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Research Articles

Study of simply connected convex domain and its geometric properties (II)

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Pages 597-603 | Received 30 Nov 2018, Accepted 17 Apr 2019, Published online: 09 May 2019

Abstract

In this paper, making use of a new integral operator defined in the open unit disk we introduce and study a new class of simply connected convex domain of analytic and univalent functions. We derive some inclusion results and its geometric properties. Moreover, we discuss analytic criteria for a member of the simply connected convex domain to be a member of the class of functions with positive real part greater than μ.

2010 Mathematics Subject Classification:

1. Introduction

We study simply connected domain including starlike and convex domain in geometric function theory. Miller and Mocanu (cf. [Citation1]) not only enlarged the scope of a simply connected domain but also worked to spread out in the area. In recent years, more and more researchers are interested to study the simply connected domain including starlike and convex domain (cf. [Citation2–10]). They not only introduced certain new simply connected domain including starlike and convex domain but also used them for the study of geometric properties of analytic and univalent functions. We are motivated by the research works based on analytic and univalent functions (cf. [Citation11] and [Citation12]). The articles provided an idea to introduce new subclasses of analytic and univalent functions in the open unit disk.

The functions analytic and univalent in U={zC:|z|<1} of the form (Equation1) with pN are said to form the class Ap. (1) f(z)=zp+k=0akzk.(1) For 0ρ<1 and k2, let Pk(ρ) (cf. [Citation13]) denote the class of analytic function h(z) with positive real part such that h(0)=1 and satisfy the analytic criterion (2) 02π(p(z))ρ1ρdθkπ,z=reiθ.(2) Moreover, Pk(0)=Pk. For the classes Pk(ρ) and Pk (cf.[Citation14].) Note that P2(β)=P(β), where P(β) is the class of functions with positive real part greater than β and P2(0)=P is the class of functions with positive real part. We can write (Equation2) in Riemann–Stieltjes sense as (3) p(z)=1202π1+(1ρ)zeit1zeitdμ(t),(3) where μ(t) is a function with bounded variation on [0,2π] such that (4) 02πdμ(t)=2&02πdμ(t)k.(4) By using (Equation2), we deduce that hPk(ρ) if and only if there exist h1,h2P(ρ) such that (5) h(z)=k4+12h1(z)k412h2(z),zU=zC:z<1.(5)

2. Materials and methods

For f given by (Equation1), γ0 and β0, we introduce a new integral operator as follows: (6) Ψp0(β,γ)f(z)=f(z);Ψp1(β,γ)f(z)=p+β1+γzβ1+γoztβ1+γ1f(t)dt;Ψp2(β,γ)f(z)=p+β1+γzβ1+γ×oztβ1+γ1Ψp1(β,γ)f(t)dt;Ψpn(β,γ)f(z)=p+β1+γzβ1+γ×oztβ1+γ1Ψpn1(β,γ)f(t)dt.(6) By using (Equation6), for the function f given in (Equation1), we get (7) Ψpm(β,γ)f(z)=zp+k=2(1+γ)p+β(1+γ)k+βmakzk,nN{0}.(7) Further, a straightforward calculation reveals that many differential operators introduced in other papers are special cases of the differential operator defined by (Equation7). By specializing the parameters β and γ, we obtain the following operators studied by various authors:

For λ0 and ω>0, let Ψp(λ,ω,β,γ) denote the class of functions f defined by (Equation1) and satisfy (8) (1λ)Ψpn+1(β,γ)f(z)zpω+λΨpn(β,γ)f(z)Ψpn+1(β,γ)f(z)×Ψpn+1(β,γ)f(z)zpωPk(ρ).(8) The analytic function g(z)=z(β/1+γ+β)z2/(1z)2, for zU belongs to the class Ψp(λ,ω,β,γ).

Next, we introduce definitions which will be used in our main results.

