Abstract
In this paper we study convolution properties of spirallike multivalent functions defined by using a differential operator and higher order derivatives. Using convolution product relations we determine necessary and sufficient conditions for multivalent functions to belong to these classes, and our results generalized many previous results obtained by different authors. We obtain convolution and inclusion properties for new subclasses of multivalent functions defined by using the Dziok-Srivatava operator. Moreover, using a result connected with the Briot-Bouquet differential subordination, we obtain an inclusion relation between some of these classes of functions.
Keywords:
analytic function; convolution product; generalized hypergeometric function; Dziok-Srivastava linear operator; spirallike function; differential subordination; Briot-Bouquet differential subordination MSC:
30C45; 30C80
1. Introduction
With the aid of the convolution (Hadamard) product Dziok and Srivastava [1] defined a well-known operator that will help us to define a few general subclasses of multivalent functions that generalize and unify many of some previous ones introduced and studied by different authors. We determine necessary and sufficient conditions in term of convolution relations such that a multivalent function belong to these classes, that generalize some of the previous results of Sarkar et al. [2] (Theorems 2.1 and 2.2), of Ahuja [3] (Theorems 2.1 and 2.2) and of Padmanabhan and Ganesan [4] (Theorems 1–4). Moreover, using the Briot-Bouquet differential subordination we found an inclusion property for these classes. The importance of the results besides in the wide generalities of the convolution and the inclusion theorems that could be useful for those that will follow a special study of some special cases of the subclasses we defined. We would like to emphasize that some other fundamental results regarding introductory and new results of the Geometric Function Theory could be found in the recent monographs [5,6].
Let denote the class of functions of the for
which are analytic in the open unit disc .
If f and g are analytic functions in we say that f is subordinate to g, written , if there exist a function w which is analytic in , with and , , such that for all .
Furthermore, if the function g is univalent in , then we have the equivalence
For the function given by (1), and g given by
the Hadamard (or convolution) product of f and g is defined by
For complex parameters and , with for all , we now define the generalized hypergeometric function
where is the Pochhammer symbol, defined in terms of the Gamma function by
Corresponding to a function given by
Dziok and Srivastava [1] considered the linear operator
defined by the following Hadamard product
It can be easily verified from the definition (2) that
It should be remarked that the linear operator is a generalization of other linear operators (see [7,8,9,10,11,12,13]).
In the following two definitions we will recall some previously defined subclasses of that we will use in our article.
Definition 1
(see [14]). Let , , with , and .
(i) We say the function is in the class if it satisfies the inequality
(ii) We say the function is in the class if it satisfies the inequality
From the above definition it follows that
and we note that and (see [15]).
Definition 2
(see [14]). Let , with , , , with , and .
(i) We say that the function is in the class if it satisfies the inequality
(ii) We say the function is in the class if it satisfies the inequality
It is easy to check that
and we remark that , , and (see [16]).
The next three definitions will introduce some classes connected to the studies of this article.
Definition 3.
Let , with , , , with , and . Suppose that ϕ is a univalent function in the unit disk with , such that
We define the classes and by
and
From the above definition we have
Remark 1.
The subclasses defined in the above definition generalize some previous ones like we could see for the next special cases of ϕ.
(i) For , we obtain the subclasses
which, for reduces to
(see Libera [17], Srivastava et al. [16]).
| (see Aouf [14]), |
(ii) For , with , we get the next extensions of the above subclasses, i.e.,
and
| (see Aouf [18]), |
| (see Aouf [15]). |
To define the next generalizations of the above subclasses, we shall prove the following lemma:
Lemma 1.
Suppose the function .
(i) Then, is a λ-spirallike (univalent) function in .
(ii) Consequently, for all the multivalued functions has an analytic branch in with .
Proof.
If we let , then and differentiating the definition formula of F we get
Therefore, the subordination (6) yields that
and this last inequality shows that F is -spirallike and univalent function in (for details, see [19]).
It follows for all , i.e., , , and this implies the second part of our lemma. □
Definition 4.
Let , with , , , with , and . Suppose that ϕ is a univalent function in the unit disk with , and satisfies the condition (4), and let be an arbitrary complex number.
(i) The function is said to be in the class if there exists a function such that
(ii) The function is said to be in the class , if where , such that , .
Remark that, according to the second part of Lemma 1, since then the multivalued functions has an analytic branch in with , hence the power function of the above definition is correctly defined.
Remark 2.
