Certain Problem for Starlike Functions with Respect to Other Points ()
1. Introduction
Let S denote the class of functions
(1.1)
which is analytic and univalent in
.
Let T denote the subclass of S consisting of functions of the form
(1.2)
Let
be the subclass of S consisting of functions given by (1.1) satisfying
These functions are called starlike with respect to symmetric points [1].
The aim of this work is to study the class
which consists of functions f of T, satisfies
(1.3)
for
,
,
.
and a class
of functions f of T, satisfies
(1.4)
for
,
,
.
and a class
of functions f of T, satisfies
(1.5)
for
,
,
.
Using fractional calculus to study distortions cahractaristics and estimated coefficients is obtained.
2. The Class
Theorem 2.1 Let the function f be defined by (1.2), then
if and only if
(2.1)
where
,
,
and
.
the result (2.1) is sharp for the function
Proof: Assume that the inequality (2.1) holds true and
. Then obtain
Thus, by maximum modulus principle [2],
Now assume that
then
Then
i.e.
Thus
And the proof is complete.
3. The Class
Theorem 3.1 Let the function f be defined by (1.2), then
if and only if
(3.1)
where
,
,
and
.
The result (3.1) is sharp for the function
Proof: Assume that the inequality (3.1) holds true and
. Then obtain
Thus, by maximum modulus principle [2],
.
Now assume that
then
Then
i.e.
Thus
And the proof is complete.
4. The Class
Theorem 4.1 Let the function f be defined by (1.2), then
if and only if
(4.1)
where
,
,
and
.
The result (4.1) is sharp for the function
Proof: Assume that the inequality (4.1) holds true and
. Then obtain
Now assume that
then
Then
i.e.
Thus
And the proof is complete.
5. Application of the Fractional Calculus
Several operators of fractional calculus (i.e., fractional derivative and fractional integral) have been rather extensively studied by many researchers (c.f. [3] [4] [5] ). Making use of the following Lemma (given by Srivastava et al. [6] and used by Gh. Esa and Darus [7] ) stated as
Lemma 5.1 Let
, then
.
to prove the following theorem:
Theorem 5.2 Let
. If f(z) defined by (1.2) in the class
,
then
(5.1)
and
(5.2)
for
, where
the result is sharp and is given by
(5.3)
Proof. By using Lemma 5.1, we have
. (5.4)
Setting
where
. (5.5)
It is easily verified that h(k)is non-decreasing for k ≥ 2, and thus we have
(5.6)
Now, noting that δ(n, 2)is increasing function of n, we have
or
(5.7)
Hence, using (5.6) and (5.7), we have
(5.8)
which proves (5.1), and other parts (5.2) we can find that
(5.9)
and the prove is complete.
Using the same technique for the functions f(z) in the classes
and
.
Now, taking
and
in the Theorem 5.1, and using the definition given by Owa [8] which is stated as:
Definition 5.3 (Fractional Integral Operator) [8]. The fractional integral of order λ is defined, for a function f(z),by
(5.10)
where f(z) is an analytic function in a simply-connected region of the z-plane containing the origin, and the multiplicity of
is removed by requiring log(z − ζ) to be real when
(z − ζ)>0.
We get two seperated corollaries which are contained in;
Corollary 5.4 Let the fuction f(z) defined by (1.2) be in the class
, then we have
(5.11)
and
(5.12)
for
,
. The result is sharp for the function
(5.13)
Corollary 5.5 Let the fuction f(z) defined by (1.2) be in the class
, then we have
(5.14)
and
(5.15)
for
. The result is sharp for the function
(5.16)
Again the same technique uses for the function in the classes
and
.
6. Conclusion
The classes
,
,
of analytic and univalent functions are investigated. The estimated coefficients are studied and obtaind respectively and shown in the Equations (2.1), (3.1) and (4.1). The application of the fractional calculus is studied on the class
and obtained in Equations (5.1) and (5.2) and concluded for other classes
and
. Fractional Integral Operator is studied and obtained on the class
and concluded for other classes by using the same mathematical techniques.