consensus was proved to be equiv-
alent to the asymptotical stability of a class of switched linear systems by a double-tree-form transformation. In [17],
another consensus protocol was designed to solve the couple-group consensus problem of multi-agent systems with direc-
ted fixed topology. In [18], two different kinds of consensus protocols were given to deal with the group consensus problem
for double-integrator dynamic multi-agent systems. In [19], the sampled-data control method was employed to deal with
the group consensus problem for multi-agent systems, where the interaction topology is undirected. However, to the best
of authors knowledge, the group consensus problems for discrete-time multi-agent systems and the multi-agent systems
with stochastic switching topologies have not been fully studied.
In this paper, we study the couple-group consensus problems for second-order discrete-time multi-agent systems with the
fixed topology and the stochastic switching topology, respectively. The stochastic switching topologies are assumed to be gov-
erned by a homogeneous Markov chain. For the fixed topology case,we obtaina sufficient condition for couple-group consensus
based on the eigenvalues of the matrix and a necessary and sufficient condition for couple-group consensus in form of matrix
inequality, respectively. For the stochastic switching topology case, we obtain a necessary and sufficient condition for mean-
square couple-group consensus in form of matrix inequality. Algorithms are given to design the feasible control gains.
Notation. Let C; R and N represent, respectively, the complex number set, the real number set and the nonnegative integer
set. Denote the spectral radius of the matrix M by
q
ðM Þ. Suppose that A; B 2 R
pp
. Let A B (respectively, A B) denote that
A B is symmetric positive semi-definite (respectively, symmetric positive definite). Given XðkÞ2R
p
, define
kXðkÞk
E
, kE½XðkÞX
T
ðkÞk
2
, where E½ is the mathematical expectation. I
n
denotes the n n identity matrix. ReðÞ and ImðÞ rep-
resent, respectively, the real part and imaginary part of a number. Let 0 denote the zero matrix with appropriate dimensions.
2. Graph theory notations
Let G¼ðV; E; AÞ be a directed graph of order n, where V¼f
v
1
; ...;
v
n
g and E represent the node set and the edge set,
respectively. A¼½a
ij
2R
nn
is the adjacency matrix associated with G, where a
ij
> 0ifð
v
i
;
v
j
Þ2E, otherwise, a
ij
¼ 0. An edge
ð
v
i
;
v
j
Þ2E if agent j can obtain the information from agent i. We say agent i is a neighbor of agent j. Let
N
i
¼f
v
j
2V: ð
v
i
;
v
j
Þ2Egdenote the neighbor set of agent i. The (nonsymmetrical) Laplacian matrix L associated with A
and hence G is defined as L¼½l
ij
2R
nn
, where l
ii
¼
P
n
j¼1;j–i
a
ij
and l
ij
¼a
ij
;
8
i – j. A directed path is a sequence of edges
in a directed graph in the form of ð
v
i
1
;
v
i
2
Þ; ð
v
i
2
;
v
i
3
Þ; ..., where
v
i
k
2V. A directed tree is a directed graph, where every node
has exactly one parent except for one node, called the root, which has no parent, and the root has a directed path to every
other node. A directed spanning tree of G is a directed tree that contains all nodes of G. A directed graph has or contains a
directed spanning tree if there exists a directed spanning tree as a subset of the directed graph, that is, there exists at least
one node having a directed path to all of the other nodes. The union of graphs G
1
and G
2
is the graph G
1
S
G
2
with vertex set
VðG
1
Þ
S
VðG
2
Þ and edge set EðG
1
Þ
S
EðG
2
Þ.
3. Problem formulation and main results
Suppose that the multi-agent system considered consists of n þ m agents. Without loss of generality, we assume that the
first n agents achieve a consistent state while the last m agents achieve another consistent state. Let G¼ðV; E; AÞ denote the
topology of multi-agent system considered. Denote I
1
¼f1; 2; ...; ng; I
2
¼fn þ 1; n þ 2; ...; n þ mg. Let V
1
¼f
v
1
;
v
2
; ...;
v
n
g
and V
2
¼f
v
nþ1
;
v
nþ2
; ...;
v
nþm
g represent the first n agents and the last m agents, respectively. Then, V¼V
1
[V
2
; V
1
\V
2
¼
U
.
In addition, let N
1i
¼f
v
j
2V
1
: ð
v
i
;
v
j
Þ2Egand N
2i
¼f
v
j
2V
2
: ð
v
i
;
v
j
Þ2Eg. It is obvious that N
i
¼ N
1i
[ N
2i
.
In this paper, we consider the second-order discrete-time multi-agent systems as follows:
n
i
½k þ 1 ¼n
i
½kþf
i
½k
f
i
½k þ 1 ¼f
i
½kþu
i
½k
; i ¼ 1; 2; ...; n þ m; ð1Þ
where n
i
½k; f
i
½k2R are the position and the velocity of the ith agent, respectively; u
i
½k2R is the control input of agent i at
time k.
3.1. Fixed topology case
In [15–18], the continuous-time consensus algorithms were proposed. Motivated by these results, we consider the fol-
lowing discrete-time consensus algorithm
u
i
½k¼
P
j2N
1i
a
ij
½
a
ðn
j
½kn
i
½kÞ þ bðf
j
½kf
i
½kÞ þ
P
j2N
2i
a
ij
ð
a
n
j
½kþbf
j
½kÞ
8
i 2I
1
;
P
j2N
2i
a
ij
½
a
ðn
j
½kn
i
½kÞ þ bðf
j
½kf
i
½kÞ þ
P
j2N
1i
a
ij
ð
a
n
j
½kþbf
j
½kÞ
8
i 2I
2
;
(
ð2Þ
596 H. Zhao et al. / Applied Mathematics and Computation 232 (2014) 595–605