Research Article | Open Access

Mohamed Amine Ben Boubaker, Mohamed Selmi, "Sharp Estimates for Green’s Functions of Cone-Type Planar Domains", *Journal of Complex Analysis*, vol. 2016, Article ID 7208285, 11 pages, 2016. https://doi.org/10.1155/2016/7208285

# Sharp Estimates for Green’s Functions of Cone-Type Planar Domains

**Academic Editor:**Okay Celebi

#### Abstract

We establish sharp estimates for Green’s functions of cone-type planar domains. Our work generalizes all estimates given by Zhao in 1988 and Selmi in 2000. Our principal idea is to use conformal mappings.

#### 1. Introduction

We work in the Euclidean space . By , we denote the Green function for the Laplacian in a domain . We write for the distance from to the Euclidean boundary of . For two positive functions and on a set , we say that is comparable to on and we denote , if there exists , such that , . The constant will be called the constant of comparison. For , we denote and . denote the open disk of center and radius . For , we denote and . In [1], Zhao established estimates for the Green function on bounded domains in . More precisely, he proved thatThis result was generalized later by Selmi in [2]. He established inequalities for the Green function of a class of Dini-smooth Jordan domains in . He proved that if is a bounded multiply connected Dini-smooth Jordan domain in , then inequality (1) holds. In [3], under restricted conditions on , Sweers established an implicit formula for the estimate of the Green function; the estimate is expressed as a function of the first eigenfunction. The aim of the present paper is to give estimates for the Green function in some particular bounded and unbounded nonsmooth domains in . Our work generalizes all estimates given by Zhao in [1], Selmi in [2], and Sweers in [3]. Our principal idea is to use conformal mappings. Note that, by the Riemann Theorem (see [4], pages 133–136 and pages 229–233), if is a nonempty open simply connected domain in , distinct of , then there exists a conformal mapping from onto the unit disk . Consequently, for all Thus, to give estimates for the Green function, it is sufficient to give estimates for and on . The problem reduces to understanding distortion introduced by . The paper is organized as follows. In Section 2, we establish some preliminary results that will be necessary throughout this paper. In Section 3, we give estimates for the Green function in , . We put and For , we denote (resp., ), the distance from to (resp., from to ). If , we define the bracket . We prove the following results.

Theorem 1. *Let , . Then*

Corollary 2. *Let , . Then*

In Section 4, we give estimates for the Green function in , We put , , and . For , we denote (resp., ; resp., ), the distance from to (resp., from to ; resp., from to ). For , we define the brackets , , , and . We prove the following.

Theorem 3. *Let , . Then*

Corollary 4. *Let , . Then, for all ,*

In Section 5, we give estimates for the Green function in . To study the intersection of two open disk, let us remark that using translations, rotations, and homotheties we reduce the problem to study the situation of and , where , , and . The two boundary circles and intersect nontangentially at with and . The interior angle of the intersection is given by and the boundary of is formed by the arcs of circlesWe denote by the distance of to the boundary . For , we write We prove the following.

Theorem 5. *For all ,where .*

Corollary 6. *For all , *

In Section 6, we give estimates for the Green function in . We reduce the problem as in Section 5. We prove the following.

Theorem 7. *For all , where .*

In Section 7, we give estimates for the Green function in . We reduce the problem as in Section 5. We prove the following.

Theorem 8. *For all , where .*

In Section 8, we give estimates for the Green function in , . We prove the following.

Theorem 9. *For all ,where , , and .*

#### 2. Preliminaries Results

In this section, the following notations will be adopted. For and , we denote

Lemma 10. *Let , . Then, *

*Proof. *It suffices to see that is continuous and does not vanish on .

Lemma 11. *Let . Then *

*Proof. *Let . The functionis holomorphic on and continuous and does not vanish on . Then, by maximum and minimum principles, it is sufficient to control on . We have to discuss three cases.*Case 1*. If , then The result follows from Lemma 10.*Case 2*. If , then *Case 3*. If , thenSo, we get Since the constants are independent of , we obtain, for all ,

Lemma 12. *Let . Then*

*Proof. *The proof is similar to that of Lemma 11. Let . The function is holomorphic on and continuous and does not vanish on . By maximum and minimum principles, it is sufficient to control on . We have to discuss four cases.*Case 1*. If , thenwhere . The result follows from Lemma 10.*Case 2*. If , then*Case 3*. If , then *Case 4*. If , then As the constants are independent of , we obtain for all Now let ; then . By using the previous relation, we get, for all ,

By using the conjugate expression in the last lemma, we obtain the following.

Lemma 13. *Let . Then *

Lemma 14. *Let , . Then, *

*Proof. *We have to discuss two cases.*Case 1*. If , assume that and put . Then, and . Sincethen the result follows by Lemma 11.*Case 2*. If , assume that and put . Then, and , or . Since then the result follows by Lemmas 12 and 13.

Lemma 15. *Let , . Then *

*Proof. *The function is continuous and even and does not vanish on . To study on a neighborhood of , we put . Hence As , the function can be extended by continuity on and the compacity argument finishes the proof.

#### 3. Estimates for the Green Function on the Sector

It is clear that

Lemma 16. *Let , , Then*

*Proof. *First, we remark that On the other hand, the bracket satisfiesLetObviously, is continuous and . Assume that ; we see that and . In consequence, . This implies the contradiction ( and ). Also, implies the contradiction ( and ). By compacity argument, with . This leads us to

Lemma 17. *Let , . Then*

*Proof. *It is clear that for all , . MoreoverThe functionis continuous and even and does not vanish. In fact allows us to conclude that with , where the contradiction is clear enough. Then .

For , , , we denote by the distance of to the boundary of .

*Proof of Theorem 1. *If , then : As, for , the function is a conformal mapping from onto , then we have By Lemma 15, we remark that and . Then, On the other hand, by Lemmas 11, 14, and 16, we getSo, we obtain

Now we present two proofs of Corollary 2.

*First Proof of Corollary 2*. By using Lemma 17, we get, for all ,By substituting the previous expression in relation (3), we get

*Second Proof of Corollary 2*. As, for , the conformal mapping , from onto , is the only singular point of order and in a neighborhood of 0. Moreover for We observe thatand for all Consequently,This implies that for all

#### 4. Estimates of the Green Function on

It is clear thatand . We see that on and on In the same way in Lemmas 11, 12, and 13, we prove the following lemma.

Lemma 18. *Consider*

By using the last Lemma, we get the following.

Lemma 19. *Let , . Then*

Lemma 20. *Let , . Then*

*Proof. *By symmetry, it is sufficient to prove the result for . We see that, for all and ,