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Research Articles

Study of simply connected domain and its geometric properties

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Pages 993-997 | Received 12 Oct 2018, Accepted 17 Sep 2019, Published online: 24 Sep 2019

ABSTRACT

By using composition of a differential operator and a subclass of analytic functions, we introduce a new application of a differential operator for starlike and convex functions. Moreover, we are dealing with starlikeness and convexity properties of hypergeometric and related functions.

2000 MATHEMATICS SUBJECT CLASSIFICATION:

1. Introduction

The Geometric Function Theory (GFT) deals with the geometric properties of analytic functions of a complex variable. Geometrically, analytic functions are divided into starlike, convex and close to convex functions. The open unit disk plays an important role in the construction of simply connected domains including starlike and convex domains. The fundamental rule of open unit disk is applied in the foundation of Riemann mapping theorem. It is a classical result in GFT, which states that for every non-empty simply connected (not contain any hole) open subset of the complex plane which is not all of the complex plane, then there exists one-to-one, onto and holomorphic mapping (implies conformal map and angle-preserving) from onto U (the open unit disk). The differential operators defined in the open unit disk has attracted the attention of many researchers.

We are motivated by the research works based on mapping properties of hypergeometric functions, convolutions of starlike and convex functions (cf. [Citation1]). This article provides an idea to introduce a new subclass of analytic functions with the help of differential operator given by (Equation2).

The p-valent functions analytic and univalent in U={zC:|z|<1} of the form (Equation1) with pN are said to form the class A(p). (1) f(z)=zp+k=p+1akzk.(1) A function f of the form (Equation1) is called starlike function of order ξ if zf(z)f(z)>ξ,0ξ<p. We denote the class of starlike functions of order ξ by S(ξ,p).

A function f of the form (Equation1) is called convex function of order ξ if 1+zf(z)f(z)>ξ. We denote the class of convex functions of order ξ by C(ξ,p).

The class S(ξ,p) was introduced by Patil and Thakare [Citation2] and C(ξ,p) was introduced by Owa [Citation3].

The differential operator Υλn(p,α,β,μ)f given by (Equation2) has been considered in [Citation4] and [Citation5] and is defined as (2) Υλn(p,α,β,μ)f(z)=zp+k=p+1α+(μ+λ)(kp)+βα+βnakzk,(2) where α, β, μ and λ having the same constraints as discussed before in [Citation6] and [Citation7]. Further, a straightforward calculation reveals that many differential operators introduced in other papers (see for example [Citation8–19]) are special cases of the differential operator (Equation2).

Now by using (Equation2) if we consider n=0, β=0, μ=0, p=1, λ=1 and α=1 then Υ10(1,1,0,0)f(z) =f(z) and for n=1, β=0, μ=0, p=1, λ=1 and α=1, Υ11(1,1,0,0)f(z) =zf(z), it follows that zf/fS(ξ).

Similarly if we set β=0, μ=0, ξ=0, n=0, p=1, λ=1 and α=1 then f(z)=z/((1z)2) belongs to S(0) and putting ξ=12, β=0, μ=0, n=0, p=1, λ=1 and α=1 then f(z)=z/(1z) belongs to S(12).

Setting n=1, p=1, λ=1, α=1, β=0 and μ=0 then Υ12(1,1,0,0)f(z) =zf(z)+z2f(z) and Υ11(1,1,0,0)f(z) =zf(z) implies 1+(zf/f)Cp(ξ).

In this case, g(z)=(1/(2(1ξ)))(((1/(1z))2(1ξ))1) belongs to Cp(ξ), 0ξ<1 and f(z)=z/(1z) belongs to Cp(0).

From (Equation2), we deduce that Υλn(p,α,β,μ)f(z)= g(z) =(zp(1(μ+λ/α+β))zp+1)/((1z)2) and g(z)=zpαμ+βλα+βzp+1(1z)2=zpαμ+βλα+βzp+11z2=zp+1+μ+λα+βzp+1+1+2μ+λα+βzp+2+=zp+k=p+1α+(μ+λ)(kp)+βα+βzk.

Let Φp,nα,β(ξ,μ) denotes the class of all functions of the form given by (Equation1) and satisfying (3) Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)>ξp,0ξ<1.(3)

After computation, we notice that if we set β=0, μ=0, n=0, p=1, λ=1 and α=1, then f(z)=z/((1z)2(1ξ)) belongs to Φp,nα,β(ξ,μ).

Consider β=0, μ=0, n=0, ξ=0, p=1, λ=1 and α=1, then f(z)=z/((1z)2) belongs to the class Φp,nα,β(ξ,μ). Moreover, for ξ=12, β=0, μ=0, n=0, p=1, λ=1 and α=1 then f(z)=z/(1z) belongs to Φp,nα,β(ξ,μ). For further discussion, we refer to [Citation1].

