2 Abstract and Applied Analysis
a second-order active leader and variable interconnection
topology. A distributed consensus protocol was proposed for
rst-order agent with distributed estimation of the general
active leader’s unmeasurable state variables in [15], while [16]
extended the results of [15] to the case of communication
delays among agents under switching topology. In [17], the
authors considered leader-following problem in the mul-
tiagent system with general linear dynamics in both xed
topology case and switching topology case, respectively. e
cooperative output regulation of linear multiagent systems
canbeviewedasageneralizationofsomeresultsoftheleader-
followingconsensusproblemofmultiagentsystems[27, 28].
In many practical systems, the state variables cannot be
obtained directly. To achieve the state consensus, the agent
has to estimate those unmeasurable state variables by output
variables. In [8], the authors proposed a distributed observer-
based tracking protocols for each rst-order following agent.
Under the assumption that the active leader’s velocity cannot
be measured directly, [29]proposedadistributedobserver-
based tracking protocol for each second-order following-
agent. To track the accelerated motion leader, [30]proposed
an observer-based tracking protocol for each second-order
follower agent to estimate the acceleration of the leader.
A robust adaptive observer based on the response system
was constructed to practically synchronize a class of uncer-
tain chaotic systems [31]. In [23], the author proposed an
observer-type consensus protocol to the consensus problem
for a class of fractional-order uncertain multiagent systems
with general linear dynamics. For the multiagent system with
general linear dynamics, [32]establishedauniedframework
and proposed an observer-type consensus protocol, and [33]
proposed a framework including full state feedback control,
observer design, and dynamic output feedback control for
leader-following consensus problem. e leader-following
consensus problem was investigated under a class of directed
switching topologies in [34]. In [35], distributed reduced-
order observer-based consensus protocols were proposed
for both continuous- and discrete-time linear multiagent
systems. Other observer-based previous works include [36–
38].
Motivated by the previous works, especially by [35], we do
some further investigations on the reduced-order observer-
based consensus protocol problem which had been studied
by [35] under directed xed interconnection topology. We
rst correct some errors and propose a new proof of the main
result established in the aforementioned paper based on the
Jordan decomposition method. Moreover, by constructing a
parameter-dependent common Lyapunov function, we prove
that the proposed protocol can guarantee the multiagent
consensus system to achieve consensus under undirected
switching topology. Although the Lyapunov function method
is conservative and is not easy to be constructed, it is t
to solve the problem under the switching interconnection
topology. We propose distributed protocol to solve leader-
following consensus with a little simple modication to the
reduced-order observer-based consensus protocol. Similarly,
we can prove that all following agents can track the leader
under a class of directed interaction topologies. As the special
cases, the consensus conditions for balanced and undirected
interconnection topology cases can be obtained directly. Al-
though the leader-following consensus problem in this paper
has been studied in many papers with the aid of internal
model principle, we obtain a low-dimensional controller in
our model.
e paper is organized as follows. In Section 2,someno-
tations and preliminaries are introduced. en, in Section 3
and Section 4, the main results on the consensus stability
are obtained for both leaderless and leader-following cases,
respectively. Following that, Section 5 provides a simulation
example to illustrate the established results, and nally, the
concluding remarks are given in Section 6.
2. Preliminaries
To make this paper more readable, we rst introduce some
notations and preliminaries, most of which can be found in
[35]. Let
×
and
×
be the set of ×real matrices and
complex matrices, respectively. Re()denotes the real part of
∈. is the identity matrix with compatible dimension.
and
represent transpose and conjugate transpose of
matrix ∈
×
,respectively.1
=[1,...,1]
∈
.For
symmetric matrices and , >(≥)means that −
is positive (semi-) denite. ⊗denotes the Kronecker product,
which satises (⊗)(⊗)=()⊗()and (⊗)
=
⊗
. A matrix is said to be Hurwitz stable if all of its
eigenvalues have negative real parts.
A weighted digraph is denoted by G ={V,,},where
V ={V
1
,V
2
,...,V
}is the set of vertices, ⊂V ×V is the
set of edges, and a weighted adjacency matrix =[
]has
nonnegative elements
. e set of all neighbor nodes of
node V
is dened by N
={|(V
,V
)∈}.edegreematrix
={
1
,
2
,...,
}∈R
×
of digraph G is a diagonal matrix
with diagonal elements
=∑
∈N
𝑖
. en, the Laplacian
matrix of G is dened as =−∈
×
, which satises
1
=0. e Laplacian matrix has following interesting
property.
Lemma (see [14]). e Laplacian matrix associated with
weighted digraph G has at least one zero eigenvalue and all
of the non-zero eigenvalues are located on the open right half
plane. Furthermore, has exactly one zero eigenvalue if and
only if the directed graph G has a directed spanning tree.
A weighted graph is called undirected graph if for all
(V
,V
)∈,wehave(V
,V
)∈and
=
.Itiswellknown
that Laplacian matrix of weighted undirected graph is sym-
metric positive semidenite, which can be derived from
Lemma 1 bynoticingthefactthatalleigenvaluesofsymmet-
ric matrix are real and nonnegative. Furthermore, Laplacian
matrix has exactly one zero eigenvalue if and only if the
undirected graph G is connected.
To establish our result, the well-known Schur Comple-
ment Lemma is introduced.
Lemma (see [39]). Let be a symmetric matrix of the par-
titioned form =[
]with
11
∈
×
,
12
∈
×(−)
,and