4. The pair (E,A) is said to be admissible if (E,A) is
impulse free and stable.
The concepts of R-controllability and R-observabil-
ity of a descriptor system (1) can be referenced to
Duan.
41
For the regular descriptor system (1), (E,A,B)
is R-controllable if and only if rank[sE 2 AB]=m for
any finite s 2C, and (E,A,C)isR-observable if and
only if rank
sE A
C
= m for any finite s 2C.
From a practical standpoint, a descriptor system
should be stable and impulse-free. The admissibility of
descriptor systems has been probed by many research-
ers. Before establishing our main results, some prelimi-
nary results for descriptor system are introduced, which
will be used later.
Lemma 1 If all pairs (E
i
,A
i
), i = 1,2,.,M, are admissi-
ble, then the pair (E
add
,A
add
) is admissible, where
E
add
=
E
1
E
2
.
.
.
E
M
0
B
B
B
B
@
1
C
C
C
C
A
,
A
add
=
A
1
A
12
A
1M
A
2
A
2M
.
.
.
.
.
.
A
M
0
B
B
B
B
@
1
C
C
C
C
A
ð2Þ
Proof. Obviously, we have
det (sE
add
A
add
) = det (sE
1
A
1
)
det (sE
2
A
2
) det (sE
M
A
M
)
ð3Þ
Then, according to Definition 1, it is self-evident.
Lemma 2 (Ishihara and Terra
29
and Duan
41
) For
E,A 2R
m 3 m
, assume that (E,A) is regular. We have the
following.
1. If there exist X = X
T
ø 0 and Y = Y
T
. 0 satisfying
E
T
XA + A
T
XE = E
T
YE ð4Þ
then (E,A) is admissible.
2. If (E,A) is admissible, then there exist X = X
T
. 0
and Y = Y
T
. 0 satisfying the Lyapunov equation
(4).
Lemma 3 (Duan
41
) For E,A 2R
m 3 m
, B 2R
m 3 r
, and
C 2R
q 3 m
, assume that (E,A) is regular and impulse-
free, and (E,A,B) is R-controllable. Then, for any given
positive definite matrices W
b
and R
b
, there exists a posi-
tive definite V
b
satisfying the following generalized
Riccati equation
E
T
V
b
A + A
T
V
b
E E
T
V
b
BR
1
b
B
T
V
b
E + E
T
W
b
E =0
ð5Þ
Problem formulation
Consider a descriptor multi-agent system consisting of
n identical agents indexed by i = 1,2,.,n. The dynamics
of agent i is modeled by the following descriptor system
E
_
x
i
= Ax
i
+ Bu
i
y
i
= Cx
i
, i =1, ..., n ð6Þ
where x
i
2R
m
is the state of agent i, u
i
2R
p
is its
control input, y
i
2R
q
is its measured output, and
E,A,B,C are constant matrices with appropriate
dimensions.
Assumption 1 For descriptor system (6), we always
assume that (E,A) is regular and impulse-free,(E,A,B) is
R-controllable and (E,A,C) is R-observable.
Considering the limited capability of the agent, we
are interested in the distributed consensus protocol for
each agent, which only depends on the information of
the agent itself, and its neighbors. The neighbor rela-
tions can be modeled by a weighted digraph, G , which
is denoted by G = fV , e, Ag, where V = fv
1
, v
2
, ..., v
n
g
is the set of vertices, e V 3 V is the set of edges, and
the weighted adjacency matrix A =[a
ij
] has nonnegative
adjacency elements a
ij
. The neighbor set of node v
i
is
defined by N
i
= fjj(v
i
, v
j
) 2 eg. The degree matrix
D =diagfd
1
, d
2
, ..., d
n
g2R
n 3 n
of digraph G is a
diagonal matrix with diagonal elements d
i
=
P
j2N
i
a
ij
.
Then the Laplacian matrix of G is defined as
L = D 2 A 2R
n 3 n
, which satisfies L1
n
= 0. The
Laplacian matrix L associated with the weighted
digraph G has at least one zero eigenvalue, and all of
the nonzero eigenvalues are located on the open right
half-plane. Furthermore, L has exactly one zero eigen-
value if and only if the directed graph G has a directed
spanning tree. Let r =(r
1
, r
2
, ., r
n
)
T
2R
n
be the right
eigenvector associated with the only zero eigenvalue,
which satisfies r
T
1 = 1. It is well-known that r is a non-
negative vector. Conveniently, let l
i
(L), i =1, 2, ., n,
be the ith eigenvalue of L, with l
1
(L)=0.
Assume that fv
i
g[N
i
= fj
1
, j
2
, ..., j
l
g. Then, a
state feedback
u
i
= k
i
(x
j
1
, ..., x
j
l
) ð7Þ
is said to be a protocol with topology G .
The descriptor multi-agent system is said to have
achieved consensus if the states of all agents satisfy
lim
t!‘
(x
i
(t) x
j
(t)) = 0, i, j =1,2, ..., n
for any initial state x
i
(0) (i =1, 2, ., n). We say that
the protocol (7) can solve the consensus problem, if the
closed-loop feedback system achieves consensus.
Chen et al. 929
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