Definition 1

Let H be the set of complex valued functions h(r,s,t):C3C such that

  1. h(r,s,t) is continuous in a domain DC3,

  2. (1,1,1)D and |h(1,1,1)|<1,

then heiθ,eiθ+m,eiθ+2m+γ+1β+p(γ+1)m+L1, whenever eiθ,eiθ+m,eiθ+2m+γ+1β+p(γ+1)m+LD, with (L)m(m1) for real θ and m1 where L=(γ+1/β+p(γ+1))2z2w′′(z).

Definition 2

Let H be the set of complex valued functions h(r,s,t):C3C such that

  1. h(r,s,t) is continuous in a domain DC3,

  2. (1,1,1)D and |h(1,1,1)|<1,

then hmeiθ+γ+1β+p(γ+1)mm2+Lm+eiθeiθ,m+eiθ,m+eiθ+meiθ+γ+1β+p(γ+1)mm2+Lm+eiθ1, whenever meiθ+γ+1β+p(γ+1)mm2+Lm+eiθeiθ,m+eiθ,m+eiθ+meiθ+γ+1β+p(γ+1)mm2+Lm+eiθD, with (L)m(m1) for real θ and m1 where L=(γ+1)2z2/(β+p(γ+1))2w(z)/w(z).

Lemma 1

[Citation1] Let w(z)=a+wkzk+ be analytic in U={z:|z|<1} with w(z)a and k1. If z0=r0eiθ (0<r0<1) and |w(z0)|=max|z|r0|w(z)|. Then z0w(z0)=mw(z0) and 1+z0w(z0)w(z0)m, where m is real number and mkw(z0)a2w(z0)2a2kw(z0)aw(z0)+a.

Lemma 2

[Citation21] Let u=u1+iu2, v=v1+iv2 and Φ(u,v) be a complex valued function satisfy the conditions:

  1. Φ(u,v) is continuous in a domain DC2,

  2. (1,0)D and Φ(1,0)>0,

  3. {Φ(iu2,v1)}>0 whenever (iu2,v1)D and v11/2(1+u22).

If h(z)=1+cnzn+cn+1zn+1+ is analytic in U such that (h(z),zh(z))D and (h(z),zh(z))>0 then {h(z)}>0 in U.

3. Results and discussion

Theorem 3.1

If the function f given by (Equation1) belongs to the class Ψp(λ,ω,β,γ), then (Ψpn+1(β,γ)f(z)/zp)ωPk(μ), where μ=2ωρβ+p(1+γ)+λ(1+γ)2ωβ+p(1+γ)+λ(1+γ).

Proof.

By using (Equation2), if (Ψpn+1(β,γ)f(z)/zp)ωPk(μ), then it must satisfy the condition given by (Equation5). Let (9) Ψpn+1(β,γ)f(z)zpω=G(z)=(1μ)h(z)+μ=k4+12(1μ)h1(z)+μ+k4+12(1μ)h2(z)+μ,(9) where h1,h2P(μ). By using (Equation9) and (Equation7), we get Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)=(1+γ)(1μ)zh(z)ω(1μ)h(z)+μβ+p(γ+1)+1, this implies (1λ)Ψpn+1(β,γ)f(z)zpω+λΨpn(β,γ)f(z)Ψpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)zpω=λ(1+γ)(1μ)zh(z)ωβ+p(γ+1)(1μ)h(z)+μ+λ(1+γ)(1μ)zh(z)ωβ+p(γ+1)Pk(μ). By using (Equation2), we can see 11ρλ(1+γ)(1μ)zhi(z)ωβ+p(γ+1)(1μ)hi(z)+μρ+λ(1+γ)(1μ)zhi(z)ωβ+p(γ+1)P(μ),i=1,2. For u=hi(z) and v=zhi(z), we define Φ(u,v) such that Φ(u,v)=(1μ)u+μρ+λ(1+γ)(1μ)vωβ+p(γ+1). Since first two conditions of Lemma 1.2. are obvious so we proceed for third condition as follows: Φ(iu2,v1)=μρ+λ(1+γ)(1μ)v1ωβ+p(γ+1),μρλ(1+γ)(1μ)(1+u22)2ωβ+p(γ+1)=A+Bu22C, where A=2ω(μρ)β+p(γ+1)λ(1+γ)(1μ),B=λ(1+γ)(1μ),C=2ωβ+p(γ+1). We concluded that {Φ(iu2,v1)}<0, ⇔ A=0, B<0 and C>0. Hence by using Lemma 2, we get (Ψpn+1(β,γ)f(z)/zp)ωPk(μ).

