## Boundary Value Problems

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# Dirichlet problem for the Schrödinger operator on a cone

Boundary Value Problems20122012:59

DOI: 10.1186/1687-2770-2012-59

Accepted: 2 May 2012

Published: 18 June 2012

## Abstract

In this article, a solution of the Dirichlet problem for the Schrödinger operator on a cone is constructed by the generalized Poisson integral with a slowly growing continuous boundary function. A solution of the Poisson integral for any continuous boundary function is also given explicitly by the Poisson integral with the generalized Poisson kernel depending on this boundary function.

MSC:31B05, 31B10.

### Keywords

Dirichlet problem stationary Schrödinger equation cone

## 1 Introduction and results

Let R and be the set of all real numbers and the set of all positive real numbers respectively. We denote the n-dimensional Euclidean space by (). A point in is denoted by , where . The Euclidean distance between two points P and Q in is denoted by . Also with the origin O of is simply denoted by . The boundary and the closure of a set S in are denoted by S and respectively.

We introduce a system of spherical coordinates , , in which are related to Cartesian coordinates by .

The unit sphere and the upper half unit sphere in are denoted by and , respectively. For simplicity, a point on and the set for a set Ω, , are often identified with Θ and Ω, respectively. For two sets and , the set in is simply denoted by .

For and , let denote an open ball with a center at P and radius r in . . By , we denote the set in with the domain Ω on . We call it a cone. We denote the sets and with an interval on R by and . By we denote . By we denote which is . We denote the -dimensional volume elements induced by the Euclidean metric on by .

Let denote the class of nonnegative radial potentials , i.e., , , such that with some if and with if or .

(1.1)

where , Δ is the Laplace operator and . These solutions called a-harmonic functions or generalized harmonic functions are associated with the operator . Note that they are (classical) harmonic functions in the case . Under these assumptions, the operator can be extended in the usual way from the space to an essentially self-adjoint operator on (see [13]). We will denote it as well. This last one has a Green’s function . Here is positive on and its inner normal derivative . We denote this derivative by , which is called the Poisson a-kernel with respect to . We remark that and are the Green’s function and Poisson kernel of the Laplacian in respectively.

Given a domain and a continuous function u on , we say that h is a solution of the Dirichlet problem for the Schrödinger operator on D with u if in D and

for every . Note that h is a solution of the classical Dirichlet problem for the Laplacian in the case .

Let be a Laplace-Beltrami operator (the spherical part of the Laplace) on and () be the eigenvalues of the eigenvalue problem for on Ω (see, e.g., [4], p. 41])
Corresponding eigenfunctions are denoted by (), where is the multiplicity of . We set , norm the eigenfunctions in and . Then there exist two positive constants and such that
(1.2)

for (see Courant and Hilbert [5]), where .

In order to ensure the existences of (). We put a rather strong assumption on Ω: if , then Ω is a -domain () on surrounded by a finite number of mutually disjoint closed hypersurfaces (e.g., see [6], pp. 88-89] for the definition of -domain). Then () and on Ω (here and below, denotes differentiation along the interior normal).

Hence well-known estimates (see, e.g., [7], p. 14]) imply the following inequality:
(1.3)

where the symbol denotes a constant depending only on n.

Let and stand, respectively, for the increasing and nonincreasing, as , solutions of the equation
(1.4)

normalized under the condition .

We shall also consider the class , consisting of the potentials such that there exists a finite limit ; moreover, . If , then the solutions of Equation (1.1) are continuous (see [8]).

In the rest of the article, we assume that and we shall suppress this assumption for simplicity. Further, we use the standard notations , , is the integer part of d and , where d is a positive real number.

Denote
It is known (see [9]) that in the case under consideration the solutions to Equation (1.4) have the asymptotics
(1.5)

where and are some positive constants.

If , it is known that the following expansion for the Green function (see [10], Ch. 11], [1, 11])

where and , is their Wronskian. The series converges uniformly if either or ().

For a nonnegative integer m and two points , , we put
where
We introduce another function of and
The generalized Poisson kernel (, ) with respect to is defined by
In fact,

We remark that the kernel function coincides with the one in Yoshida and Miyamoto [12] (see [10], Ch. 11]).

Put

where is a continuous function on and is a surface area element on .

With regard to classical solutions of the Dirichlet problem for the Laplacian, Yoshida and Miyamoto [12], Theorem 1] proved the following result.

Theorem A If u is a continuous function on satisfying
then is a classical solution of the Dirichlet problem on with g and satisfies

Our first aim is to give growth properties at infinity for .

Theorem 1 Let (resp. ), (resp. ) and

If u is a measurable function on satisfying
(1.6)
then
(1.7)
(1.8)

Next, we are concerned with solutions of the Dirichlet problem for the Schrödinger operator on .