Lemma 1.1

[Citation20]

Suppose ω(z) be a nonconstant analytic function in U with ω(0)=0. If |ω(z)| attains its maximum value at a point z0U on the circle r<1, then z0ω(0)=ζω(z0), where ζ1 is some real number.

Theorem 1.2

For η1,η20, if the function f given in (Equation1) satisfy the analytic criterion Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1η1Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1η2  <1ξpη11ξp+μ+λ2(α+β)η20ξp21ξpη1+η21+μ+λ(α+β)η2p2ξp. then f belongs to Φp,nα,β(ξ,μ).

Proof.

By using (Equation2) (4) (μ+λ)z(Υλn+1(p,α,β,μ)f(z))(α+β)Υλn+1(p,α,β,μ)f(z)=Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)+p(μ+λ)(α+β)1,(4) implies (5) (μ+λ)z(Υλn(p,α,β,μ)f(z))(α+β)Υλn(p,α,β,μ)f(z)=Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)+p(μ+λ)(α+β)1(5)

Subtracting last two equations and after doing some calculations, we get (6) (μ+λ)z(Υλn+1(p,α,β,μ)f(z))Υλn+1(p,α,β,μ)f(z)(μ+λ)z(Υλn(p,α,β,μ)f(z))Υλn(p,α,β,μ)f(z)=(α+β)Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)(α+β)Υλn+1(p,α,β,μ)f(z)(Υλn(p,α,β,μ)f(z))(6)

Let for 0ξp/2, we define ω(z) such that (7) pΥλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)=p+(p2ξ)ω(z)1ω(z).(7) Differentiating (Equation7) with respect to z and multiplying the resulting equation by z, we obtain (8) z(Υλn+1(p,α,β,μ)f(z))Υλn+1(p,α,β,μ)f(z)z(Υλn(p,α,β,μ)f(z))Υλn(p,α,β,μ)f(z)=(p2ξ)zω(z)(p+(p2ξ)ω(z))+zω(z)1ω(z).(8) Combining (Equation6) and (Equation8), then (9) (α+β)Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)(α+β)Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)(μ+λ)[((p2ξ12ξ2)zω(z))(1ω(z))]+(μ+λ)[zω(z)(p+(p2ξ12ξ2)ω(z))](p+(p2ξ12ξ2)ω(z))(1ω(z)).(9) Therefore from (Equation7) and (Equation9), we conclude that (10) (α+β)Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)=(μ+λ)[((2p2ξ)zω(z))](p+(p2ξ)ω(z))(1ω(z))+(α+β)(p+(p2ξ)ω(z))p(1ω(z))(10) After simplification (11) Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)=(μ+λ)[(2p2ξ)zω(z)](α+β)(p+(p2ξ)ω(z))(1ω(z))+(ppω(z)+(2p2ξ)ω(z))p(1ω(z)).(11) Or (12) Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1=(μ+λ)[(2p2ξ)zω(z)](α+β)(p+(p2ξ)ω(z))(1ω(z))+(2p2ξ)ω(z))p(1ω(z)).(12) Similarly (Equation7) implies (13) pΥλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)p=(2p2ξ)ω(z)1ω(z).(13) So using (Equation12) and (Equation13) we get Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1η1Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1η2=(2p2ξ)ω(z)p(1ω(z))η1×Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)η2=(2p2ξ)ω(z)p(1ω(z))η1+η2×p(μ+λ)[zω(z))](α+β)(p+(p2ξ)ω(z))ω(z)+1η2. Let there exists a point z0U such that max|z||z0||ω(z)|=|ω(z0)|=1. Then by using Lemma 1.1, we have ω(z0)=eiθ, 0<θ2π and z0ω(z0)=ζω(z0),ζ1. Therefore Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1η1×Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1η2=(2p2ξ)ω(z0)p(1ω(z0))η1+η2×p(μ+λ)[ζω(z0))](α+β)(p+(p2ξ)ω(z0))ω(z0)+1η2.=(2p2ξ)η1+η2p|1eiθ|p(μ+λ)ζ(α+β)(p+(p2ξ)eiθ+1η2.(2p2ξ)η1+η2pη1+η2×p(μ+λ)ζ(α+β)(p+p2ξ)+1η2.1ξpη11ξp+(μ+λ)ζ2(α+β)η2. Which is contradiction for 0ξp/2, so our supposition is wrong, therefore |ω(z)|<1 and hence fΦp,nα,β(ξ,μ).

