# New Stability and Exact Observability Conditions for Semilinear Wave Equations

## Abstract

The problem of estimating the initial state of 1-D wave equations with globally Lipschitz nonlinearities from boundary measurements on a finite interval was solved recently by using the sequence of forward and backward observers, and deriving the upper bound for exact observability time in terms of Linear Matrix Inequalities (LMIs) [5]. In the present paper, we generalize this result to n-D wave equations on a hypercube. This extension includes new LMI-based exponential stability conditions for n-D wave equations, as well as an upper bound on the minimum exact observability time in terms of LMIs. For 1-D wave equations with locally Lipschitz nonlinearities, we find an estimate on the region of initial conditions that are guaranteed to be uniquely recovered from the measurements. The efficiency of the results is illustrated by numerical examples.

^{1}

footnoteinfo]This work was supported by Israel Science Foundation (grant no. 1128/14).

a]Emilia Fridman, a]Maria Terushkin

istributed parameter systems; wave equation; Lyapunov method; LMIs; exact observability.

## 1 Introduction

Lyapunov-based solutions of various control problems for finite-dimensional systems can be formulated in the form of Linear Matrix Inequalities (LMIs) [3]. The LMI approach to distributed parameter systems is capable of utilizing nonlinearities and of providing the desired system performance (see e.g. [4, 7, 12]). For 1-D wave equations, several control problems were solved by using the direct Lyapunov method in terms of LMIs [8, 5]. However, there have not been yet LMI-based results for n-D wave equations, though the exponential stability of the n-D wave equations in bounded spatial domains has been studied in the literature via the direct Lyapunov method (see e.g. [18, 9, 1, 6]).

The problem of estimating the initial state of 1-D wave equations with globally Lipschitz nonlinearities from boundary measurements on a finite interval was solved recently by using the sequence of forward and backward observers, and deriving the upper bound for exact observability time in terms of LMIs [5]. In the present paper, we generalize this result to n-D wave equations on a hypercube. This extension includes new LMI-based exponential stability conditions for n-D wave equations. Their derivation is based on n-D extensions of the Wirtinger (Poincare) inequality [10] and of the Sobolev inequality with tight constants, which is crucial for the efficiency of the results. As in 1-D case, the continuous dependence of the reconstructed initial state on the measurements follows from the integral input-to-state stability of the corresponding error system, which is guaranteed by the LMIs for the exponential stability. Some preliminary results on global exact observability of multidimensional wave PDEs will be presented in [?].

Another objective of the present paper is to study regional exact observability for systems with locally Lipschitz in the state nonlinearities. Here we restrict our consideration to 1-D case, and find an estimate on the region of initial conditions that are guaranteed to be uniquely recovered from the measurements. Note that our result on the regional observability cannot be extended to multi-dimensional case (see Remark 4 below for explanation and for discussion on possible n-D extensions for different classes of nonlinearities). The efficiency of the results is illustrated by numerical examples.

The presented simple finite-dimensional LMI conditions complete the theoretical qualitative results of e.g. [15] (where exact observability of linear systems in a Hilbert space was studied via a sequence of forward and backward observers) and [2] (where local exact observability of abstract semilinear systems was considered).

Notation: denotes the -dimensional Euclidean space with the norm , is the space of real matrices. The notation with means that is symmetric and positive definite. For the symmetric matrix , and denote the minimum and the maximum eigenvalues of respectively. The symmetric elements of the symmetric matrix will be denoted by . Continuous functions (continuously differentiable) in all arguments, are referred to as of class (of class ). is the Hilbert space of square integrable , where , with the norm . For the scalar smooth function denote by () the corresponding partial derivatives. For define , . is the Sobolev space of absolutely continuous functions with the square integrable . is the Sobolev space of scalar functions with absolutely continuous and with .

## 2 Observers and exponential stability of n-D wave equations

### 2.1 System under study and Luenberger type observer

Throughout the paper we denote by the n-D unit hypercube with the boundary . We use the partition of the boundary:

Here subscripts D and N stand for Dirichlet and for Neumann boundary conditions respectively.

We consider the following boundary value problem for the scalar n-D wave equation:

(2.1) |

where is a function, denotes the outer unit normal vector to the point and is the normal derivative. Let be the known bound on the derivative of with respect to :

(2.2) |

Since is a unit hypercube, the boundary conditions on can be rewritten as

Consider the following initial conditions:

(2.3) |

The boundary measurements are given by

(2.4) |

Similar to [5], the boundary-value problem (2.1) can be represented as an abstract differential equation by defining the state and the operators

where is defined as so that it is continuous in for each . The differential equation is

(2.5) |

in the Hilbert space , where

and . The operator has the dense domain

where . Here the boundary condition holds in a weak sense (as defined in Sect. 3.9 of [16]), i.e. the following relation holds:

The operator is m-dissipative (see Proposition 3.9.2 of [16]) and hence it generates a strongly continuous semigroup. Due to (2.2), the following Lipschitz condition holds:

(2.6) |

where Then by Theorem 6.1.2 of [14], a unique continuous mild solution of (2.5) in initialized by

exists in . If , then this mild solution is in and it is a classical solution of (2.1) with (see Theorem 6.1.5 of [14]).

