Abstract

The object of the present investigation is to solve Fekete-Szegö problem and determine the sharp upper bound to the second Hankel determinant for a new class of analytic functions in the unit disk. We also obtain a sufficient condition for an analytic function to be in this class.

1. Introduction and Preliminaries

Let be the class of functions of the form: which are analytic in the open unit disk .

A function is said to be starlike function of order and convex function of order , respectively, if and only if and , for and for all . By usual notations, we denote these classes of functions by and , respectively. We write and , the familiar subclasses of starlike functions and convex functions in .

Furthermore, a function is said to in the class , if it satisfies the inequality: Note that is a subclass of close-to-convex functions of order in .

Let denote the class of analytic functions of the form: satisfying the condition in .

Let the functions and be analytic in . We say that is subordinate to , written as or , if there exists a Schwarz function , which (by definition) is analytic in with and . Furthermore, if the function is univalent in , then we have the following equivalence relation (cf., e.g., [1]):

For the functions analytic in and given by the power series their Hadamard product (or convolution), denoted by is defined as Note that is analytic in .

The Gauss hypergeometric function is defined by the infinite series where denotes the Pochhammer symbol (or shifted factorial) given, in terms of the Gamma function , by We note that the series, given by (7), converges absolutely for and hence the function represents an analytic function in the unit disc [2].

We further observe that the Gauss hypergeometric function plays an important role in the study of various properties and characteristics of subclasses of univalent/multivalent functions in geometric function theory (cf., e.g. [3–5]). In our present investigation, we consider the incomplete beta function , defined by

By making use of the Hadamard product and the function , Carlson and Shaffer [6] defined the linear operator by If is given by (1), then it follows from (10) that The operator extends several operators introduced and studied by earlier researchers in geometric function theory.  For example, , the well-known Ruscheweyh derivative operator [7] of and , the familiar Owa-Srivastava fractional differential operator [8] of .

With the aid of the linear operator , we introduce a subclass of as follows.

Definition 1. A function is said to be in the class , if it satisfies the following subordination relation: where the power in the right hand side of (13) indicates the principal branch. Note that if , then by (13)

We denote by , the class of functions satisfying the subordination condition: In fact, by suitably specializing the parameters , and in the class , we can obtain several subclasses of .

Remark 2. To bring out the geometrical significance of the class , we set and note that which gives or . Letting , we deduce that which on simplification reduces to . Thus, is the interior of the right half branch of the hyperbola . Hence, if , then the set of values for lie in , where is given by (16).

Fekete and Szegö [9] defined the Hankel determinant of a function , given by (1) as In our present investigation, we also consider the second Hankel determinant of , given by It is known [10] that if given by (1) is analytic and univalent in , then the sharp inequality holds. For a family of functions in of the form (1), the more general problem of finding the sharp upper bounds for the functionals is popularly known as Fekete-Szegö problem for the class . The Fekete-Szegö problem for the known classes of univalent functions, starlike functions, convex functions, and close-to-convex functions has been completely settled [9, 11–18]. Recently, Janteng et al. [19, 20] have obtained the sharp upper bounds to the second Hankel determinant for the family of functions in whose derivatives have positive real part in . For initial work on the class , one may refer to the paper by MacGregor [21].

Our objective in the present paper is to solve the Fekete-Szegö problem and also to determine the sharp upper bound to the second Hankel determinant for the class by following the techniques devised by Libera and Złotkiewicz [22, 23]. The criteria for functions in to be in this class are also obtained.

To establish our main results, we will need the following results about the functions belonging to the class .

Lemma 3. Let the function , given by (3), be a member of the class . Then for some complex numbers satisfying and . The estimates in (21) and (22) are sharp.

We note that the estimate (21) is contained in [10]; the estimate (22) is obtained by Ma and Minda [24]; the results in (23) and (24) are due to Libera and Złotkiewicz [23] (see also [22]).

2. Main Results

Unless otherwise mentioned, we assume throughout the sequel that

Now, we determine the sharp upper bound for the functional for functions of the form (1) belonging to the class .

Theorem 4. Let and . If the function , given by (1), belongs to the class , then for any The estimate in (26) is sharp.

Proof. Since , by (14) we have where is given by (3). It is easily seen that Writing the series expansion of given by (11), , in (27) and equating the coefficients of in the resulting equation, we obtain Thus for any , and by using (22) in the above expression, we get which, upon simplification, gives the required assertion of Theorem 4.
Equality in (26) holds for the function defined in by where the function is given by (16). This completes the proof of Theorem 4.

Theorem 5. Let and . If the function , given by (1), belongs to the class , then The estimates are sharp.

Proof. First, we assume that . Then so that by (26), we obtain Next, let Then, a routine calculation yields and by using (26) again, we get Finally, if , then Thus, by (26), we have
The estimates are sharp for the function defined in by where the function is given by (16) and the proof of Theorem 5 is completed.

Using (21) in (29) and putting and , respectively, in Theorem 5, we get the following.

Corollary 6. Let . If the function , given by (1), belongs to the class , then The estimates in (44) and (46) are sharp for the function defined by whereas the estimate in (45) is sharp for the function given by where the function is given by (16).

Letting and in Theorem 8, we obtain the following.

Corollary 7. If the function , given by (1), belongs to the class , then The estimates are sharp for the function defined in by where is given by (16).

Next, we find the sharp upper bound for the fourth coefficient of functions in the class .

Theorem 8. Let the function , given by (1), belong to the class . Then and the estimate in (51) is sharp.

Proof. From (31), we have Since the functions and are in the class simultaneously, we assume without loss of generality that . For convenience of notation, we write . Now, by using (23) and (24) in (52), we deduce that for some complex numbers and .
Applying the triangle inequality in the above expression followed by the replacement of with in the resulting equation, we obtain
We next maximize the function on the closed rectangle . Since we have for and . Thus, cannot have a maximum in the interior on the closed rectangle . Therefore, for fixed where A routine calculation yields for or . Since and , we conclude that the maximum of is attained at . Thus, the upper bound of the function corresponds to . Putting in (54), we get our desired estimate (51).
Equality in (51) holds for the function defined by where is given by (16).