Research Article | Open Access

Xiaofei Fan, Yantao Wang, Ligang Wu, Xian Zhang, "An *r*-Order Finite-Time State Observer for Reaction-Diffusion Genetic Regulatory Networks with Time-Varying Delays", *Complexity*, vol. 2018, Article ID 7257083, 15 pages, 2018. https://doi.org/10.1155/2018/7257083

# An *r*-Order Finite-Time State Observer for Reaction-Diffusion Genetic Regulatory Networks with Time-Varying Delays

**Academic Editor:**Mohamed Boutayeb

#### Abstract

It will be settled out for the open problem of designing an -order finite-time (F-T) state observer for reaction-diffusion genetic regulatory networks (RDGRNs) with time-varying delays. By assuming the Dirichlet boundary conditions, aiming to estimate the mRNA and protein concentrations via available network measurements. Firstly, sufficient F-T stability conditions for the filtering error system have been investigated via constructing an appropriate Lyapunov–Krasovskii functional (LKF) and using several integral inequalities and (reciprocally) convex technique simultaneously. These conditions are delay-dependent and reaction-diffusion-dependent and can be checked by MATLAB toolbox. Furthermore, a method is proposed to design an -order F-T state observer, and the explicit expressions of observer gains are given. Finally, a numerical example is presented to illustrate the effectiveness of the proposed method.

#### 1. Introduction

Recently, due to a great many applications in the real world, genetic regulatory networks have become one of the hot topics in many fields. Much significant results (see [1–10] and the references therein) have been obtained. Usually, owing to the highly complexity of genetic regulatory networks, spatial homogeneity (i.e., the mRNA and protein concentrations are independent on their space positions) is assumed in the process of modelling genetic regulatory networks. However, this assumption is sometimes unreasonable, for example, the concentrations of proteins in ribosomal gathering are higher than ones of other parts of the cell in the process of translation. So, bringing the diffusing phenomenon into the models of genetic regulatory networks is urgent and necessary, which results in RDGRNs. Generally, the models of genetic regulatory networks are divided into discrete-time models and continuous-time ones [11]. A continuous-time model has wide applications in studying the complex features and the nonlinear behaviors of genetic regulatory networks. Moreover, due to the slow processes of transcription and translation, time delays should be considered in the continuous-time models of RDGRNs. It should be emphasized that time delays may lead to poor network performance, even instability. To the best our knowledge, those works in [12–17] have researched the problem of stability analysis of delayed RDGRNs. The asymptotic stability analysis of delayed RDGRNs have been involved in [13–15] by constructing an appropriate LKF and applying some inequality techniques. In [16], a sufficient condition of F-T stability for delayed RDGRNs has been given by constructing an LKF including quad-slope integrations and applying the Gronwall inequality and Wirtinger-type integral inequality. Related research on uncertain stochastic time-delay RDGRNs and impulsive stochastic time-delay RDGRNs can be found in [12, 17], respectively.

Generally speaking, with the change of environment, not all mRNA and protein concentrations are measurable. So, it is important and necessary to estimate the mRNA and protein concentrations. Currently, one of effective approaches to estimate system states is to design observers based on the available measurement. As we all know, the problem of estimating the states of delayed RDGRNs is only studied in [18, 19], although some scholars have addressed the diffusion-free case (see [20–22] and the references therein). A full-order observer based on available measurement has been designed in [18] by introducing an LKF and employing several integral inequalities, convex approach and Green’s second identity. The existence condition and design method of a full-order F-T observer have been given in [19]. It is worth emphasizing that all these results are about the full-order observers, and all approaches proposed in these literatures are not available for designing reduced-order observers. But, the design of a reduced-order observer is necessary, since it can be more easily realized than the full-order one in engineering practise.

