asterisk ð nÞ is used to represent a term that is induced by symmetry. λ
max
ðPÞ and λ
min
ðPÞ denote
the maximum and minimum eigenvalues of the real symmetric matrix P, respectively. Matrices
in this paper are assumed to be with appropriate dimensions.
2. Problem formulation and preliminaries
In [19], the following simplified genetic oscillator model which contains only one increasing
and one decreasing nonlinear terms has been proposed:
_
yðtÞ¼AyðtÞþBf
1
ðyðtÞÞ þ Cf
2
ðyðtÞÞ ð1Þ
where yðtÞA R
n
represents the concentratio ns of proteins, RNAs and chemical complexes; A, B,
and C are const ant matrices in R
nn
; f
1
ðyðtÞÞ¼½f
11
ðy
1
ðtÞÞ; …; f
1n
ðy
n
ðtÞÞ
T
with f
1j
ðy
j
ðtÞÞ as a
monotonic increasing function of the Hill form f
1j
ðy
j
ðtÞÞ¼ðy
j
ðtÞ=β
1j
Þ
H
1j
=½1 þðy
j
ðtÞ=β
1j
Þ
H
1j
and
f
2
ðyðtÞÞ¼½f
21
ðy
1
ðtÞÞ; …; f
2n
ðy
n
ðtÞÞ
T
with f
2j
ðy
j
ðtÞÞ as a monotonic decreasing function of the
form f
2j
ðy
j
ðtÞÞ¼1=½1 þðy
j
ðtÞ=β
2j
Þ
H
2j
, where H
1j
and H
2j
are the Hill coef ficients; β
1j
and β
2j
are
positive constants. Since
f
2j
ðy
j
ðtÞÞ¼1
ðy
j
ðtÞ=β
2j
Þ
H
2j
1 þðy
j
ðtÞ=β
2j
Þ
H
2j
1g
j
ðy
j
ðtÞÞ; ð2Þ
by letting f ðyðtÞÞ ¼f
1
ðyðtÞÞ, Li et al. [19] have rewritten Eq. (1) as follows:
_
yðtÞ¼AyðtÞþBf ðyðtÞÞCgðyðtÞÞ þ Ce
n
ð3Þ
where gðyðtÞÞ¼½g
1
ðy
1
ðtÞÞ; g
2
ðy
2
ðtÞÞ; …; g
n
ðy
n
ðtÞÞ
T
and e
n
¼[1,…,1]
T
n 1
. Then, the following
network of N linearly c oupled genetic oscillators has been proposed [19]:
_
x
i
ðtÞ¼Ax
i
ðtÞþBf ðx
i
ðtÞÞCgðx
i
ðtÞÞþ Ce
n
þ ∑
N
j ¼ 1
G
ij
Dx
j
ðtÞ; i ¼1; …; N ð4Þ
where x
i
ðtÞA R
n
is the state vector of the i-th genetic oscillator; D A R
nn
defines the coupling
between two genetic oscillators; G ¼ðG
ij
Þ
NN
is a coupling symmetric matrix of the network, in
which G
ij
is defined as follows: if there is a link from oscillator j to oscillator i ðja iÞ, then G
ij
equals to a positive constant denoting the coupling strength of this link, otherwise, G
ij
¼0; G
ii
¼
∑
N
j ¼ 1;j a i
G
ij
.
To help the readers have a better understanding of the above models and our new model below
from a biological perspective, the repressilator [28], as a typical genetic oscillator which can be
described by model (1) or (3), is introduced here. The repressilator is a network of three genes
lacI, tetR, and cI, and the protein product s of these genes are LacI, TetR, and CI, respectively.
Each of these protein products inhibits the transcription of the gene of the other protein in a
cyclic way. Speci fically, the protein LacI inhibits the transcription of the gene tetR. The protein
TetR inhibits the transcription of the gene cI and the protein CI in turn inhibits the expression of
lacI. Thus, a feedback cycle is formed.
For coupled repressilators described by model (4), quorum sensing (QS) is used as a
mechanism for coupling purpose [20,23,29]. The QS is accomplished by diffusive exchange of
small auto-inducer (AI) molecules which can diffuse freely through the cell membrane and
participate in the intercellular coupling as well as in self-feedback processes. There are two
proteins which play important roles in the above processes. The first protein, LuxI, synthesizes
the AI molecule under the contr ol of the repressilator protein LacI. The second protein, LuxR,
X. Wan et al. / Journal of the Franklin Institute 351 (2014) 4395–44144398