We note that for appropriate choices of the parameters in the above defined classes of functions we obtain a few earlier studied classes as follows:
(i) For the special case the classes and coincide with the classes and , respectively;
(ii) For , and , with , we have which was studied by Ahuja [20];
(iii) For we have and which was studied by Sarkar et al. [2].
Definition 5.
Let , with , , , with , and . Suppose that ϕ is a univalent function in the unit disk with , and satisfies the condition (4).
For , with , we define the following two subclasses of , that are:
and
In particular, for , we denote
Moreover, by choosing appropriately the parameters in the above defined two classes of functions we obtain the following special cases introduced and studied by different authors:
(i) For , and , with , we obtain
(see Aouf and Seoudy [21]);
(ii) For , and , with , we have
(see Seoudy [22]);
(iii) For , , , and , with , , we get
and
where the operator was introduced by Saitoh [12];
(iv) For , , with , and we obtain
and
where is the generalized Bernardi-Libera-Livingston operator (see [7]);
(v) For , , , with , we have
and
where is the -th order Ruscheweyh derivative (see Goel and Sohi [9]);
(vi) For , , , and , with , , we get
and
where the operator was defined by Liu and Noor [10];
(vii) For , , with , and , with , we obtain
and
where the operator was considered by Cho et al. [8];
(viii) For , , , and we have
and
where the operator was introduced by Srivastava and Aouf [13] for , and was investigated by Patel and Mishra [11] for , .
2. Convolution and Inclusion Properties
Our first result represent a necessary and sufficient condition, in term of convolution product, for a function to belongs to class .
Theorem 1.
Let , such that for all and . We will define the function , such that , where the power function is considered to the main branch, i.e., .
Then, if and only if
where
Proof.
For any function we can easily verify that
and from the definition formula of the function g we get
First, according to the above identity we have that is equivalent to
and using the definition of the subordination, from (10) it follows that
From the convolution relations (9) a simple computation shows that (11) could be rewritten in the following form
which is equivalent to (7), where C given by (8).
Reversely, like it was shown in the first part of the proof, the assumption (7) is equivalent to (11). Denoting
and
these functions are analytic in and is univalent since is univalent in . The relation (11) shows that and therefore the simply-connected domain is included in a connected component of . From here, and using the fact that together with the univalence of the function , it follows that which represents in fact the subordination (10), i.e., . □
The next result that is also a necessary and sufficient condition for a function to belongs to class generalizes some other previously results like we mention after its proof.
Theorem 2.
Let , such that for all . Then, with if and only if
where
Proof.
First, from Definition 4 we have that if there exists a function such that , where .
Differentiating the above definition formula we get
and using the fact that if and only if it follows that is equivalent to
Since the right-hand side of the above subordination is a univalent function in it follows that
Finally, from the convolution relations (9), a simple computation shows that (14) is equivalent to (12) where D is given by (13).
Reversely, since for all , it follows that the function given by
where is correctly defined. The above definition relation is equivalent to
where and by similar computations like in the first part of the proof we deduce that is equivalent to (14). Now, denoting
and
and using the same arguments as in the second part of the proof of Theorem 1 we deduce that , that is . □
Remark 3.
(i) For the special case Theorems 1 and 2 reduces to the convolution results obtained by Sarkar et al. [2] (Theorems 2.1 and 2.2);
(ii) Putting , , , with , and writing as x in Theorems 1 and 2 we obtain the convolution results of Ahuja [3] (Theorems 2.1 and 2.2);
(iii) For we have , where g is defined like in Theorem 1 and . Moreover, for this special case the additional assumptions and for all of these theorems are not necessary, hence both of the results of Theorems 1 and 2 coincide with the following corollary:
Corollary 1.
If , then if and only if (7) holds for
Example 1.
Let consider in Theorems 1 and 2 the function
and . Then, the assumption (4) is equivalent to
which holds for all , and from the above mentioned theorems we obtain the next particular cases, respectively:
1. Let such that for all , and . We will define the function such that , where the power function is considered to the main branch.
Then, if and only if
where
2. Let , such that for all . Then, with , if and only if
where
The next theorem is a necessary and sufficient condition for a function to belongs to class and it extends a few previously results obtained by different authors.
Theorem 3.
Proof.
If we let
then
From (5) and according to the Corollary 1 we deduce that if and only if
Remark 4.
