#### Abstract

The optimal boundary control problem for () infinite order distributed parabolic systems with multiple time delays given in the integral form both in the state equations and in the Neumann boundary conditions is considered. Constraints on controls are imposed. Necessary and suffacient optimality conditions for the Neumann problem with the quadratic performance functional are derived.

#### 1. Introduction

Distributed parameters systems with delays can be used to describe many phenomena in the real world. As is well known, heat conduction, properties of elastic-plastic material, fluid dynamics, diffusion-reaction processes, the transmission of the signals at a certain distance by using electric long lines, and so forth, all lie within this area. The object that we are studying (temperature, displacement, concentration, velocity, etc.) is usually referred to as the state.

During the last twenty years, equations with deviating argument have been applied not only in applied mathematics, physics, and automatic control, but also in some problems of economy and biology. Currently, the theory of equations with deviating arguments constitutes a very important subfield of mathematical control theory.

Consequently, equations with deviating arguments are widely applied in optimal control problems of distributed parameter system with time delays [1].

The optimal control problems of distributed parabolic systems with time-delayed boundary conditions have been widely discussed in many papers and monographs. A fundamental study of such problems is given by [2] and was next developed by [3, 4]. It was also intensively investigated by [1, 5–16] in which linear quadratic problem for parabolic systems with time delays given in the different form (constant, time delays, time-varying delays, time delays given in the integral form, etc.) was presented.

The necessary and sufficient conditions of optimality for systems consist of only one equation and for systems governed by different types of partial differential equations defined on spaces of functions of infinitely many variables and also for infinite order systems are discussed for example in [9, 11, 15–18] in which the argument of [19, 20] was used.

Making use of the Dubovitskii-Milyutin Theorem in [13, 21–28] the necessary and sufficient conditions of optimality for similar systems governed by second order operator with an infinite number of variables and also for infinite order systems were investigated. The interest in the study of this class of operators is stimulated by problems in quantum field theory.

In particular, the papers of [1, 8] present necessary and sufficient optimality conditions for the Neumann problem with quadratic performance functionals, applied to a single one equation of second-order parabolic system with fixed time delay and with multiple time delays given in the integral form both in the state equations and in the Neumann boundary conditions, respectively. Such systems constitute a more complex case of distributed parameter systems with time delays given in the integral form.

Also in [9, 11] time-optimal boundary control for a single one equation distributed infinite order parabolic and hyperbolic systems in which constant time lags appear in the integral form both in the state equation and in the Neumann boundary condition is present. Some specific properties of the optimal control are discussed.

In this paper we recall the problem in a more general formulation. A distributed parameter for infinite order parabolic systems with multiple time delays given in the integral form both in the state equations and in the Neumann boundary conditions is considered. Such an infinite order parabolic system can be treated as a generalization of the mathematical model for a plasma control process. The quadratic performance functionals defined over a fixed time horizon are taken and some constraints are imposed on the initial state and the boundary control. Such a system may be viewed as a linear representation of many diffusion processes, in which time-delayed signals are introduced at a spatial boundary, and there is a freedom in choosing the controlled process initial state. Following a line of the Lions scheme, necessary and sufficient optimality conditions for the Neumann problem applied to the above system were derived. The optimal control is characterized by the adjoint equations.

This paper is organized as follows. In Section 1, we introduce spaces of functions of infinite order. In Section 2, we formulate the mixed Neumann problem for infinite order parabolic operator with multiple time delays given in the integral form. In Section 3, the boundary optimal control problem for this case is formulated, then we give the necessary and sufficient conditions for the control to be an optimal. In Section 4, we generalized the discussion to two cases, the first case: the optimal control for coupled infinite order parabolic systems is studied. The second case: the optimal control for coupled infinite order parabolic systems was to be formulated.

#### 2. Sobolev Spaces with Infinite Order

The object of this section is to give the definition of some function spaces of infinite order and the chains of the constructed spaces which will be used later.

Let be a bounded open set of with a smooth boundary , which is a manifold of dimension . Locally, is totally on one side of . We define the infinite order Sobolev space of infinite order of periodic functions defined on [29–31] as follows: where is the space of infinitely differentiable functions, is a numerical sequence, and is the canonical norm in the space , and being a multi-index for differentiation, .

The space is defined as the formal conjugate space to the space , namely: where and .

The duality pairing of the spaces and is postulated by the formula: where

From above, is everywhere dense in with topological inclusions and denotes the topological dual space with respect to , so we have the following chain of inclusions: We now introduce which we will denoted by , where denotes the space of measurable functions such that endowed with the scalar product , is a Hilbert space.

In the same manner we define the spaces , and , as its formal conjugate.

