International Journal of Mathematics and Mathematical Sciences

Volume 2011 (2011), Article ID 451492, 14 pages

http://dx.doi.org/10.1155/2011/451492

## Existence and Uniqueness of Generalized Solutions to a Telegraph Equation with an Integral Boundary Condition via Galerkin's Method

^{1}Laboratory of Advanced Materials, Badji Mokhtar University, P.O. Box 12, 23000 Annaba, Algeria^{2}Department of Mathematics, Motilal Nehru National Institute of Technology Allahabad, Allahabad 211 004, India^{3}Department of Mathematics and Statistics, Indian Institute of Technology Kanpur, Kanpur 208 016, India

Received 28 December 2010; Revised 1 March 2011; Accepted 4 March 2011

Academic Editor: Christian Corda

Copyright © 2011 Assia Guezane-Lakoud et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We consider a telegraph equation with nonlocal boundary conditions, and using the application of Galerkin's method we established the existence and uniqueness of a generalized solution.

#### 1. Introduction

The initial work devoted to the second-order partial differential equations with nonlocal integral conditions goes back to Cannon [1]. Subsequent investigations include [1–10] and many other references therein.

Nonlocal problems for some classes of partial differential equations have attracted much interest in the last few years, and several papers have been devoted to this subject. The problem of existence and uniqueness of solutions is of growing interest, as well as methods of obtaining explicit or approximate solutions for this kind of problems. The main reason of such a wide interest in this type of problems is that the non-local conditions occur in various equations of mathematical physics and in mathematical biology.

Nonlocal conditions come up when values of the function on the boundary are connected to values inside the domain. There are various types of non-local integral conditions for hyperbolic, parabolic, or elliptic equations; we point out an important non-local problem (see [11, 12]), studied for a parabolic equation with the following non local conditions. connected with quasistationary thermoelasticity theory. The non-local boundary conditions of type arise in modeling the technology of integral circuits, studied in [13].

Problems with nonlocal integral conditions for multidimensional hyperbolic equations have been intensively studied in recent years. However, the nonlocal integral conditions for equations were in the form of second kind as or where is a bounded domain in with a smooth boundary, is the inward normal vector to ; see [14, 15]. Non-local integral conditions of first kind as are studied in [15]. Under some assumptions on the kernel , existence and uniqueness of the generalized solution are established.

The present work deals with the application of the Galerkin method to determine a function , is a bounded domain with a smooth boundary , , which satisfies, in weak sense, the telegraph equation subject to the initial conditions and the integral condition where is the inward normal vector to . This equation arises in the analysis of propagation of electrical signals in a cable transmission line. Both the current and voltage satisfy an equation of this form. This equation arises also in the propagation of pressure waves in the study of pulsate blood flow in arteries.

Cannon et al. [5] have developed and analyzed finite-difference and finite-element Galerkin’s methods for the solution of one-dimensional heat equation with the Neumann and integral conditions. Cannon and van der Hoek [4] presented numerical schemes based on the finite difference method. In [16], the authors have applied the Galerkin method to a parabolic problem with an integral boundary condition and established the existence, uniqueness, and continuous dependence upon the data of a weak solution. In [17–21], the authors have used the method of semidiscretization in time and have established the existence and uniqueness of a weak solution. In [22], the authors have discussed some existence uniqueness results for a neutral functional differential equation with a nonlocal initial condition via the Galerkin approximation.

The paper is organized as follows. In Section 2, we specify notations, state some inequalities and make the sense of the desired solution precise. In Section 3, we establish the uniqueness of the solution. Finally, Section 4, is devoted to the construction of the approximate solution and its existence via the Galerkin method.

#### 2. Notation and Definition

Let be the usual space of Lebesgue square integrable real functions on whose inner product and norm will be denoted by and , respectively.

is the Sobolev space consisting of functions such that all derivatives lower than one belong to equipped with the norm

We define the space .

##### 2.1. Some Useful Inequalities: Gronwall Inequalities [23]

Let and be two integrable nonnegative functions on the interval with nondecreasing. If where is a positive constant, then .

*Cauchy Schwarz inequality:*

*Cauchy inequality:*

*Trace inequality:*

where is a positive constant that depends only on and on the domain .

##### 2.2. Definition of Generalized Solution

We define the generalized solution of the problem (1.6)–(1.8) as a weak solution that satisfies for all Integrating by parts, we get Regrouping the identities (2.7), then (2.6) becomes

*Definition 2.1. **By a generalized solution of problem *(1.6)*–*(1.8) *one means a function ** such that the identity (2.8) holds for all function *.

