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Symmetry
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7 January 2022

Third-Order Differential Subordination Results for Analytic Functions Associated with a Certain Differential Operator

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,
and
1
Department of Mathematics, Faculty of Education for Girls, University of Kufa, Najaf 54001, Iraq
2
Department of Mathematics, College of Science, University of Al-Qadisiyah, Diwaniyah 58001, Iraq
3
Department of Mathematics and Computer Science, University of Oradea, 410087 Oradea, Romania
*
Author to whom correspondence should be addressed.
This article belongs to the Section Mathematics

Abstract

In this research, we study suitable classes of admissible functions and establish the properties of third-order differential subordination by making use a certain differential operator of analytic functions in U and have the normalized Taylor–Maclaurin series of the form: f ( z ) = z + n = 2 a n z n ,   ( z U ) . Some new results on differential subordination with some corollaries are obtained. These properties and results are symmetry to the properties of the differential superordination to form the sandwich theorems.

1. Introduction

Let S be the class of functions which are analytic in the open unit disk U = { z C :   | z | < 1 } . Also let S τ [ a , n ]   ( n = { 1 , 2 , 3 , } ;   a C ) be the subclass of S in which the functions satisfy the following form:
f ( z ) = a + a n z n + a n + 1 z n + 1 + ,                   ( z U ) .
Let G be a subclass of S which are analytic in U and have the normalized Taylor-Maclaurin series of the form:
f ( z ) = z + n = 2 a n z n                                                   ( z U ) .
Suppose that f   and   g are analytic functions in S . We say that f is subordinate to g , written as follows:
f g   in     U     or     f ( z ) g   ( z ) ,               ( z U )
if there exists a Schwarz function ω S , which is analytic in U , with ω ( 0 ) = 0   and   | ω ( z ) | < 1   ( z U ) , such that f ( z ) = g ( ω ( z ) ) ,   ( z U ) .
Furthermore, if g is univalent in U , we have [1]:
g ( z ) f ( z ) g   ( 0 ) = f ( 0 )       and       g ( U ) f ( U ) ,           ( z U ) .
For a function f ( z ) G given by (1) and g ( z ) G , defined by:
g   ( z ) = z + n = 2 b n z n
the Hadamard product (or convolution) of f ( z ) ,   g ( z ) denoted by f g is defined by
( f g ) ( z ) = z + n = 2 a n b n z n = ( g f ) ( z ) .
We’ll go over some additional terms and concepts from the differential subordination theory here.
Definition 1.
[2] Let Π : C 4 × U C and suppose that the function h ( z ) is univalent in U . If the function p ( z ) is analytic in U and satisfies the following third-order differential subordination:
Π ( p ( z ) , z p ( z ) , z 2 p ( z ) , z 3 p ( z ) ; z ) h ( z )
then p ( z ) is called a solution of the differential subordination ( 4 ) . Furthermore, a given univalent function q ( z ) is called a dominant of the solutions of ( 4 )   or more simply, a dominant if p ( z ) q ( z ) for all p ( z )   satisfying (4). A dominant q ^ ( z ) that satisfies q ^ ( z ) q ( z ) for all dominants q ( z ) of (4) is said to be the best dominant.
Definition 2.
[2] Let Q be the set of all functions q that are analytic and univalent on U ¯ E ( q ) , where
E ( q ) = { ξ : ξ U : lim z ξ { q ( z ) } = }
and m i n | q ( z ) | = p > 0   f o r   ξ U E ( q ) . Further, let the subclass of Q for which q ( 0 ) = a ,   be denoted by Q ( a ) with
Q ( 0 ) = Q 0     and     Q ( 1 ) = Q 1 .
The subordination methodology is applied to appropriate classes of admissible functions.
The following class of admissible functions was given by Antonino and Miller [2].
Definition 3.
[2] Let Ω be a set in C . Also q Q and n { 1 } . The class Ψ n [ Ω , q ] of admissible functions consists of those functions Π : C 4 × U C , which satisfy the following admissibility conditions:
Π ( r , s , t , u ;   z ) Ω
whenever
r = q ( ξ ) ,   s = k ξ q ( z ) , ( t s + 1 ) k ( ξ q ( z ) q ( z ) + 1 )
and
( u s ) k 2 ( ξ 2 q ( z ) q ( z ) ) ,
where z U ,   ξ U E ( q )   and   k n .
Lemma 1.