We suggest a Luenberger type observer of the form:

(2.7) |

under the initial conditions and the boundary conditions

(2.8) |

where is the injection gain.

The well-posedness of (2.7), (2.8) will be established by showing the well-posedness of the estimation error . Taking into account (2.1), (2.3) we obtain the following PDE for the estimation error :

(2.9) |

under the boundary conditions

(2.10) |

Here and

The initial conditions for the error are given by

The boundary conditions on can be presented as

Let be a mild solution of (2.1). Then is continuous and, thus, the function defined as

satisfies the Lipschitz condition (2.6), where is replaced by . By the above arguments, where in the definition of we have , the error system (2.9), (2.10) has a unique mild solution initialized by Therefore, there exists a unique mild solution to the observer system (2.7), (2.8) with the initial conditions . If then is a classical solution of 2.9), (2.10) with for . Hence, if and , there exists a unique classical solution to the observer system (2.7), (2.8) with for .

### 2.2 Lyapunov function and useful inequalities

We will derive further sufficient conditions for the exponential stability of the error wave equation (2.9) under the boundary conditions (2.10). Let

(2.11) |

be the energy of the system. Consider the following Lyapunov function for (2.9), (2.10):

with some constant . Note that the above Lyapunov function without the last term was considered in [1, 6, 18]. The time derivative of this new term of cancels the same term with the opposite sign in the time derivative of (cf. (2.23) below) leading to LMI conditions for the exponential convergence of the error wave equation.

We will employ the following n-D extensions of the classical inequalities:

###### Lemma 1

Consider such that . Then the following n-D Wirtinger’s inequality holds:

(2.12) |

Moreover,

(2.13) |

Proof : Since , by the classical 1-D Wirtinger’s inequality [10]

Integrating the latter inequality in we obtain

with . Clearly the latter inequality holds for all , which after summation in yields (2.12).

Since we have by Sobolev’s inequality

that after integration in leads to

with . The latter inequality holds leading after summation in to (2.13).

### 2.3 Exponential stability of n-D wave equation

In this section we derive LMI conditions for the exponential stability of the estimation error equation. We start with the conditions for the positivity of the Lyapunov function:

###### Lemma 2

Let there exist positive scalars and such that

(2.14) |

Then the Lyapunov function is bounded as follows:

(2.15) |

where .

Proof : By Cauchy-Schwarz inequality we have

(2.16) |

Then

leading to

(2.17) |

Taking into account the n-D Wirtinger inequality (2.12), we further
apply S-procedure [17] ^{2}

(2.18) |

with :

where .

Similarly

(2.19) | |||||

with .

We are looking next for conditions that guarantee along the classical solutions of the wave equation initiated from . Then and, thus, (2.15) yields

(2.20) | |||

Since is dense in the same estimate (2.20) remains true (by continuous extension) for any initial conditions . For such initial conditions we have mild solutions of (2.1), (2.3).

###### Theorem 1

Given and , assume that there exist positive constants and that satisfy the LMI (2.14) and the following LMIs:

(2.21) |

Then, under the condition (2.2), solutions of the boundary-value problem (2.9), (2.10) satisfy (2.20), where and are given by (2.15), i.e. the system governed by (2.9), (2.10) is exponentially stable with a decay rate .

Proof : Differentiating in time we obtain

We have

Applying Green’s formula to the first integral term, substituting and taking into account (2.2), we find

Furthermore, we have

Then Green’s formula leads to (see (11.35) of [13])

(2.22) |

Noting that on and taking into account the boundary conditions we obtain

(2.23) |

By inequalities (2.16) and (2.2) we have

Further due to (2.16)

Then by completion of squares we find

Summarizing we obtain

(2.24) |

Therefore, employing (2.19) we arrive at

(2.25) |

By taking into account Wirtinger’s inequality (2.12), we add to (2.25) the nonnegative term (2.18), where is replaced by . Denote . Then after employing the bound (2.13) we arrive at

if the LMIs (2.21) are feasible.

###### Remark 1

For the term of leads to in (cf. (2.22)).

## 3 Exact observability of n-D wave equation

Our next objective is to recover (if possible) the unique initial state (2.3) of the solution to (2.1)-(2.3) from the measurements on the finite time interval

(3.1) |

###### Definition 1

(i) for any initial condition it is possible to find a sequence from the measurements (3.1) such that (i.e. it is possible to recover the unique initial state as );

(ii) there exists a constant such that for any initial conditions and leading to the measurements and and to the corresponding sequences and , the following holds:

(3.2) |

The time is called the observability time.

The system is called regionally exactly observable if the above conditions hold for all with for some .

Note that (3.2) means the continuous in the measurements recovery of the initial state. In this section we will derive LMI sufficient conditions for n-D wave equations with globally Lipschitz in the first argument , where (2.2) holds globally in . In Section 4, we will present LMI-based conditions for the regional observability for 1-D wave equation, where (2.2) holds locally in .