Also, we have the following chain of inclusions:

The construction of the Cartesian product of -times to the above Hilbert spaces can be constructed, for example with norm defined by: where is a vector function and .

Finally, we have the following chain of inclusions: where are the dual spaces of . The spaces considered in this paper are assumed to be real.

#### 3. Mixed Neumann Problem for Infinite Order Parabolic System with Multiple Time Lags

The object of this section is to formulate the following mixed initial boundary value Neumann problem for infinite order parabolic system with multiple time delays which defines the state of the system model [1, 5–11, 18, 24, 26]. where has the same properties as in Section 1. We have (i)is a specified positive number representing a finite time horizon,(ii) are time delays, such that and where , , for and , , for ,(iii) are given real functions defined on ,(iv) are given real functions defined on ,(v),(vi) is a function defined on such that ,(vii) are functions defined on and such that and ,(viii) are initial functions defined on and , respectively, such that . .

The parabolic operator in the state equation (3.1) is an infinite order parabolic operator and [17, 21, 29–31] is given by: is an infinite order self-adjoint elliptic partial differential operator maps onto .

For this operator we define the bilinear form as follows.

*Definition 3.1. *For each , we define a family of bilinear forms on by:
where maps onto and takes the above form. Then

Lemma 3.2. *The bilinear form is coercive on , that is,
*

* Proof. *It is well known that the ellipticity of is sufficient for the coerciveness of on :
Then
Also we have

Equations (3.1)–(3.5) constitute a Neumann problem. Then the left-hand side of the boundary condition (3.4) may be written in the following form:
where is a normal derivative at , directed towards the exterior of , and is the direction cosine of , with being the normal at exterior to .

Then (3.4) can be written as:

*Remark 3.3. *We will apply the indication appearing in (3.14) to prove the existence of a unique solution for (3.1)–(3.5).

We will formulate sufficient conditions for the existence of a unique solution of the mixed boundary value problem (3.1)–(3.5) for the cases where the boundary control .

For this purpose, we introduce the Sobolev space [20, Vol. 2, page 6] defined by: which is a Hilbert space normed by where the space denotes the Sobolev space of order of functions defined on and taking values in [20, Vol. 1].

The existence of a unique solution for the mixed initial-boundary value problem (3.1)–(3.5) on the cylinder can be proved using a constructive method, that is, solving at first equations (3.1)–(3.5) on the subcylinder and in turn on and so forth, until the procedure covers the whole cylinder . In this way, the solution in the previous step determines the next one.

For simplicity, we introduce the following notation:

Making use of the results of [7, 20] we can prove that the following result holds.

Theorem 3.4. * Let , , , and be given with , , , and . Then, there exists a unique solution for the mixed initial-boundary value problem (3.1)–(3.5). Moreover, .*

#### 4. Problem Formulation-Optimization Theorems

Now, we formulate the optimal control problem for (3.1)–(3.5) in the context of the Theorem 3.4, that is .

Let us denote by the space of controls. The time horizon is fixed in our problem.

The performance functional is given by where , and is a given element in ; is a positive linear operator on into .

* Control Contraints*

We define the set of admissible controls such that

Let denote the solution of the mixed initial-boundary value problem (3.1)–(3.5) at corresponding to a given control . We note from Theorem 3.4 that for any the performance functional (4.1) is well-defined since .

Making use of the Loins's scheme we will derive the necessary and sufficient conditions of optimality for the optimization problem (3.1)–(3.5), (4.1), (4.2). The solving of the formulated optimal control problem is equivalent to seeking a such that

From the Lion's scheme [19, Theorem 1.3, page 10], it follows that for a unique optimal control exists. Moreover, is characterized by the following condition: For the performance functional of form (4.1) the relation (4.4) can be expressed as

In order to simplify (4.5), we introduce the adjoint equation, and for every , we define the adjoint variable as the solution of the equations: where

As in the above section with change of variables, that is, with reversed sense of time. that is, , for given and any , there exists a unique solution for problem (4.6)–(4.11).

The existence of a unique solution for the problem (4.6)–(4.11) on the cylinder can be proved using a constructive method. It is easy to notice that for given and , the problem (4.6)–(4.11) can be solved backwards in time starting from , that is, first solving (4.6)–(4.11) on the subcylinder and in turn on , and so forth until the procedure covers the whole cylinder . For this purpose, we may apply Theorem 3.4 (with an obvious change of variables).

Hence, using Theorem 3.4, the following result can be proved.