[2]: Let p S [ a , n ]   w i t h   n 2   and   q Q ( a ) satisfying the following conditions:
( ξ 2 q ( ξ ) q ( ξ ) ) 0     and         | z p ( z ) q ( ξ ) | k ,
where z U ,   ξ U \ E ( q )   and   k n . If Ω is a set in C ,   Π Ψ n [ Ω , q ] , and
Π ( p ( z ) , z p ( z ) , z 2 p ( z ) , z 3 p ( z ) ;   z ) Ω ,
then
p ( z ) q ( z )                     ( z U ) .
The geometric function theory relies heavily on the study of operators. Convolution of certain analytic functions may be used to express several differential and integral operators. This formalism, it is noticed, facilitates further mathematical research and also aids in a better understanding of the geometric aspects of such operators. The importance of convolution in the theory of operators may be understood by the following set of examples given by Barnard and Kellogg [3] and Carlson–Shaffer [4], etc.
Now, we introduce new operator by using the convolution in our this study.
Definition 4.
Let f , g G ,   σ 0   and   δ , λ , we define the operator:
D σ , δ , λ 1 : G G
by
D σ , δ , λ 1 ( f g ) ( z ) = σ σ + δ + λ D σ , δ , λ 0 ( f g ) ( z ) + δ + λ σ + δ + λ z ( D σ , δ , λ 1 ( f g ) ( z ) ) = z + n = 2 ( σ + ( δ + λ ) n σ + δ + λ ) a n z n ,
where
D σ , δ , λ 0 ( f g ) ( z ) = ( f g ) ( z ) .
Let f , g G ,   σ 0   and   δ , λ . Then
D σ , δ , λ 1 ( f g ) ( z ) = σ σ + δ + λ D σ , δ , λ 0 ( f g ) ( z ) + δ + λ σ + δ + λ z ( D σ , δ , λ 1 ( f g ) ( z ) ) = σ σ + δ + λ [ z + n = 2 a n b n z n ] + δ + λ σ + δ + λ z [ z + n = 2 a n b n z n ] = σ σ + δ + λ z + n = 2 σ σ + δ + λ a n b n z n + δ + λ σ + δ + λ z [ 1 + n = 2 n a n b n z n 1 ] = σ + δ + λ σ + δ + λ z + n = 2 σ + ( δ + λ ) n σ + δ + λ a n b n z n = z + n = 2 ( σ + ( δ + λ ) n σ + δ + λ ) a n b n z n .
In general
D σ , δ , λ m ( f g ) ( z ) = σ σ + δ + λ D σ , δ , λ m + 1 ( f g ) ( z ) + δ + λ σ + δ + λ z ( D σ , δ , λ m ( f g ) ( z ) ) = z + n = 2 ( σ + ( δ + λ ) n σ + δ + λ ) m a n b n z n .
By simple calculation, we obtain
( δ + λ ) z ( D σ , δ , λ m ( f g ) ( z ) ) = ( σ + δ + λ ) ( D σ , δ , λ m + 1 ( f g ) ( z ) ) σ ( D σ , δ , λ m ( f g ) ( z ) ) D σ , δ , λ m ( f g ) ( z ) .
The notion of the third-order differential subordination can be found in the work of Ponnusamy and Juneja [5]. The recent work by several authors (see for example, [6,7]; see also [8,9]) on the differential subordination attracted many researchers in this field. For example, see [8,10,11,12,13,14,15,16,17,18,19,20].
In this research, we investigate suitable classes of admissible functions associated with the new differential operator D σ , δ , λ m ( f g ) ( z ) and establish the properties of third-order differential subordination by making use a certain new differential operator of analytic functions in U and have the normalized Taylor–Maclaurin series of the form: f ( z ) = z + n = 2 a n z n ,   ( z U ) . Some new results on differential subordinations with some corollaries are obtained. Here, we obtain the symmetry of the differential superordination results.

3. Discussion

We study classes of admissible functions and establish the properties of third-order differential subordination using certain differential operator of analytic functions in U and have the normalized Taylor–Maclaurin series of the form: f ( z ) = z + n = 2 a n z n ,   ( z U ) . Some new results on differential subordination with some corollaries are obtained. These properties and results are symmetry to the properties of the differential superordination to form the sandwich theorems. Our results are different from the previous results for the other authors. We opened some windows for authors to generalize our new subclasses to obtain some new results in univalent and multivalent function theory using the results in the paper.

Author Contributions

Conceptualization, methodology, software by A.A.L., validation, formal analysis, investigation, resources, by A.H.B., data curation, writing—original draft preparation, writing—review and editing, visualization by W.G.A., supervision, project administration, funding acquisition, by A.M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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