Abstract and Applied Analysis

Volume 2015, Article ID 276810, 11 pages

http://dx.doi.org/10.1155/2015/276810

## The Dirichlet Problem for Second-Order Divergence Form Elliptic Operators with Variable Coefficients: The Simple Layer Potential Ansatz

Department of Mathematics, Computer Science and Economics, University of Basilicata, Viale dell'Ateneo Lucano 10, 85100 Potenza, Italy

Received 9 June 2015; Accepted 18 October 2015

Academic Editor: Giovanni P. Galdi

Copyright © 2015 Alberto Cialdea 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 investigate the Dirichlet problem related to linear elliptic second-order partial differential operators with smooth coefficients in divergence form in bounded connected domains of () with Lyapunov boundary. In particular, we show how to represent the solution in terms of a simple layer potential. We use an indirect boundary integral method hinging on the theory of reducible operators and the theory of differential forms.

#### 1. Introduction

As remarked in [1, p. 121], elliptic operators with variable coefficients naturally arise in several areas of physics and engineering. In this paper, we study the Dirichlet problem related to a scalar elliptic second-order differential operator with smooth coefficients in divergence form in a bounded simply connected domain of () with Lyapunov boundary.

This is a classical problem which nowadays can be treated in several ways. In particular, different potential methods have been developed for such operators (see, e.g., [1–6]).

In the present paper, we obtain the solution of the Dirichlet problem by means of a simple layer potential instead of the classical double layer potential (see, e.g., [6, pp. 73–75]). We use an indirect boundary integral method introduced for the first time in [7] for the -dimensional Laplacian. It requires neither the knowledge of pseudodifferential operators nor the use of hypersingular integrals, but it hinges on the theory of singular integral operators and the theory of differential forms (for details of the method, see, e.g., [8, Section ]). The method has been also used to treat different boundary value problems in simply connected domains: the Neumann problem for Laplace equation (via a double layer potential), the Dirichlet problem for the Lamé and Stokes systems, the four boundary value problems of the theory of thermoelastic pseudooscillations, the traction problem for Lamé and Stokes systems, the four basic boundary value problems arising in couple-stress elasticity, and the two boundary value problems of the linear theory of viscoelastic materials with voids (see [9, 10] and the references therein). The method can be applied also in multiply connected domains, as shown for the Laplacian, the linearized elastostatics, and the Stokes system (see [11] and the references therein).

The present paper is organized as follows.

In Section 2, after giving preliminary results, we make use of Fichera’s construction of a principal fundamental solution [12] and we prove some identities for the related nuclear double form.

Section 3 is devoted to the study of the Dirichlet problem. It contains the main results concerning the reduction of a certain singular integral operator acting in spaces of differential forms and the integral representation of the solution of the Dirichlet problem by means of a simple layer potential.

#### 2. Preliminary Results

Let be a bounded domain (open connected set) of ().

In this paper, we deal with the Dirichlet problem:where is a scalar second-order differential operator (throughout this paper, we use the Einstein summation convention): We suppose that the coefficients are defined on , being an open ball containing , and we assume that they belong to , .

Moreover, assume that is a symmetric contravariant positive-definite tensor. Then, is a uniform elliptic operator; that is, there exists such that , for every and for any .

For the sake of simplicity, we suppose that the determinant of is equal to 1.

It is known that to the contravariant tensor there corresponds a covariant tensor such that being the Kronecker delta.

A differential form of degree (in short a -form) on is a function defined on whose values are in the -covectors space of . A -form can be represented aswith respect to an admissible coordinate system , where are the components of a skew-symmetric covariant tensor (for details about differential forms, we refer to [13, 14]).

The symbol means the space of all -forms whose components are continuously differentiable up to the order in a coordinate system of class (and then in every coordinate system of class ).

If , the differential of is a -form defined asFurther, if , the adjoint of is the following -form:where is the generalized Kronecker delta (). We recall that (see, e.g., [15, p. 127])

We remark that (see, e.g., [13, p. 285])

If , we define the codifferential of as the following -form:A differential double form of degree with respect to and of degree with respect to (in short a double -form) is represented asIf , we denote it briefly by .

We proceed to introduce the following double -form (see [13, p. 204]) defined, for every , , aswhere, for , ( being the hypersurface measure of the unit sphere in ) is a parametrix in the sense of Hilbert and E.E. Levi for the operator . We recall that (if we write , being a double -form, we mean that all its components are )(see [13, Section ]).

