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# Open Mathematics

### formerly Central European Journal of Mathematics

Editor-in-Chief: Gianazza, Ugo / Vespri, Vincenzo

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# Simple sufficient conditions for starlikeness and convexity for meromorphic functions

Pranay Goswami
/ Teodor Bulboacă
/ Rubayyi T. Alqahtani
Published Online: 2016-08-12 | DOI: https://doi.org/10.1515/math-2016-0050

## Abstract

In this paper we investigate some extensions of sufficient conditions for meromorphic multivalent functions in the open unit disk to be meromorphic multivalent starlike and convex of order α. Our results unify and extend some starlikeness and convexity conditions for meromorphic multivalent functions obtained by Xu et al. [2], and some interesting special cases are given.

MSC 2010: 30C45

## 1 Introduction and definitions

Let Σp, n denote the class meormorphic multivalent functions of the form $f(z)=z−p+∑k=n∞ak−pzk−p(p,n∈N:={1,2,3…}),$(1)

which are analytic in the punctured unit disk ${\mathcal{U}}^{\ast }:=u\mathrm{\setminus }\left\{0\right\}$, where $\mathcal{U}:=\left\{z\in \mathbb{C}:\left|z\right|<1\right\}$.

1. A function f ∈ Σp, n is said to be meromorphic starlike functions of order α, if it satisfies the inequality $−Rezf′(z)f(z)>α,z∈U,$

for some real α (0 ≤ α < p), and we denote this subclass by $\mathrm{\Sigma }{S}_{p,n}^{\ast }\left(\alpha \right)$.

2. A function f ∈ Σp, n is said to be meromorphic convex functions of order α, if it satisfies the inequality $−Re1+zf″(z)f′(z)>α,z∈U,$

for some real α (0 ≤ α < p), and we denote this subclass by ΣKp, n(α).

Let ${\mathcal{A}}_{n}$ denote the class of analytic functions in $\mathcal{U}$ of the form $f(z)=z+∑k=n∞ak+1zk+1,z∈U(n∈N).$

1. Let ${S}_{n}^{\ast }$ denote the class of n-starlike functions in $\mathcal{U}$, i.e. $f\in {\mathcal{A}}_{n}$ and satisfies $Rezf′(z)f(z)>0,z∈U.$

2. Further, we denote by ${\mathcal{K}}_{n}$ the class of n-convex functions in $\mathcal{U}$, i.e. $f\in {\mathcal{A}}_{n}$ and satisfies $Re1+zf″(z)f′(z)>0,z∈U.$

In the recent papers of Goyal et al. [1] and Xu et al. [2], the authors obtained some sufficient conditions for multivalent and meromorphic starlikeness and convexity, respectively. In this paper we will derive some extensions of these sufficient conditions for starlikeness and convexity of order α for meromorphic multivalent functions.

## 2 Main results

In order to find some simple sufficient conditions for the starlikeness and convexity of order α for a function f ∈ Σp, n, we will recall the following lemma due to P. T. Mocanu (see also [3]):

#### ([4, Corollary 1.2])

If $f\in {\mathcal{A}}_{n}$ and satisfies the inequality $f′(z)−1

then $f\in {S}_{n}^{\ast }$.

Remark that, for the special case n = 1, this result was previously obtained in [5, Theorem 3].

If f ∈ Σp, n, with f(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, satisfies the inequality $zpf(z)1α−pzf′(z)f(z)+α+p−α<(n+1)(p−α)(n+1)2+1,z∈U,$(2)

for some real values of α (0 ≤ α < p), then $f\in \mathrm{\Sigma }{S}_{p,n}^{\ast }\left(\alpha \right)$. (The power is the principal one).

For f ∈ Σp, n, with f(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, let’s define the function h by $h(z)=zzpf(z)1α−p,z∈U(0≤α(5)

If h satisfies the inequality $h″(z)≤n+1(n+1)2+1,z∈U,$(6)

then $f\in \mathrm{\Sigma }{S}_{p,n}^{\ast }\left(\alpha \right)$. (The power is the principal one).

Next, we will give some sufficient conditions for a function f ∈ Σp, n to be a convex function of order α.

If f ∈ Σp, n, with f′(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, satisfies the inequality $zp+1f′(z)−p1α−p1+zf″(z)f′(z)+α+p−α<(n+1)(p−α)(n+1)2+1,z∈U,$(7)

for some real values of α (0 ≤ α < p), then f ∈ ΣKp, n(α). (The power is the principal one).

If f ∈ Σp, n, with f′(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, satisfies the inequality $1zzp+1f′(z)−p1α−p1+zf″(z)f′(z)+p≤(n+1)(p−α)2(n+1)2+1,z∈U,$(10)

for some real values of α (0 ≤ α < p), then f ∈ ΣKp, n(α). (The power is the principal one).