#### 3. Uniqueness of the Generalized Solution

Now we will show that the generalized solution of problem (1.6)–(1.8), if it exists, is unique.

Theorem 3.1. *Assume that , ,, , and**
then the generalized solution of problem (1.6)–(1.8), if it exists, is unique.*

*Proof. *Suppose that there exist two different generalized solutions and for the problem (1.6)–(1.8); then obviously their difference is a generalized solution of the problem (1.6)–(1.8) with homogeneous equation and homogeneous initial and non local conditions, that is, . We will prove that in . Let , and denote . Consider the function
The identity (2.8) becomes
Substituting into (3.3), integrating by parts, and then using the fact that , it follows that
Using the assumption on the function , we get
where . Now, applying Cauchy Schwarz inequality to the first term in the right-hand side of (3.5), then Cauchy inequality with , yields
Using the trace inequality, we obtain
Considering the function
it is easy to see that , , and . Consequently, substituting in (3.7), we get
Seting
it follows that
Since is arbitrary chosen, let ; then (3.11) becomes
Applying Gronwall Lemma, we get
Consequently, we obtain , for all and .

If , then in . In the case where , we see that , where , is the entire part of ; then repeating the preceding reasoning for , we get , for all , and then in . Thus, the uniqueness is proved.

*Remark 3.2. *It should be noted that we can prove the uniqueness theorem by using a priori estimates, so we establish that the generalized solution, if it exists, satisfies the inequality

#### 4. Existence of Generalized Solution

Theorem 4.1. *Assume that the assumptions of Theorem 4.1 hold; then the non-local problem (1.6)–(1.8) has a unique solution .*

*Proof. *In order to prove the existence of the generalized solution we apply Galerkin’s method. Let be a fundamental system in , such that . Now we will try to find an approximate solution of the problem (1.6)–(1.8) in the form
The approximates of the functions and are denoted, respectively, by
Substituting the approximation solution in (1.6), multiplying both sides by , and then integrating according to on , we get
Substituting (4.1) in (4.3), we obtain
This implies that
Denote
Then (4.5) becomes
We obtain a system of differential equations of second order according to the variable with smooth coefficients and the initial conditions
Consequently, we get a Cauchy problem of linear differential equations with smooth coefficients that is uniquely solvable. So it has a unique solution satisfying (4.3).

Lemma 4.2. *The sequence is bounded.*

*Proof. *Multiplying (4.4) by and summing over from 1 to , we get
Integrating by parts the left-hand side of (4.9) over on yields
Substituting the four last identities into (4.9), we get
Applying Cauchy Schwarz inequality, Cauchy inequality, the hypothesis on the operator , and the fact that to the second term in the right hand side of (4.11), we get
We may obtain similar inequalities for the third and the fourth terms in the right-hand side of (4.11). Now if we apply Cauchy Schwarz inequality to the last term in (4.11) we get
Using the result inequalities in (4.11) and then regrouping the same terms yield
Choose such that and and let
then (4.14) becomes
Now, we apply Gronwall Lemma to get
Since and are the orthogonal projections of and , respectively, from the Pythagorean relation we have that and . Integrating (4.17) according to on yields
Consequently, the sequence is bounded.

*Remark 4.3. *We have proved that the sequence is bounded, so we can extract a subsequence which we denote by that is weakly convergent; then we prove that its limit is the desired solution of the problem (1.6)–(1.8).

Lemma 4.4. *The limit of the subsequence is the solution of the problem (1.6)–(1.8).*

*Proof. *For this we prove that the limit of the subsequence satisfies the identity (2.8) for all functions . Since the set is such that is dense in , it suffices to prove (2.8) for . Multiplying (4.3) by the function , , and then taking the sum from to , we obtain
Integrating by parts on , we get
Denote the weak limit of the subsequence by . When tends to infinity, we get
Indeed, using Cauchy Schwarz inequality then Cauchy inequality, we obtain
and then by passing to the limit in (4.20) we get that the limit satisfies(2.8).

*Example 4.5. *Now we present an example to demonstrate the applications of the results established in the earlier sections. We consider the following:
with the initial conditions
and the nonlocal boundary condition

The results of the earlier sections guarantee the existence and uniqueness of a solution. It may be noted that is the unique solution of (4.23)–(4.25).

#### Acknowledgment

The authors thank the referees for their helpful suggestions and corrections to improve the paper.

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