Lemma 4.1. *Let the hypothesis of Theorem 3.4 be satisfied. Then for given and any , there exists a unique solution for the adjoint problem (4.6)–(4.11).*

We simplify (4.5) using the adjoint equation (4.6)–(4.11). For this purpose denoting by and , respectively, setting in (4.6)–(4.11), multiplying both sides of (4.6) and (4.7) by , then integrating over and , respectively and then adding both sides of (4.6), (4.11), we get Using (3.1), the first integral on the right-hand side of (4.13) can be written as: Using Green’s formula, the second integral on the right-hand side of (4.13) can be written as: Using the boundary condition (3.2), one can transform the second integral on the right-hand side of (4.15) into the form:

The last component in (4.15) can be rewritten as Substituting (4.16) and (4.17) into (4.15) and then the results into (4.13), we obtain

Afterwards, using the facts that for and and for and , and consequently for , we obtain

Substituting (4.19) into (4.5) gives

The foregoing result is now summarized.

Theorem 4.2. *For the problem (3.1)–(3.5), with the performance functional (4.1) with and and with constraints on controls (4.2), there exists a unique optimal control which satisfies the maximum condition (4.20).*

##### 4.1. Mathematical Examples

*Example 4.3. *Consider now the particular case where (no constraints case). Thus the maximum condition (4.20) is satisfied when
If is the identity operator on , then from Lemma 4.1 it follows that .

*Example 4.4. *We can also consider an analogous optimal control problem where the performance functional is given by:
where .

From Theorem 3.4 and the Trace Theorem [20, Vol. 2, page 9], for each , there exists a unique solution with . Thus, is well defined. Then, the optimal control is characterized by: We define the adjoint variable as the solution of the equations: As in the above section, we have the following result.

Lemma 4.5. *Let the hypothesis of Theorem 3.4 be satisfied. Then, for given and any , there exists a unique solution to the adjoint problem (4.24).*

Using the adjoint equations (4.24)in this case, the condition (4.23) can also be written in the following form: The following result is now summarized.

Theorem 4.6. *For the problem (3.1)–(3.5) with the performance function (4.22) with and , and with constraint (4.2), and with adjoint equations (4.24), there exists a unique optimal control which satisfies the maximum condition (4.25).*

*Example 4.7 (). *We can also consider an analogous optimal control problem where the performance functional is given by:
where .

From Theorem 3.4 and the Trace Theorem [20, Vol. 2, page 9], for each , there exists a unique solution . Thus, is well defined. Then, the optimal control is characterized by: We define the adjoint variable as the solution of the equations: As in the above section, we have the following result.

Lemma 4.8. *Let the hypothesis of Theorem 3.4 be satisfied. Then, for given and any , there exists a unique solution to the adjoint problem (4.28).**Using the adjoint equations (4.28) in this case, the condition (4.27) can also be written in the following form:
**
The following result is now summarized.*

Theorem 4.9. *For the problem (3.1)–(3.5) with the performance function (4.26) with and , and with constraint (4.2), and with adjoint equations (4.28), there exists a unique optimal control which satisfies the maximum condition (4.29).*

*Example 4.10. *We can also consider an analogous optimal control problem where the performance functional is given by:
where .

From Theorem 3.4 and the Trace Theorem [20, Vol. 2, page 9], for each , there exists a unique solution with . Thus, is well defined. Then, the optimal control is characterized by:

The above inequality can be simplified by introducing an adjoint equation, the form of which is identical to (4.24). Then using Theorem 3.4 we can establish the existence of a unique solution for (4.24).

As in the above section, we have the following result.

Lemma 4.11. *Let the hypothesis of Theorem 3.4 be satisfied. Then, for given and any , there exists a unique solution to the adjoint problem (4.24)–(37).**Using the adjoint equations (4.24)–(37) in this case, the condition (4.31) can also be written in the following form:
**
The following result is now summarized.*

Theorem 4.12. *For the problem (3.1)–(3.5) with the performance function (4.30) with and , and with constraint (4.2), and with adjoint equations (4.24), there exists a unique optimal control which satisfies the maximum condition (4.32).*

#### 5. Generalization

The optimal control problems presented here can be extended to certain different two cases. Case 1: optimal control for coupled infinite order parabolic systems with multiple time delays. Case 2: optimal control for coupled infinite order parabolic systems with multiple time delays. Such extension can be applied to solving many control problems in mechanical engineering.

*Case 1 (optimal control for coupled infinite order parabolic systems with multiple time delays). *We can extend the discussions to study the optimal control for coupled infinite order parabolic systems with multiple time delays. We consider the case where , the performance functional is given by [15, 16]:
where .

The following results can now be proved.

Theorem 5.1. *Let , ,, , and be given with , , and . Then, there exists a unique solution for the following mixed initial-boundary value problem:
**
where
*

Lemma 5.2. *Let the hypothesis of Theorem 5.1 be satisfied. Then for given and any , there exists a unique solution for the adjoint problem:
*