For (p/2)ξp, we define ω(z) by (14) Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)=11pξ1ω(z),(14) implies (15) z(Υλn+1(p,α,β,μ)f(z))Υλn+1(p,α,β,μ)f(z)z(Υλn(p,α,β,μ)f(z))Υλn(p,α,β,μ)f(z)=pξ1ω(z)1pξ1ω(z)(15) Using (Equation6) we get (16) (α+β)Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)(α+β)Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)=(μ+λ)pξ1ω(z)1pξ1ω(z)(16) Now by using (Equation14) and (Equation16), we have (17) Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)=11pξ1ω(z)+(μ+λ)pξ1ω(z)(α+β)[1pξ1ω(z)](17) After simple calculation, we get (18) Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1=pξ1ω(z)1pξ1ω(z)+(μ+λ)pξ1ω(z)(α+β)[1pξ1ω(z)].(18) Similarly (19) Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1=pξ1ω(z)1pξ1ω(z).(19) Now by using (Equation18) and (Equation19) and after simplification, we have Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1η1×Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1η2=pξ1ω(z)1pξ1ω(z)η1+η21+(μ+λ)zω(z)(α+β)[ω(z)]η2. We suppose that there exists a point z0U such that max|z||z0||ω(z)|=|ω(z0)|=1. Then by using Lemma 1.1, we have ω(z0)=eiθ, 0<θ2π and z0ω(z0) =ζω(z0),ζ1. Implies Υλn+1(p,α,β,μ)f(z0)Υλn(p,α,β,μ)f(z0)1η1×Υλn+2(p,α,β,μ)f(z0)Υλn+1(p,α,β,μ)f(z0)1η2=pξ1ω(z0)1pξ1ω(z0)η1+η21+(μ+λ)zω(z0)(α+β)[ω(z0)]η2.=pξ1η1+η2|1pξ1eiθ|1+(μ+λ)ζ(α+β)η2.1ξpη1+η21+(μ+λ)ζ(α+β)η2, which is contradiction for (p/2)ξ2ξ1pξ2, so our supposition is wrong, therefore |ω(z)|<1 and hence fΦp,nα,β(ξ,μ).

Consider η1=η2=1 in Theorem 1.2, then we have the following result.

Corollary 1.3

If fA(p) satisfy the following criterion (20) Υλn+1(p,α,β,μ)f(z)Υλn(p,α,β,μ)f(z)1Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1<1ξp1ξp+μ+λ2(α+β)0ξp21ξp1+μ+λ(α+β)p2ξp..(20) then fΦp,nα,β(ξ,μ).

Taking η1=0, η2=1 in Theorem 1.2, then we have the following

Corollary 1.4

If fA(p) satisfy the following Υλn+2(p,α,β,μ)f(z)Υλn+1(p,α,β,μ)f(z)1<1ξp+μ+λ2(α+β)0ξp21ξp1+μ+λ(α+β)p2ξp., then fΦp,nα,β(ξ,μ).

Substitute β=l, μ=0, α=p in Theorem 1.2, we get

Corollary 1.5

If fA(p) and satisfy (21) Ipn+1(λ,l)f(z)Ipn(λ,l)f(z)1η1Ipn+2(λ,l)f(z)Ipn+1(λ,l)f(z)1η2  <1ξpη11ξp+λ2(p+l)η20ξp21ξpη1+η21+λ(p+l)η2p2ξp..(21) then fΦp,nα,β(ξ,μ).

Consider β=0, μ=0, α=p, λ=1 in Theorem 1.2, we get

Corollary 1.6

If fA(p) and satisfy (22) Dpn+1f(z)Dpnf(z)Dpnf(z)η1Dpn+2f(z)Dpn+1f(z)Dpn+1f(z)η2<1ξpη11ξp+12pη20ξp21ξpη1+η21+1pη2p2ξp..(22) then fΦp,nα,β(ξ,μ).

Setting β=0, μ=0, α=p, λ=1, η1=0 in Theorem 1.2, we get

Corollary 1.7

If fA(p) and satisfy Dpn+2f(z)Dpn+1f(z)Dpn+1f(z)η2<1ξp+12pη20ξp21ξpη21+1pη2p2ξp,. then fΦp,nα,β(ξ,μ).

2. Conclusions

In this paper, a linear differential operator is used to introduce new simply connected domain of analytic functions in the open unit disk U. It has been discussed that new simply connected domain consist of both members of simply connected starlike domain of order zero, simply connected starlike domain of order ξ and simply connected convex domain. Further, an analytic criterion for p-valent analytic function to be a member of family of new simply connected domain has been discussed. Moreover, our results have been discussed and compared with the earlier one.

Acknowledgments

The author is profoundly appreciative to the refs for contributing productive observations and benefits in updating the content of the aforementioned article.

Disclosure statement

No potential conflict of interest was reported by the author.

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