The next results provide other properties of and .

Lemma 1. *For every ,where .*

*Proof. *Taking definition (13) into account, we haveOn the other hand,and this yields the claim.

The identities proved in the next proposition generalize the ones obtained by Colautti [16, p. 309] for the Laplacian.

Proposition 2. *Let be the double -form defined by (11). Then, for every , the following properties hold:wherewhere ; andwhere*

*Proof. *First, we prove (19). It follows from (12), (3), and (7) thatOn the other hand,From (12), (3), and (7), we have thatwhere satisfies (20) on account ofand (8).

Now we pass to show (21). With calculations analogue to (26), we have thatwhere thanks to (15) and (27).

Moreover,Arguing again as in (26) and taking Lemma 1 into account, we getwhere both and are . Then, we obtain the claim by settingFinally, we prove (22). Thanks to (9) and (19), we havewhere . Now, by using (21) and (8), we getand hence the claim with

Proposition 3. *If , thenwhere is a linear first-order differential operator whose coefficients depend only on first- and second-order derivatives of entries of the tensor .**In particular,*

*Proof. *We begin by observing thatSince is symmetric and , we getand, keeping in mind (7), we getOn the other hand,Then,and this proves (35). Finally, (36) follows from (35).

Finally, following Fichera we employ the parametrix to construct a principal fundamental solution of the differential operator (see [12]).

Lemma 4. *There exists such that the function is a principal fundamental solution of . In particular, we haveMoreover, for every ,for some .*

*Proof. *The existence of can be obtained as the solution of a certain integral equation (see [12, §]). In [12] properties (43) are proved and is written aswhere is a smooth function on and, for some ,Then, by (15),

#### 3. The Dirichlet Problem

In this section, we suppose that the domain is such that is connected and such that its boundary is a Lyapunov surface (i.e., , ).

By , we denote the outwards unit normal vector at the point and by we denote the conormal vector at the point associated with the operator and defined as (). By we denote the conormal derivative

As usual, the symbols and () stand for the classical Lebesgue and Sobolev spaces, respectively.

By , we denote the space of all -forms whose components are real-valued functions in a coordinate system of class (and then in every coordinate system of class ).

We will look for the solution of the Dirichlet problem for the operator in the domain in the form of a simple layer potential. To this end, we introduce the space .

*Definition 5. *The function belongs to if and only if there exists such that it can be represented by means of a simple layer potential; that is,

Specifically our aim is to give an existence and uniqueness theorem for the Dirichlet problemFirst, we prove the following formula.

Proposition 6. *For any ,where is the linear first-order differential operator considered in Proposition 3 and*

*Proof. *Set, for every ,On account of (22) and (36), we getand (52) follows from (23).

On the other hand, if is the minor of obtained deleting the th row and the th column, for , we getTherefore,Then, if ,and this concludes the proof.

*Remark 7. *We note that (51) generalizes the following identity (see [7] [8, Proposition ]):where and denote the fundamental solution for Laplace equation and the double -form associated with , respectively.

We recall that if and are two Banach spaces and is a continuous linear operator, we say that can be reduced on the left if there exists a continuous linear operator such that , where stands for the identity operator on and is compact. One of the main properties of such operators is that equation has a solution if and only if for any such that , being the adjoint of (see [17, 18]).

Theorem 8. *Let be the singular integral operator defined as**Then, can be reduced on the left by the operator : where the symbol means that if is an -form on , then .*

*Proof. *We start with the observation that and then The operator is compact because of (44). Concerning , keeping in mind Proposition 6 and setting , we getSince and in view of (52), is a compact operator from into itself.

In view of the Stokes formula for and on account of known properties of potentials (see, e.g., [6, p. 35]), we getThen,Since , is a compact operator.

Thus,is a Fredholm operator and the assertion is proved.

Theorem 9. *Given , there exists a solution of the singular integral equationif and only iffor every weakly closed form .*

*Proof. *Denote by the adjoint of ; that is,From Theorem 8, it follows that operator can be reduced on the left; therefore, (67) admits a solution if and only ifOn the other hand, if and only if is a weakly closed form; that is,In fact, ifwe have and then for any smooth solution of in . Therefore, we have Let us consider If and are such that in and on , we have From the Green formulas we haveIn view of (72), we find