#### Abstract

Let , we take up the existence, the uniqueness and the asymptotic behavior of a positive continuous solution to the following nonlinear problem in , , , , where , is a positive differentiable function in and is a positive continuous function in such that there exists satisfying for each in , , and such that .

#### 1. Introduction

Let and . The following differential equation: has been studied with various boundary conditions, where is a continuous function in , differentiable and positive in , is a nonnegative continuous function in , and , (see [1–15]).

For a function depending only on , , and , the operator is the radially symmetric -Laplacian in and the equation of type (1) arises also as radial solutions of the Monge-Ampère equation (see [13]).

In this paper, our main purpose is to obtain the existence of a unique positive solution to the following boundary value problem: and to establish estimates on such solution under an appropriate condition on .

The study of this type of (1) is motivated by [5, 15]. Namely, in the special case , and , , the authors in [15] studied (1) and gave some uniqueness results. In this work, we consider a wider class of weights and we aim to extend the study of (1) in [15], to .

For the case , the problem has been studied in [5]. Indeed, the authors of [5] proved the following existence result.

Theorem 1. *The problem has a unique positive solution satisfying for each **
where is a positive constant and is the function defined on by
*

We shall improve in this paper the above asymptotic behavior of the solution of problem and we extend the study of to .

The pure elliptic problem of type has been investigated by several authors with zero Dirichlet boundary value; we refer the reader to [16–24] and the references therein. More recently, applying Karamata regular variation theory, Chemmam et al. gave in [17] the asymptotic behavior of solutions of problem . In this work, we aim to extend the result established in [17] to the radial case associated to problem .

To simplify our statements, we need to fix some notations and make some assumptions. Throughout this paper, we shall use , to denote the set of Karamata functions defined on by where is a positive constant and such that .

It is clear that if and only if is a positive function in such that For two nonnegative functions and on a set , we write , , if there exists a constant such that , for each .

The letter will denote a generic positive constant which may vary from line to line.

Furthermore, we point out that if is a nonnegative continuous function in , then the function defined on by is the solution of the problem

As it is mentioned above, our main purpose in this paper is to establish existence and global behavior of a positive solution of problem . Let us introduce our hypotheses.

Here, the function is continuous in , differentiable and positive in such that with .

The function is required to satisfy the following hypothesis.

() is a positive measurable function on such that with and the function such that .

*Remark 2. *We need to verify condition
in hypothesis , only if , (see Lemma 6 below).

As a typical example of function satisfying , we quote the following.

*Example 3. *Put . Then for and or and , the function satisfies .

Now, we are ready to state our main result.

Theorem 4. *Assume . Then problem has a unique positive continuous solution satisfying for ,
**
where is the function defined on by
*

The main body of the paper is organized as follows. In Section 2, we establish some estimates and we recall some known results on functions belonging to . Theorem 4 is proved in Section 3. The last Section is reserved to some applications.

#### 2. Key Estimates

In what follows, we are going to give estimates on the functions and , where is a function satisfying and is the function given by (12). First, we recall some fundamental properties of functions belonging to the class , taken from [17, 22].

Lemma 5. *Let and . Then one has and .*

Lemma 6 (Karamata’s theorem). *Let and be a function in . Then one has the following properties:*(i)*If , then converges and
*(ii)*If , then diverges and
*

Lemma 7. *Let . Then there exists such that for and *

Lemma 8. *Let , then one has
**
If further converges, one has
*

Now, we are able to prove the following propositions which play a crucial role in this paper.

Proposition 9. *Let be a function satisfying . Then one has for **
where is the function defined by
*

*Proof. *For , we have
Put
To prove the result, it is sufficient to show that for .

Since the function is continuous and positive in , we have
Now, assume that , then we have
It follows from Lemma 7 that
To reach our estimates, we consider the following cases.

(i) If , then it follows from Lemma 6 that
Now, using Lemma 5 and again Lemma 6, we deduce that

(ii) If , then it follows from Lemma 6, that . So, since , we have

(iii) If , then for each , we have
Since the function is in , then using the fact that and Lemma 6, we obtain that

(iv) If , we have by Lemma 6 that
this yields to
Hence, we reach the result by combining (22) with the estimates stated in each case above. This completes the proof.

Proposition 10. *Let be a function satisfying and let be the function given by (12). Then for , one has
*

*Proof. *Let and , we obtain by simple calculus that for ,
where
So, one can see that
where . Then, using Lemmas 5, 7, and 8, we obtain that and . Hence, it follows from Proposition 9 that
where is the function defined in (19) by replacing by and by . This ends the proof.

#### 3. Proof of Theorem 4

##### 3.1. Existence and Asymptotic Behavior

Let be a function satisfying and let be the function given by (12). By Proposition 10, there exists a constant such that for each We look now at the existence of positive solution of problem satisfying (11).

For the case , the existence of a positive continuous solution to problem is due to [5]. Now, we look to the existence result of problem when and we give precise asymptotic behavior of such solution for . For that, we split the proof into two cases.

*Case 1 (). *Let be a positive continuous solution of problem . So, in order to obtain estimates (11) on the function , we need the following comparison result.

Lemma 11. *Let and such that
**
Then .*

*Proof. *Suppose that for some . Then there exists , such that and for with or .

By an elementary argument, we have
On the other hand, since , then , for each . This yields to

Using further (40), we deduce that the function is nondecreasing on with . Hence, from the monotonicity of , we obtain that the function is nondecreasing on with and . This yields to a contradiction, which completes the proof.

Now, we are ready to prove (11). Put and . It follows from (7) that the function satisfies According to (37), we obtain by simple calculation that and satisfy, respectively, (38) and (39). Thus, we deduce by Lemma 11 that This implies (11) by using (37).

*Case 2 (). *Put and let
Obviously, the function belongs to and so is not empty. We consider the integral operator on defined by
We shall prove that has a fixed point in , to construct a solution of problem . For this aim, we look at first that . Let , then we have for each
This together with (37) implies that
Since and , then leaves invariant the convex . Moreover, since , then the operator is nondecreasing on . Now, let be the sequence of functions in defined by
Since , we deduce from the monotonicity of that for , we have
Thanks to the monotone convergence theorem, we deduce that the sequence converges to a function which satisfies
We conclude that is a positive continuous solution of problem satisfying (11).

##### 3.2. Uniqueness

Assume that satisfies . For , the uniqueness of solution to problem follows from Lemma 11. Thus in the following, we look at the case . Let Let and be two positives solutions of problem in . Then there exists a constant such that This implies that the set is not empty. Now, put , then we aim to show that .

Suppose that , then we have So, we have in , which implies that the function is nondecreasing on with . Hence from the monotonicity of , we obtain that the function is nondecreasing on with . This implies that . On the other hand, we deduce by symmetry that . Hence . Now, since and , we have . This yields to a contradiction with the fact that . Hence, and then .

#### 4. Applications

##### 4.1. First Application

Let be a positive measurable function in satisfying for where the real numbers and satisfy one of the following two conditions:(i) and ,(ii) and . Using Theorem 4, we deduce that problem has a positive continuous solution in satisfying the following.(i)If , then for (ii)If and , then for (iii)If and , then for (iv)If and , then for (v)If , then for (vi) If and , then for

##### 4.2. Second Application

Let be a function satisfying and let . We are interested in the following nonlinear problem: Put , then by a simple calculus, we obtain that satisfies Using Theorem 4, we deduce that problem (64) has a unique solution such that Consequently, we deduce that (63) has a unique solution satisfying