Research Article | Open Access

Rosihan M. Ali, Naveen Kumar Jain, V. Ravichandran, "On the Largest Disc Mapped by Sum of Convex and Starlike Functions", *Abstract and Applied Analysis*, vol. 2013, Article ID 682413, 12 pages, 2013. https://doi.org/10.1155/2013/682413

# On the Largest Disc Mapped by Sum of Convex and Starlike Functions

**Academic Editor:**Ferhan M. Atici

#### Abstract

For a normalized analytic function defined on the unit disc , let be a function of positive real part in , need not have that property in , and . For certain choices of and , a sharp radius constant is determined, , so that maps onto a specified region in the right half-plane.

#### 1. Introduction

Letbe the class of functionsanalytic inand normalized by. Letbe its subclass consisting of univalent functions. For two analytic functionsand, the functionis subordinate to, written, if there is an analytic self-mapwithsatisfying. Given an analytic functionwithandin, denote byandthe subclasses ofconsisting, respectively, ofsatisfyingand.

For various choices of, these classes reduce to well-known subclasses of starlike and convex functions. For instance, with,, thenandare, respectively, the subclasses consisting of *starlike functions of order*and *convex functions of order*. The classesandare the familiar subclasses ofof starlike and convex functions. For,, is the class of functionssatisfying
studied by Uralegaddi et al. [1]. Various subclasses ofhave been investigated in [2–5]. For,, the classis the class of *strongly starlike functions of order*. The classintroduced by Sokół and Stankiewicz [6] consists of functionssatisfying

Thus, a functionis in the classiflies in the region bounded by the right-half of the lemniscate of Bernoulli given by. Results related to the classcan be found in [3, 7–9].

In investigating the classof uniformly convex functions, Rønning [10] introduced a classof *parabolic starlike functions*. These are functionssatisfying

It is important to keep in mind that the qualifier “parabolic” refers to the geometry of the image ofunder the map; that is, the domain necessarily lies in a parabolic region of the-plane. It does not convey the interpretation that the functionmaps the diskonto a parabolic region. This terminology of *parabolic starlike functions* is however widely accepted and used by authors. Ali and Ravichandran [11] recently surveyed works on uniformly convex and *parabolic starlike functions*.

This paper finds radius estimates for classes of functions in. The radius of a propertyin a given set of functions[12, page 119] is the largest numbersuch that every function in the sethas the propertyin each discfor every. For example, the Koebe function, which mapsonto the domain, is starlike but not convex. However,maps the disconto a convex domain for every. Indeed, every univalent functionmapsonto a convex domain for[13, Theorem 2.13, page 44]. This number is known as the radius of convexity for.

It is known that. The functionis convex and therefore starlike of order; it is clear that the function has real part greater than 1/2. Now the function takes values in, and therefore does not have positive real part for all. Their sum takes values inand therefore the sumdoes not have positive real part in. This motivates us to determine the largest radiussuch that

More generally, letandbe functions satisfyingin, whileneed not necessarily be positive in the whole unit discFor certain choices ofand, a sharp radius constantis determined,, so that whenever, the sumtakes values in specified regions in the complex plane. The results obtained are shown to reduce those of Singh and Paul [14] in certain special cases.

#### 2. Main Results

Forand, with*∈*, several radius results for the sumto be in certain regions in the complex plane are obtained in the following result.

Theorem 1. *Let; letbe defined by
**
Then* (a)*, , whereis given by
* (b)*, whereis given by
* (c)*, , whereis given by
* (d)*, , whereis given by
* (e)*,whereis the root of the equation in:
* *In particular,
* (f)*Also, whereis the root of the following equation in:
* *andis the root of the equation in:
**Each radius constantis sharp.*

For two analytic functions, their *convolution or Hadamard product*, denoted by, is defined by. The following results are needed in the sequel.

Lemma 2 ([15, Lemma 2.7, page 126; Lemma 3.5, page 130]). *Ifand, orandbelong to, then
**
for any functionanalytic in, wheredenotes the closed convex hull of.*

Lemma 3 ([7, Lemma, page 6559]). *For, letbe given by
**
and for, letbe given by
**Then,
*

*Proof of Theorem 1. *Letbe defined by

First, for each, will be shown to, respectively, satisfy, and. Then, using Lemma 2,is deduced to satisfy the required condition.

As in [14], let
so that

(a) By (21), (22), and (23), it follows that
*Case (i)*. Suppose thatWe assert thatfor, where the minimum is taken over all. Let. Then
if,, and that for,

Thus, for,

On the other hand, if, then it can be shown that
*Case (ii)*. For, thenin,. Indeed for, as in the case (i),

The previously mentioned two cases show thatin. Figure 1 illustrates sharpness of the radiusin the case.

(b) Forgiven by (21), a calculation shows that

By Lemma 3, the functionsatisfies
provided
that is,

This inequality holds if. Figure 2 illustrates sharpness of the radius.

(c) From (30), it follows that
provided
holds, which occurs whenever. Sharpness of the radiusin the caseis illustrated in Figure 3.

(d) Inequality (30) also yields
provided
that is, when. Figure 4 illustrates sharpness of the radiusin the case.

(e) For the functiongiven by (21), it follows from (22) and (23) that
. A calculation shows thatwhere

Nowfor,if, and

Thus

Evidently, (38) and (41) give
provided

Figure 5 illustrates sharpness of the radiusin the case.

(f) The inequality
holds if
or, with,. Then,

Let. Since
there exists a uniquesuch thatand.

Thus,for. When,
hence,forFigure 6 illustrates sharpness of the radius.

Next, consider,. Then,

Lemma 2, together with (50) and the corresponding inequality for the function, shows that each functionsatisfies the required condition. For sharpness, consider the function. Then,
Sharpness of the numbersis now evident in view of the definition.

For, Theorem 1(a) reduces to the following corollary.

Corollary 4 ([14, Theorem, page 724]). *If, then
**
inThe numberis sharp.*

Theorem 5. *Letandbe defined by
**
Then* (a)*,, whereis given by
* (b)*,, whereis the root of the equation
* *In particular,
* (c)*Also, whereis given by the equation in* *andis given by the equation in**Each radius constantis sharp.*

*Proof. *Let

Each, , is shown to, respectively, satisfy, , and. Then, it follows from Lemma 2 thatsatisfies the required condition.

(a) We claim thatin. By (22) and (23),
*Case (i)*. SupposeIn this case, it is shown thatforover allin. Let. It can be verified that
if,, and that for,

Thus for,

On the other hand, if, then

Sinceis a decreasing function in,
*Case (ii)*. For, we prove thatin,Let. As in Case (i), then

It is evident from the previous two cases thatinFigure 7 shows that, for, the radiusis sharp.

(b) Let. Then,

By (67), it follows that

Letbe defined by
(The caseis similar.) A calculation shows that

Let

Then,,for, andfor. Thus,

Now (68) and (72) show that
provided
that is,

Thus,inFigure 8 shows that, for, the radiusis sharp.

(c) Proceeding similarly as in part (a),

provided

Letbe defined by

Now

LetSince
there exists a uniquesuch thatand.

Then, for,

When,

Evidently,forand henceinFigure 9 shows that the radiusis sharp.

Now, with,,

Lemma 2, together with (83) and the corresponding inequality for the function, shows that each functionsatisfies the required condition. For sharpness, consider the function. Clearly
hence the fact that the numberis sharp follows from the definition of.