Research Article | Open Access

Sarika Verma, Sushma Gupta, Sukhjit Singh, "Bounds of Hankel Determinant for a Class of Univalent Functions", *International Journal of Mathematics and Mathematical Sciences*, vol. 2012, Article ID 147842, 6 pages, 2012. https://doi.org/10.1155/2012/147842

# Bounds of Hankel Determinant for a Class of Univalent Functions

**Academic Editor:**Teodor Bulboaca

#### Abstract

The authors study the coefficient condition for the class defined as the family of analytic functions and , which satisfy , where is a real number.

#### 1. Introduction

Let be the class of functions of the following form: which are analytic in the unit disc , and let be the subclass of consisting of functions which are univalent in . A function is said to be close to convex in the open unit disc if there exists a convex function (not necessarily normalized) such that For fixed real numbers , let denote the family of functions in which satisfy

In 2005, V. Singh et al. [1] established that, for , functions in satisfy in and so are close to convex in .

In [2], Noonan and Thomas defined the Hankel determinant of the function for and by

The determinant has been investigated by several authors with the subject of inquiry ranging from rate of growth of as , to the determination of precise bounds on for specific and for some special classes of functions. In a classical theorem, Fekete and Szeg [3] considered the Hankel determinant of for and

The well-known result due to them states that if , then where and is a real number. In the present paper, we obtain a sharp bound for when .

#### 2. Preliminary Results

We denote by the family of all functions given by analytic in for which for . It is well known that for , for each .

Lemma 2.1 (See [4]). * The power series for p(z) given in (2.1) converges in to a function in if and only if the Toeplitz determinants
** and = are all nonnegative. They are strictly positive except for
**
and for ; in this case, for and for .*

Lemma 2.2 (See [5, 6]). * Let . Then
**
for some such that and .*

#### 3. Main Result

Theorem 3.1. * Let , , be a real number. If , then
**
where is the root of the equation and
*

*Proof. * Since , it follows from (1.3) that there exists a function such that
Equating coefficients in (3.3) yields

Thus, we can easily establish that

Using (2.4), in view of Lemma 2.2, we obtain that

Since , so . Letting , we may assume without restriction that . Thus, applying the triangle inequality on (3.6), with , we obtain

Differentiating , we get the following:

Using elementary calculus, one can show that for . It implies that is an increasing function, and, thus, the upper bound for corresponds to , in which case

Then,

Setting , since , we have
provided , where is the root of the equation .

*Case 1. * When , then the maximum value of corresponds to . Therefore, we have

*Case 2. *When , the maximum value of corresponds to . Therefore, we have
where is given by (3.2). This completes the proof of the Theorem.

Setting in above theorem, we get the following result of Janteng et al. [7].

Corollary 3.2. *If an analytic function is such that , , then
**
The result is sharp.*

#### References

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#### Copyright

Copyright © 2012 Sarika Verma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.