Theorem 2 Let γ andbe as in Theorem 1. If u is a continuous function onsatisfying (1.6), thenis a solution of the Dirichlet problem for the Schrödinger operator onwith u and (1.7) (resp. (1.8)) holds.

If we take , then we immediately have the following corollary, which is just Theorem A in the case .

Corollary If u is a continuous function on satisfying
(1.9)
then is a solution of the Dirichlet problem for the Schrödinger operator on with u and satisfies
(1.10)

By using Corollary, we can give a solution of the Dirichlet problem for any continuous function on .

Theorem 3 If u is a continuous function onsatisfying (1.9) andis a solution of the Dirichlet problem for the Schrödinger operator onwith u satisfying
(1.11)
then

whereandare constants.

## 2 Lemmas

Throughout this article, let M denote various constants independent of the variables in questions, which may be different from line to line.

Lemma 1
(2.1)
(2.2)
for anyand anysatisfying (resp. );
(2.3)

for anyand any.

Proof (2.1) and (2.2) are obtained by Kheyfits (see [10], Ch. 11]). (2.3) follows from Azarin (see [13], Lemma 4 and Remark]). □

Lemma 2 (see [1])

For a nonnegative integer m, we have
(2.4)

for anyandsatisfying (), whereis a constant dependent of n, m and s.

Lemma 3 (see [2], Theorem 1])

Ifis a solution of Equation (1.1) onsatisfying

(2.5)
then
Lemma 4 Obviously, the conclusion of Lemma 3 holds true if (2.5) is replaced by
(2.6)
Proof Since
from (1.5) and

(2.6) gives that (2.5) holds, from which the conclusion immediately follows. □

## 3 Proof of Theorem 1

We only prove the case , the remaining case can be proved similarly.

For any , there exists such that
(3.1)
The relation implies this inequality (see [14])
(3.2)
For and any fixed point satisfying , let , , , , , and , we write
where
By , (1.6), (2.1) and (3.1), we have the following growth estimates
(3.3)
(3.4)
(3.5)
We obtain by , (2.2) and (3.1)
(3.6)
By (2.3) and (3.2), we consider the inequality
where
We first have
(3.7)

which is similar to the estimate of .

Next, we shall estimate . Take a sufficiently small positive number such that for any , where

and divide into two sets and .

If , then there exists a positive such that for any , and hence
(3.8)

which is similar to the estimate of .

We shall consider the case . Now put
Since , we have

where is a positive integer satisfying .

Since we see from (1.2)
for . Similar to the estimate of , we obtain

for .

So
(3.9)
We only consider in the case , since for . By the definition of , (1.3) and Lemma 2, we see
where
To estimate , we write
where
Notice that
Thus, by , (1.5) and (1.6) we conclude
Analogous to the estimate of , we have
Thus we can conclude that
which yields
(3.10)
By , (1.5), (2.4) and (3.1) we have
(3.11)

Combining (3.3)–(3.11), we obtain that if is sufficiently large and ϵ is sufficiently small, then as , where . Then we complete the proof of Theorem 1.

## 4 Proof of Theorem 2

For any fixed , take a number satisfying (). By , (1.4), (1.6) and (2.4), we have

Thus is finite for any . Since is a generalized harmonic function of for any fixed , is also a generalized harmonic function of . That is to say, is a solution of Equation (1.1) on .

Now we study the boundary behavior of . Let be any fixed point and l be any positive number satisfying .

Set is a characteristic function of and write
where

Notice that is the Poisson a-integral of , we have . Since (; ) as , we have from the definition of the kernel function . , and therefore tends to zero.

So the function can be continuously extended to such that

for any from the arbitrariness of l. Thus we complete the proof of Theorem 2 from Theorem 1.

## 5 Proof of Theorem 3

From Corollary, we have the solution of the Dirichlet problem on with u satisfying (1.9). Consider the function . Then it follows that this is the solution of Equation (1.1) in and vanishes continuously on .

Since
for any , we have

from (1.10) and (1.11). Then the conclusions of Theorem 3 follow immediately from Lemma 4.

## Declarations

### Acknowledgements

The authors would like to thank anonymous reviewers for their valuable comments and suggestions about improving the quality of the manuscript. This work is supported by The National Natural Science Foundation of China under Grant 11071020 and Specialized Research Fund for the Doctoral Program of Higher Education under Grant 20100003110004.

## Authors’ Affiliations

(1)
Department of Mathematics and Information Science, Henan University of Economics and Law
(2)
School of Mathematical Science, Laboratory of Mathematics and Complex Systems, MOE Beijing Normal University

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