Theorem 3.2

If the function f given by (Equation1) belongs to the class Ψp(λ,ω,β,γ), then (Ψpn+1(β,γ)f(z)/zp)ω/2Pk(μ), where μ=λ(1+γ)+λ2(1+γ)2+4ωρβ+p(1+γ)ω(β+p(1+γ))+λ(1+γ)2ω(β+p(1+γ))+λ(1+γ).

Proof.

By using (Equation2), if (Ψpn+1(β,γ)f(z)/zp)ω/2Pk(μ) implies satisfy the condition given by (Equation5). We consider (10) Ψpn+1(β,γ)f(z)zpω=G(z)=[(1μ)h(z)+μ]2=k4+12(1μ)h1(z)+μ2+k4+12(1μ)h2(z)+μ2,(10) where h1,h2P(μ). By using (Equation10) and (Equation7), we have Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)=2(1+γ)(1μ)zh(z)ω(1μ)h(z)+μβ+p(γ+1)+1, this implies (1λ)Ψpn+1(β,γ)f(z)zpω+λΨpn(β,γ)f(z)Ψpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)zpω=2λ(1+γ)(1μ)zh(z)ωβ+p(γ+1)(1μ)h(z)+μ2+(1μ)h(z)+μ×2λ(1+γ)(1μ)zh(z)ωβ+p(γ+1)Pk(μ). By using (Equation2) for i=1,2, we have 11ρ2λ(1+γ)(1μ)zhi(z)ωβ+p(γ+1)[(1μ)hi(z)+μ]2ρ+[(1μ)h(z)+μ]×2λ(1+γ)(1μ)zhi(z)ωβ+p(γ+1)P(μ). For u=hi(z) and v=zhi(z), we define Φ(u,v) such that Φ(u,v)=[(1μ)hi(z)+μ]2ρ+[(1μ)h(z)+μ]2λ(1+γ)(1μ)zhi(z)ωβ+p(γ+1).

Since first two conditions of Lemma 1.2. are obvious so we proceed for third condition as follows: Φ(iu2,v1)=[μ]2(1μ)2u22ρ+2λμ(1+γ)(1μ)v1ωβ+p(γ+1),[μ]2(1μ)2u22ρλμ(1+γ)(1μ)(1+u22)ωβ+p(γ+1)=A+Bu22C, where A=ω(μ2ρ)β+p(γ+1)λμ(1+γ)(1μ),B=(1μ)[ωβ+p(γ+1)(1μ)+λμ(1+γ)],C=ωβ+p(γ+1). We concluded that {Φ(iu2,v1)}<0, ⇔ A=0, B<0 and C>0. Hence by using Lemma 2, we get (Ψpn+1(β,γ)f(z)/zp)ω/2Pk(μ).

Theorem 3.3

Let h(r,s,t)H and f belongs to Ap satisfy the following criterion: Ψpn+1(β,γ)f(z)zp,Ψpn(β,γ)f(z)zp,Ψpn1(β,γ)f(z)zpD and hΨpn+1(β,γ)f(z)zp,Ψpn(β,γ)f(z)zp,Ψpn1(β,γ)f(z)zp<1. Then Ψpn+1(β,γ)f(z)zp<1.

Proof.