1. If we put n = 1 in Theorem 2.2 and Theorem 2.3, we get the results established by Xu et al. [2].

2. For the special case n = 1, Theorem 2.4 and Theorem 2.5 represent the results of Xu et al. [2].

For f ∈ Σp, n, with f(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, let’s define the function F by $F(z)=∫0z[tpf(t)]γdt,z∈U(γ∈C),$

where the power is the principal one. Thus, $F\left(z\right)=z+\frac{\gamma }{n+1}{a}_{n-p}{z}^{n+1}+\cdots \in {\mathcal{A}}_{n}$, and considering this integral operator we derive the next result:

If f ∈ Σp, n, with f(z) ≠ 0 for all $z\in {\mathcal{U}}^{\ast }$, satisfies the inequality $γ[zpf(z)]γzzf′(z)f(z)+p≤n+12(n+1)2+1,z∈U,$(12)

for $\gamma \le \frac{1}{p}$, then $f\in \mathrm{\Sigma }{S}_{p,n}^{\ast }\left(p+\frac{1}{\gamma }\right)$. (The power is the principal one).

## 3 Special cases

Let’s consider the function f defined by $f(z)=1zp1−λ+λsin⁡zzα−p,z∈U,$(15)

where 0 ≤ α < p, the power is the principal one, and assuming that the parameter λ ∈ ℂ is chosen such that $1λ≠1−sin⁡zz,z∈U.$(16)

Using MAPLE™ software, from Figure 1a we may see that $maxz≤11−sin⁡zz<0.18,$(17)

therefore (16) holds whenever $\left|\lambda \right|\le \frac{50}{9}=5.555...$, and consequently, if λ ∈ ℂ satisfies this inequality then f ∈ Σp,2.

Using again MAPLE™ software, from Figure 1b we have that $maxz≤1sin2z2<0.28,$(18)

and a simple computation leads to $zpf(z)1α−pzf′(z)f(z)+α+p−α=2(p−α)λsin2z2<2(p−α)λ⋅0.28,z∈U.$

Thus, according to Theorem 2.2 we obtain the following special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{75}{14\sqrt{10}}=1.6941...$, then $f(z)=1zp1−λ+λsin⁡zzα−p∈ΣSp,2∗(α),$

for some real values of α (0 ≤ α < p), where the power is the principal one.

For the function f given by (15), the function h defined by (5) is of the form $h(z)=(1−λ)z+λsin⁡z,z∈U.$

Therefore, $\left|{h}^{″}\left(z\right)\right|=\left|\lambda \right|\left|\mathrm{sin}z\right|$, and using MAPLE™ software, the Figure 2a yields that $maxz≤1sin⁡z<1.2.$

Now, according to Theorem 2.3 we obtain the following special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{5}{2\sqrt{10}}=0.79057...$, then $f(z)=1zp1−λ+λsinzα−p∈ΣSp,2∗(α),$

for some real values of α (0 ≤ α < p), where the power is the principal one.

If we compare the result given by Example 3.1 with the above one, for this special choice of the function f the Example 3.1 gives a better result.

As we already proved, if $\left|\lambda \right|\le \frac{50}{9}=5.555...$ then the relation (16) holds. Therefore, there exists a function f ∈ Σp,2 such that $f′(z)=−pzp+11−λ+λsin⁡zzα−p,z∈U,$

where 0 ≤ α < p, and the power is the principal one, assuming that λ ∈ ℂ is chosen such that $\left|\lambda \right|\le \frac{50}{9}=5.555...$

A simple computation combined with (18) shows that $zp+1f′(z)−p1a−p1+zf″(z)f′(z)+α+p−α=2(p−α)λsin2z2<2(p−α)λ⋅0.28,z∈U,$

and from Theorem 2.4 we obtain the following special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{75}{14\sqrt{10}}=1.6941...$, then f ∈ Σp,2 with $f′(z)=−pzp+11−λ+λsin⁡zza−p,z∈U,$(19)

for some real values of α (0 ≤ α < p), is in ΣKp, n(α). (The power is the principal one).

Using MAPLE™ software, from Figure 2b we have that $maxz≤1zcos⁡z−sin⁡zz2<0.38.$(20)

From a simple computation combined with (20) we get $1zzp+1f′(z)−p1α−p1+zf″(z)f′(z)+p=(p−α)λzcos⁡z−sin⁡zz2<(p−α)λ⋅0.38,z∈U,$

and using Theorem 2.5 we obtain the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{75}{19\sqrt{10}}=1.2483...$, then f ∈ Σp,2 that satisfies (19) has, moreover, the property that f ∈ ΣKp, n(α), for some real values of α (0 ≤; α < p), where the power is the principal one.