(i) Putting , and , with , in Corollary 1 and Theorem 3 we obtain the convolution results due to Padmanabhan and Ganesan [4] (Theorems 1 and 2);
(ii) Putting , and , with , in Corollary 1 and Theorem 3 we obtain the convolution results due to Padmanabhan and Ganesan [4] (Theorems 3 and 4).
The following result is a necessary and sufficient condition for a function to belongs to class and for the special case it coincide with the previous theorem.
Theorem 4.
Proof.
Letting
we have
According to the Definition 4 we have
and from Theorem 2 we deduce that if and only if
Example 2.
Let the function
and . From the Theorems 3 and 4 we obtain the next two special cases, respectively:
Remark 5.
Note that for the special case we have , and using the same reasons like in Remark 3 (iii) we deduce that the results of Theorems 3 and 4 coincide.
The following result represents a necessary and sufficient condition for a function to belongs to class connected with the Dziok and Srivastava convolution operator [1].
Theorem 5.
Let of the form (1) such that for all . Then, with , if and only if
Proof.
A similar result with the previous theorem deals with a sufficient condition for a function to belongs to class define by using the Dziok and Srivastava convolution operator.
Theorem 6.
Let , such that for all . Then, with , if and only if
Proof.
From Theorem 4 we have that if and only if
where D is given by (13). It can be easily shown that
and
Example 3.
If we consider the function
and , then from the Theorems 5 and 6 we obtain the next result, respectively:
1. Let of the form (1), such that for all . Then, with , if and only if
2. Let , such that for all . Then, with , if and only if
Now we will give an inclusion relation for the classes and to prove this result we shall require the following lemma:
Lemma 2
(see [23]). Let ϕ be convex (univalent) in , with , . If θ is analytic in with , then
The above subordination is the well-known Briot–Bouquet type differential subordination and it allows us to find a simple sufficient condition such that the inclusion holds.
Theorem 7.
Suppose that the function ϕ is convex (univalent) in and satisfies the inequality
If , such that for all , then .
Proof.
Suppose that and let us define the function by
Then, is analytic in and from (3) we obtain
Example 4.
1. For the above theorem reduces to the next result:
Suppose that the function ϕ is convex (univalent) in and satisfies the inequality
If such that for all , then .
2. For
and, the above special case becomes:
Suppose that and satisfies the inequality
If such that for all , then .
Remark 6.
For special choices for and , where , we can obtain the corresponding results for different linear operators which are defined in the introduction.
3. Concluding Remarks
Using higher order derivatives we defined the classes and that for special choices of generalize some classes previously studied by Aouf [14], Libera [17], Srivastava et al. [16], and Aouf [15,18]. In the first two theorems we determine, in term of convolution product, necessary and sufficient conditions for a function to belong to these classes, respectively. For special choices of the parameters these results extend those of Sarkar et al. [2] and of Ahuja [3].
For the other new subclasses and , the Theorems 3 and 4 give us necessary and sufficient condition for a function to be in these classes, respectively, extending for particular cases of the parameters and of the function some results of Padmanabhan and Ganesan [4].
The next two theorems deal with necessary and sufficient conditions for a multivalent function to be in the new defined classes and introduced by using higher order derivatives and the well-known Dziok-Srivastava linear operator, that generalize the previous ones, while the last result gives us a sufficient condition such the inclusion holds.
The results we obtain are new and could help the researchers in the field of Geometric Function Theory to obtain other new results in this field, or for used them in some appropriate particular cases for different studies.
Author Contributions
Conceptualization, E.E.A., M.K.A., R.M.E.-A. and T.B.; methodology, E.E.A., M.K.A., R.M.E.-A. and T.B.; validation, E.E.A., M.K.A., R.M.E.-A. and T.B.; investigation, E.E.A., M.K.A., R.M.E.-A. and T.B.; resources, E.E.A., M.K.A., R.M.E.-A. and T.B.; data curation, E.E.A., M.K.A., R.M.E.-A. and T.B.; writing—original draft preparation, E.E.A., M.K.A., R.M.E.-A. and T.B.; writing—review and editing, E.E.A., M.K.A., R.M.E.-A. and T.B.; supervision, E.E.A., M.K.A., R.M.E.-A. and T.B.; project administration, E.E.A., M.K.A., R.M.E.-A. and T.B.; funding acquisition, E.E.A. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Acknowledgments
The authors are grateful to the reviewers for their valuable remarks, comments, and advice, that help us to improve the quality of the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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