The above discussion motivates us to design an -order (i.e., reduced-order) F-T state observer for delayed RDGRNs. By constructing a novel LKF and employing several integral inequalities and (reciprocally) convex technique to estimate its derivative, a F-T stability criterion in the form of linear matrix inequalities (LMIs) is established for the resulting error system. In addition, we propose a method for designing an -order F-T state observer for RDGRNs with time-varying delays, and the observer gains are parameterized by the solutions of these LMIs. Furthermore, the method proposed in this paper is explained by a numerical example.

It is worth emphasizing that the method proposed in this paper has the following advantages: (i)The -order observer is delay-dependent and reaction-diffusion-dependent, which is more practical.(ii)Compared with the full-order observers, the designed -order one can save the cost in the engineering.(iii)For delayed genetic regulatory networks without reaction-diffusion items, the method is still keeping available by removing the corresponding parts of .(iv)The method can also extend to some other time-delay models, including Markov jump neural networks [23–25] and stochastic delayed systems [26].

*Notation 1. *For given matrices and , we say and , if is real symmetric positive definite and semidefinite, respectively. The identity matrix is defined by , and the zero matrix by . and stand for the transpose matrix of and the sum of and its transpose, respectively. For an arbitrary but fixed positive integer , we denote by the set . Let with , . The set of all functions having the continuous second derivatives is defined by . and represent the norms on and are defined by
and
The symbol refers to .

#### 2. Problem Formulation

Consider the following delayed RDGRN [15]: where

, , and stand for the concentrations of mRNAs and proteins, respectively; , , and are the rate constants; and denote the diagonal diffusion rate matrices; represents the coupling matrix with elements defined as in [15]; is the Hill function, is the sum of dimensionless transcriptional rates which repress gene , and are delays subject to where , , , and are nonnegative real numbers.

The expression of indicates that for some common scalar .

Assume that is the unique equilibrium solution of (3). Set

Then the delayed RDGRN (3) turns into where

Next, it is assumed that the initial conditions and the Dirichlet boundary conditions of (8) are as follows: where , and and are functions in . Furthermore, let the network outputs be with the full-row-rank constant matrices and . Here, and represent the expression levels of mRNAs and proteins at time , respectively.

This paper aims at designing an -order F-T state observer for the delayed RDGRN (8), which is described as:

Here, and are the -order observer states, and , , , and are the observer gains.

*Remark 1. *Clearly, when the observer (12) is of full-order. So, our method is also available to establish full-order observers for the delayed RDGRN (8).

Define the augmented vectors

According to (8), (11), and (12), one can obtain the resulting error system as follows: where

*Definition 1 (see [12]). *The trivial solution of system (14) is called F-T stable with respect to positive scalars , , and , if
We say that system (14) is F-T stable with respect to positive scalars , , and , if so, it is its trivial solution.

To achieve our aim, one requires to seek -order observer gains , , , and such that system (14) is F-T stable with respect to positive scalars , , and .

#### 3. Preliminaries

The following two lemmas are needed to design an -order F-T state observer.

Lemma 1 (Jensen’s inequality) [27]. *For given scalars , an integral function and a matrix , there holds the following inequality:
*

Lemma 2 (Wirtinger-type integral inequalities) [28]. *For given scalars , a function which is derivative and a matrix , there hold the following inequalitieswhere
*

#### 4. Design Method of Observer

In this section, a method to design an -order F-T state observer for the delayed RDGRN (8) is proposed, that is, determine the observer gains , and such that the error system (14) is F-T stable. For this end, we define where , and are the same with previous ones.

Now we can provide an approach to design an *r*-order F-T state observer for the delayed RDGRN (8).

Theorem 1. *For given scalars , and satisfying (5) and positive constants , and , system (14) is F-T stable with respect to , and T, if there exist matrices , and diagonal matrices and and matrices , and of appropriate dimensions, such that the following inequalities are feasible for and κ∈{0,}:
where , and , , , and are defined previously.*

In addition, based on a feasible solution of (21), (22), and (23), an -order F-T state observer can be represented by (12) with the following gains:

*Proof 1. *Choose the following LKF functional:
where

Then, calculating the derivatives of along on the solution of system (14), one can obtain that

Firstly, it follows from Green formula that