Comparing the result given by Example 3.3 with the above one, for this special choice of the function f the Example 3.3 gives a better result.

As we proved at the beginning of this section, the function f ∈ Σp,2, where $f(z)=1zp1−λ+λsin⁡zz1γ,z∈U,$(21)

with λ ∈ ℂ, $\left|\lambda \right|\le \frac{50}{9}=5.555...$, and the power is the principal one. Using the inequality (20), we deduce that $γ[zpf(z)]γzzf′(z)f(z)+p=λzcos⁡z−sin⁡zz2<λ⋅0.38,z∈U,$

and from Theorem 2.7 we obtain the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{75}{19\sqrt{10}}=1.2483...$, then the function f given by (21) is in $\mathrm{\Sigma }{S}_{p,2}^{\ast }\left(p+\frac{1}{\gamma }\right)$. for $\gamma \le -\frac{1}{p}$. (The power is the principal one).

Using MAPLE™ software, we could check that the next inequalities hold (see Figures 3a, 3b, 4a, and 4b): $maxz≤11+z2−ez−1z<0.22,$(22) $maxz≤1ez−1−z<0.73,$(23) $maxz≤1ez−1<1.73,$(24) $maxz≤1zez−ez+1z2−12<0.51.$(25)

From (22) and (23), using Theorem 2.2 we may easily obtain the following special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{300}{73\sqrt{10}}=1.2996...$, then $f(z)=1zp1+λez−1z−1−z2α−p∈ΣSp,2∗(α),$

for some real values of α (0 ≤ α < p), where the power is the principal one.

From (22) and (24), according to Theorem 2.3 we could similarly obtain the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{300}{173\sqrt{10}}=0.54837...$, then $f(z)=1zp1+λez−1z−1−z2α−p∈ΣSp,2∗(α),$

for some real values of α (0 ≤ α < p), where the power is the principal one.

Thus, for this special choice of the function f the Example 3.6 gives a better result.

From (22) and (24), using Theorem 2.4 we easily get the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{300}{73\sqrt{10}}=1.2996...$, then f ∈ Σp,2 with $f′(z)=−pzp+11+λez−1z−1−z2α−p,z∈U,$(26)

for some real values of α (0 ≤ α < p), is in ΣKp, n(α). (The power is the principal one).

From (22) and (25), according to Theorem 2.5 we could similarly obtain the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{150}{51\sqrt{10}}=0.93008...$, then f ∈ Σp,2 that satisfies (26) has, moreover, the property that f ∈ ΣKp, n(α), for some real values of α (0 ≤ α < p), where the power is the principal one.

Consequently, for this special choice of the function f the Example 3.8 gives a better result.

Finally, from the inequalities (22) and (25), using Theorem 2.7 we obtain the next special case:

If λ ∈ ℂ and $\left|\lambda \right|\le \frac{150}{51\sqrt{10}}=0.93008...$, then $f(z)=−1zp1+λez−1z−1−z21γ,∈ΣSp,2∗p+1γ,$

for $\gamma \le -\frac{1}{p}$. (The power is the principal one).

We will omit the detailed proofs of the last three examples, since these are similar with the previous ones.

## References

• [1]

Goyal S.P., Bansal S.K., Goswami P, Extensions of sufficient conditions for starlikeness and convexity of order α for multivalent function, Appl. Math. Lett., 2012, 25(11), 1993-1998 Google Scholar

• [2]

Xu Y., Frasin B.A., Liu J., Certain sufficient conditions for starlikeness and convexity of meromorphically multivalent functions, Acta Math. Sci. Ser. B Engl. Ed., 2013, 33(5), 1300-1304 Google Scholar

• [3]

Mocanu P.T., Oros Gh., Sufficient condition for starlikeness of order α, Int. J. Math. Math. Sci., 2001, 28(9), 557-560 Google Scholar

• [4]

Mocanu P.T., Some simple criteria for starlikeness and convexity, Lib. Math. (N.S.), 1993, 13, 27-40 Google Scholar

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Singh V., Univalent functions with bounded derivative in the unit disc, Indian J. Pure Appl. Math., 1977, 8, 1370-1377 Google Scholar

## About the article

Rubayyi T. Alqahtani: Department of Mathematics and Statistics, College of Science, Al-Imam Mohammad Ibn Saud Islamic University (IMSIU), P.O.Box 65892, Riyadh 11566, Saudi Arabia

Received: 2016-01-31

Accepted: 2016-06-10

Published Online: 2016-08-12

Published in Print: 2016-01-01

Citation Information: Open Mathematics, ISSN (Online) 2391-5455,

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© 2016 Goswami et al., published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0

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