Let (11) Ψpn+1(β,γ)f(z)zp=w(z).(11) Clearly w(0)=1 and w(z)1. By using (Equation11) and (Equation7), we get (12) zΨpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)p=zw(z)w(z).(12) By using (Equation7) and doing some calculation, we have (13) zΨpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)=(βγ+1+p)Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)β1+γ.(13) After using (Equation12) and (Equation13) simultaneously, we get (14) Ψpn(β,γ)f(z)zp=w(z)+γ+1β+p(γ+1)zw(z).(14) After applying the same steps on (Equation14) and (Equation7), we get (15) Ψpn1(β,γ)f(z)zp=w(z)+2γ+1β+p(γ+1)zw(z)+γ+1β+p(γ+1)2z2w′′(z)+zw(z).(15) We claim that |w(z)|<1. On contrary, we suppose that for a=k=1, the w(z)=1+w1(z), this implies there exist z0E such that max|w(z)|=|w(z0)|=1, hence w(z0)=eiθ and m=γ+1/β+p(γ+1)zw(z)(say), then Ψpn+1(β,γ)f(z)zp=eiθ&Ψpn(β,γ)f(z)zp=eiθ+m,Ψpn1(β,γ)f(z)zp=eiθ+2m+γ+1β+p(γ+1)m+L, where L=(γ+1/β+p(γ+1))2z2w′′(z). After doing calculation, we have 1+γ+1β+p(γ+1)zw(z)w(z)m&m+mγ+1β+(γ+1)zw(z)w(z)m2 and m+γ+1β+p(γ+1)zw(z)w(z)(γ+1)(β+p(γ+1))zw(z)m2. This implies m+(γ+1)2z2w(z)(β+p(γ+1))2m2&Lm(m1). Since h(r,s,t)H, then by using definition of h(r,s,t), we have hΨpn+1(β,γ)f(z)zp,Ψpn(β,γ)f(z)zp,Ψpn1(β,γ)f(z)zp=hγ+1β+p(γ+1)eiθ,eiθ+m,eiθ+2m+γ+1β+p(γ+1)m+L1, which is contradiction and hence Ψpn+1(β,γ)f(z)zp<1.

Theorem 3.4

Let h(r,s,t)H and f belongs to Ap satisfy the following criterion: Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z),Ψpn1(β,γ)f(z)Ψpn(β,γ)f(z),Ψpn2(β,γ)f(z)Ψpn1(β,γ)f(z)D and hΨpn(β,γ)f(z)Ψpn+1(β,γ)f(z),Ψpn1(β,γ)f(z)Ψpn(β,γ)f(z),Ψpn2(β,γ)f(z)Ψpn1(β,γ)f(z)<1. Then Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)<1.

Proof.

Since, (16) Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)=w(z),(16) then clearly w(0)=1 and w(z)1. By using (Equation16) and (Equation7), we get (17) (γ+1)zΨpn(β,γ)f(z)Ψpn(β,γ)f(z)(γ+1)zΨpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)=(γ+1)zw(z)w(z).(17) By using (Equation7) and doing some calculation, we have (18) zΨpn(β,γ)f(z)Ψpn(β,γ)f(z)=βγ+1+pΨpn1(β,γ)f(z)Ψpn(β,γ)f(z)βγ+1(18) and (19) zΨpn+1(β,γ)f(z)Ψpn+1(β,γ)f(z)=βγ+1+pΨpn(β,γ)f(z)Ψpn+11(β,γ)f(z)βγ+1.(19) Simultaneously using (Equation17), (Equation18) and (Equation19) implies (β+p(γ+1))Ψpn1(β,γ)f(z)Ψpn(β,γ)f(z)=(1+γ)zw(z)w(z)+(β+p(γ+1))w(z) or (20) Ψpn1(β,γ)f(z)Ψpn(β,γ)f(z)=(1+γ)zw(z)(β+p(γ+1))w(z)+w(z).(20)

