On Certain Sufficient Condition Involving Gaussian Hypergeometric Functions

Abstract

The authors define a new subclass of of functions involving complex order in the open unit disk . For this new class, we obtain certain inclusion properties involving the Gaussian hypergeometric functions.

1. Introduction and Motivation

Let be the class of functions normalized by which are analytic in the open unit disk As usual, we denote by the subclass of consisting of functions which are also univalent in . A function is said to be starlike of order in if and only if

This function class is denoted by We also write where denotes the class of functions that are starlike in with respect to the origin.

A function is said to be convex of order in if and only if

The class of convex functions is denoted by the class Further, , the well-known standard class of convex functions. It is an established fact that

A function is said to be in the class of uniformly convex functions in if is a normalized convex function in and has the property that, for every circular arc contained in the unit disk , with center also in U, the image curve is a convex arc. The function class was introduced by Goodman [1].

For functions given by (1.1) and given by we define the Hadamard product (or Convolution) of and by

Furthermore, we denote by and two interesting subclasses of consisting, respectively, of functions which are -uniformly convex and -starlike in . Thus, we have The class was introduced by Kanas and Wiśniowska [2], where its geometric definition and connections with the conic domains were considered. The class was investigated in [3]. In fact, it is related to the class by means of the well-known Alexander equivalence between the usual classes of convex and starlike functions; see also the work of Kanas and Srivastava [4] for further developments involving each of the classes and . In particular, when , we obtain where and are the familiar classes of uniformly convex functions and parabolic starlike functions in respectively (see for details, [1, 5]). In fact, by making use of a certain fractional calculus operator, Srivastava and Mishra [6] presented a systematic and unified study of the classes and .

A function is said to be in the class if it satisfies the inequality

The class was introduced earlier by Dixit and Pal [7]. Two of the many interesting subclasses of the class are worthy of mention here. First of all, by setting the class reduces essentially to the class introduced and studied by Ponnusamy and Rønning [8], where Secondly, if we put we obtain the class of functions satisfying the inequality which was studied by (among others) Padmanabhan [9] and Caplinger and Causey [10].

Finally, many of the authors have also studied the class . For details of these works one can refer to the works of Ding Gong [11], R. Singh and S. Singh [12], Owa and Wu [13], and also the references cited by them. Although, many mapping properties of the class have been studied by these authors, they did not study any mapping properties involving the hypergeometric functions.

The Gaussian hypergeometric function , is given by is the solution of the homogeneous hypergeometric differential equation and has rich applications in various fields such as conformal mappings, quasiconformal theory, and continued fractions.

Here, , , are complex numbers such that , for , and for each positive integer , is the Pochhammer symbol. In the case of , , is defined if or where . In this situation, becomes a polynomial of degree in . Results regarding when is positive, zero, or negative are abundant in the literature. In particular when , the function is bounded. This and the zero balanced case are discussed in detail by many authors (see [14, 15]). The hypergeometric function has been studied extensively by various authors and it plays an important role in Geometric Function Theory. It is useful in unifying various functions by giving appropriate values to the parameters , and . We refer to [8, 16–19] and references therein for some important results.

In particular, the close-to-convexity (in turn the univalency), convexity, starlikeness, (for details on these technical terms we refer to [5]), and various other properties of these hypergeometric functions were examined based on the conditions on , and in [8]. For more interesting properties of hypergeometric functions, one can also refer to [20, 21].

Let and be analytic in and univalent. Then we say that is subordinate to written as if and .

For , we recall that the operator of Hohlov [22] which maps into itself defined by where denotes usual Hadamard product of power series. Therefore, for a function defined by (1.1), we have

Using the integral representation, we can write

When equals the convex function , then the operator in this case becomes . For , , with then the convolution operator turns into Bernardi operator Indeed, and are known as Alexander and Libera operators, respectively.

Let and let be of the form (1.1). If , then the following coefficient inequalities hold true (cf. [2]): where is the coefficient of in the function which is the extremal function for the class related to the class by the range of the expression where is given, as above, by (1.22).

Similarly, if of the form (1.1) belong to the class , then (cf. [3]) where is given, as above by (1.22).

2. Properties of

Theorem 2.1. Let and be of the form (1.1). If , then The estimate is sharp.

Proof. Since , we have where is analytic in and satisfies the condition and for . Hence, we have Using and , we have By equating the coefficients, we observe that the coefficient in the right-hand side depends only on on the left-hand side of the above expression. This gives By using we get Squaring both sides of (2.6) and integrating around , we obtain By letting we conclude that or By making use of the fact that we get This gives The result is sharp for the function

Theorem 2.2. Let . Then a sufficient condition for is The result is sharp for the function

Proof. In view of (2.13), which is clearly less than or equal to zero for all , Letting we get Thus, .

3. Results Involving Gaussian Hypergeometric Function

Theorem 3.1. Let Also, let c be a real number such that . Then a sufficient condition for the function to be in the class is that where

Proof. has the series representation given by In view of Theorem 2.2, it suffices to show that
From the fact that we observe that is real and positive, under the hypothesis By writing as, we get Using the fact that it is easy to see that From (1.14), By using the Gauss summation theorem we get Equation (3.4) now follows by an application of (3.1) and (3.2).