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Identification of the unknown coefficient in a quasi-linear parabolic equation by a semigroup approach
Journal of Inequalities and Applications volume 2013, Article number: 212 (2013)
Abstract
This article presents a semigroup approach to the mathematical analysis of the inverse coefficient problems of identifying the unknown coefficient in the quasi-linear parabolic equation with Dirichlet boundary conditions , . It is shown that the unknown coefficient can be approximately determined via the semigroup approach.
1 Introduction
Consider the following initial boundary value problem:
where . The left and right boundary values , are assumed to be constants. The functions and satisfy the following conditions:
(C1) ;
(C2) , , .
Under these conditions, the initial boundary value problem (1) has the unique solution [1–3].
Consider the inverse problem of determining the unknown coefficient from the following observations at the boundary :
Here represents the solution of the parabolic problem (1). It is assumed that the function is noisy free measured output data. In this context the parabolic problem (1) will be referred to as a direct (forward) problem with the inputs and . The function is assumed to belong to and satisfy the consistency conditions . Moreover, it is assumed that the function satisfies the consistency conditions , .
In inverse problems, mostly numerical methods are employed to determine the unknown function. Even using one or more measured output data in analytic methods does not help in determination of the unknown function exactly [4, 5].
The purpose of this paper is to determine the analytic representation of the approximate unknown coefficient via a semigroup approach. We determine the approximate solution of the boundary value problem (1) without determining the unknown coefficient . In the determination of it, we just use the values , which are obtained and the over measured data . However, this cannot be possible for any kind of a boundary value problem with different type of boundary conditions. The semigroup approach for inverse problems for the identification of unknown coefficient in a quasi-linear parabolic equations was studied by Demir and Ozbilge [6, 7]. Moreover, the identification of the unknown diffusion coefficient in a linear parabolic equation was studied by Demir and Hasanov [8]. The study in this paper is based on similar philosophy to that in [6–9].
Recent studies [6–9] on the identification of the unknown function via a semigroup approach [10] show that the semigroup approach is the most effective analytical method which sheds more light on the unknown function. The weak point of this method is that you cannot apply this method to the initial boundary value problems with boundary conditions other than constant or zero. Overcoming this problem in some way makes this method more effective and valuable.
The paper is organized as follows. In Section 2, determination of the unknown coefficient is given via the semigroup approach. Some concluding remarks are given in Section 3.
2 Determination of the unknown coefficient via the semigroup approach
Consider the inverse problem of determining the explicit form of the unknown coefficient via the semigroup approach. Let the parabolic equation be arranged as follows:
Here we treat the term as a right-hand side function, i.e., . In order to formulate the solution of the parabolic problem (1) in terms of a semigroup, a new function needs to be defined to make the boundary values equal to zero:
which satisfies the following parabolic problem:
We can represent the solution of this problem in terms of a semigroup of linear operators. Here, is a second-order differential operator and its domain is , where and are Sobolev spaces. Obviously, by completion since the initial value function belongs to . Hence is dense in , which is a necessary condition for being an infinitesimal generator.
In the following, despite doing the calculations in the smooth function space, by completion they are valid in the Sobolev space. Denote by the semigroup of linear operators generated by the operator −A [5, 6]. Since the initial boundary value problem (4) possesses the property of uniqueness of its solutions, then the solution of its homogeneous part can be written as follows:
Furthermore, by using the variations of parameters formula, we can write the solution of the initial boundary value problem (3) as follows:
In order to construct the semigroup , we need to identify the eigenvalues and eigenfunctions of the infinitesimal operator A. Hence, the following eigenvalue problem must first be considered:
This problem is called the Sturm-Liouville problem. The eigenvalues are determined with for all the corresponding eigenfunctions as . In this case, the semigroup can be represented in the following way:
where . It is well known that the Sturm-Liouville problem (5) generates a complete orthogonal family of eigenfunctions so that the null space of the semigroup is trivial, i.e., . The null space of the semigroup of the linear operators can be defined as follows:
The unique solution of the initial-boundary value problem (4) in terms of the semigroup can be represented in the following form:
Hence, by using identity (3) and taking the initial value into account, the solution of the parabolic problem (1) in terms of a semigroup can be written in the following form:
Now, differentiating both sides of identity (7) with respect to t yields
Using the semigroup property ,
is obtained. Taking in the above identity, we get
Taking in the above identity, we obtain
Since , . Taking this into account yields
By solving the equation for , the following explicit formula for the value of the unknown coefficient is obtained:
Now, differentiating both sides of identity (8) with respect to x yields
Taking in the above identity, we obtain
Since , . Taking this into account yields
By solving the equation for , the following explicit formula for the value of the unknown coefficient is obtained:
By making use of the values and , we can write a linear approximation for near as follows:
Substituting the linear approximation (11) into equation (8) yields
which enables us to obtain the approximate solution of the parabolic problem (1). For this, we first need to show that
by using the well-known property of the semigroup of the linear operators
which leads us to the determination of the unknown coefficient analytically.
Lemma Let be the semigroup of the linear operators, and let be the solution of the equation , where A is the infinitesimal generator of . Then the following identity holds:
for each .
Proof First of all, observe that since eigenfunctions of the infinitesimal generator A generate a complete orthogonal family, we can write in terms of eigenfunctions in the following form:
Taking derivative of both sides with respect to the variable t produces
Notice that this identity implies that is in the domain of the semigroup of linear operators . Hence we can apply to :
Now we know that
Taking derivative of both sides with respect to the variable t produces
which is exactly the same as (11). This is exactly what is desired. However, notice that the identity
does not hold. □
This lemma leads us to the solution of the parabolic problem (1) without knowing the unknown coefficient , but knowing and . Then the solution enables us to determine the unknown coefficient approximately.
Now, applying the identity (13) to (12) and using the linearity, we get
Hence we can obtain the solution from the above identity by integrating with respect to t as follows:
where denotes the integration constant. We can determine it by using the initial condition
which yields
Substituting the integration constant into the identity (15), we obtain the approximate solution of the parabolic problem (1). Hence, by using this solution and the parabolic equation, we are able to determine the unknown coefficient approximately.
3 Conclusion
The aim of this study was to determine the unknown coefficient via the semigroup approach. Lemma in Section 2 plays a crucial role in the identification of the unknown coefficient . This lemma leads us to new conclusions about the identification problems via the semigroup approach. We will present these results in our future studies….
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Acknowledgements
Dedicated to Professor Hari M Srivastava.
The research was supported by parts by the Scientific and Technical Research Council (TUBITAK) of Turkey and Izmir University of Economics.
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Ozbilge, E., Demir, A. Identification of the unknown coefficient in a quasi-linear parabolic equation by a semigroup approach. J Inequal Appl 2013, 212 (2013). https://doi.org/10.1186/1029-242X-2013-212
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DOI: https://doi.org/10.1186/1029-242X-2013-212