After differentiating (Equation20) and simplification, we get (21) zΨpn1(β,γ)f(z)Ψpn1(β,γ)f(z)zΨpn(β,γ)f(z)Ψpn(β,γ)f(z)=zw(z)+γ+1(β+p(γ+1))z2w(z)w(z)+zw(z)w(z)(zw(z))2(w(z))2w(z)+γ+1(β+p(γ+1))zw(z)w(z).(21) Now by using (Equation7) and (Equation20) for right-hand side of (Equation21), we get Ψpn2(β,γ)f(z)Ψpn1(β,γ)f(z)=w(z)+(γ+1)zw(z)(β+p(γ+1))w(z)+γ+1(β+p(γ+1))zw(z)+γ+1(β+p(γ+1))2z2w(z)w(z)+zw(z)w(z)(zw(z))2(w(z))2w(z)+γ+1(β+p(γ+1))zw(z)w(z). We claim that |w(z)|<1. On contrary, we suppose that for a=k=1, w(z)=1+w1(z), this implies there exist z0E such that max|w(z)|=|w(z0)|=1, hence w(z0)=eiθ and m=(γ+1)z0w(z0)(β+p(γ+1))w(z0)(say), then m(β+p(γ+1))eiθ=(γ+1)z0w(z0). This implies that Ψpn(β,γ)f(z0)Ψpn+1(β,γ)f(z0)=eiθ&Ψpn1(β,γ)f(z0)Ψpn(β,γ)f(z0)=m+eiθ,Ψpn2(β,γ)f(z0)Ψpn1(β,γ)f(z0)=m+eiθ+meiθ+γ+1β+p(γ+1)mm2+Lm+eiθ, where L=(γ+1)2z2w(z)/(β+p(γ+1))2w(z). After doing calculation, we have 1+γ+1β+p(γ+1)zw(z)w(z)m&m+mγ+1β+p(γ+1)zw(z)w(z)m2 and m+γ+1β+p(γ+1)zw(z)w(z)(γ+1)(β+p(γ+1))zw(z)w(z)m2. This implies m+(γ+1)2z2(β+p(γ+1))2w(z)w(z)m2&Lm(m1). Since h(r,s,t)H, then by using definition of h(r,s,t), we have hΨpn(β,γ)f(z)Ψpn+1(β,γ)f(z),Ψpn1(β,γ)f(z)Ψpn(β,γ)f(z),Ψpn2(β,γ)f(z)Ψpn1(β,γ)f(z)=hmeiθ+γ+1β+p(γ+1)mm2+Lm+eiθeiθ,m+eiθ,m+eiθ+meiθ+γ+1β+p(γ+1)mm2+Lm+eiθ1, which is contradiction and therefore Ψpn(β,γ)f(z)Ψpn+1(β,γ)f(z)<1.

Example 1

Since we have g(z)=zβ1+γ+βz21z2,zU,=z(1z)1+z21+γ1+γ+β1z2,=z+z2+z3++1+γ1+γ+βz2+2(1+γ)1+γ+βz3+,g(z)=z+(1+γ)+β1+γ+βz2+3(1+γ)+β1+γ+βz3+,g(z)=z+k=2(1+γ)k+β1+γ+βzk. After taking n-times hadamard product of g, we have g(z)=z+k=2(1+γ)k+β1+γ+βnzk. We define g(z)f(z)=z1z,implies f(z)=z+k=21+γ+β(1+γ)k+βnzkforp=1, which clearly satisfy our result.

4. Conclusion

In this paper, an integral operator is used to introduce new simply connected convex domain of analytic functions in the open unit disk U. Further, analytic criteria for p-valent analytic function belongs to simply connected convex domain to be a member of class of functions with positive real part greater than μ have been discussed.

Acknowledgements

The author is deeply indebted to the referees for providing constructive comments and helps in improving the content of this article.

Disclosure statement

No potential conflict of interest was reported by the author.

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