#### Abstract

Let be the class of functions of the form which are analytic in . By using the method of differential subordinations, we give some sufficient conditions for Carathéodory function, that is, if satisfies and where , , , ,, then . Some useful consequences of this result are also given.

#### 1. Introduction

Let be the class of functions of the following form: which are analytic in . We denote by the subclass of consisting of univalent functions and by and the usual subclasses of whose members are starlike (with respect to the origin) and close-to-convex, respectively. Finally, we denote by the family of functions which satisfy the condition . It is well known that .

Let be the class of functions of the following form: which are analytic in . If satisfies , then we say that is the Carathéodory function. For Carathéodory functions, Nunokawa et al. [1] have shown some sufficient conditions applying the differential inequalities. In the present paper, using the method of differential subordinations, we derive certain conditions under which we have , where . Our results generalize or improve some results due to [1–4].

To prove our results, we need the following lemma due to Miller and Mocanu [5].

Lemma 1. *Let be analytic and univalent in U and let and be analytic in a domain D containing , with when . Set
**
and suppose that *(i)*is starlike univalent in , and*(ii)*.**If is analytic in , with , and
**
then and is the best dominant of (4).*

#### 2. Main Results

Theorem 2. *Let , , , and . If satisfies and
**
where
**
then and is the best dominant of (5).*

*Proof. *Let , , ,
and choose
Then is analytic and univalent in , , , and satisfy the conditions of the lemma. The function
is univalent and starlike in because
Further, we have
Since , from (11) it is easy to know that
Hence the function is close-to-convex and univalent in . Now it follows from (5)–(12) that
Therefore, by virtue of the lemma, we conclude that and is the best dominant of (5). The proof of the theorem is complete.

Making use of the theorem, we can obtain a number of interesting results.

Corollary 3. *Let , . If satisfies
**
then .*

*Proof. *Let , , , , , , and in the theorem, then we have
satisfy the conditions of the lemma. Note that
Hence, similar to the proof of the theorem, we conclude that , that is, .

*Remark 4. *Note that Corollary 3 was also proved by Nunokawa et al. [1] using another method.

Taking , , , , , and in the theorem, we have the following.

Corollary 5. *Let , , satisfies , and
**
then .*

*Remark 6. *Note that the function
maps onto the -plane slit along the half-lines , and , . Hence Corollary 5 coincides with the result obtained by Nunokawa et al. [1, Theorem 2] using another method.

Letting , , in Corollary 5, we have the following.

Corollary 7. *If , , and
**
where is real and , then .*

*Remark 8. *Lewandowski et al. [3] proved if , , , and
then . We see that Corollary 7 improves this result.

Corollary 9. *Let with . If
**
for some , then and
*

*Proof. *Letting , , then ,
Taking , and , in the theorem, it follows from (5), (6), and (21) that , which implies that (see [6]).

*Remark 10. *Frassin and Darus [2] have shown that if and
for some , then .

For , the function
is analytic and convex univalent in . Since
the disk is properly contained in . Therefore, in view of (21), we see that Corollary 9 is better than the result given in [2].

Letting , , , , , , and in the theorem, we have the following.

Corollary 11. *If , , , and
**
then .**For the univalent function , we now find the image of the unit disk .**Let , where and are real. We have
**
Elimination of yields
**
Therefore, we conclude that
*

*Remark 12. *Nunokawa and Hoshino [4] have proved that if , and
then . For , properly contains the half plane . Hence Corollary 11 with is better than the result given in [4].

#### Acknowledgments

This work was partially supported by the National Natural Science Foundation of China (Grant no. 11171045). The author would like to thank the referees for their careful reading and for making some valuable comments which have essentially improved the presentation of this paper.