Note on Neighborhoods of Some Classes of Analytic Functions with Negative Coefficients
Irina Dorca,1Mugur Acu,2and Daniel Breaz3
Academic Editor: R. Avery
Received12 Apr 2011
Accepted15 May 2011
Published02 Jul 2011
Abstract
In this paper, we prove several inclusion relations associated with the neighborhoods of some subclasses of starlike and convex functions with negative coefficients.
1. Introduction
Let be the set of functions which are regular in the unit disc ,
and .
In [1], the subfamily of consisting of functions of the form
was introduced, where
For belonging to , SΔlΔgean [2] has introduced the following operator called the SΔlΔgean operator:
The object of the present paper is to derive some properties of neighborhoods for some subclasses of analytic functions with negative coefficients, which we have already studied.
2. Preliminary Results
Remark 2.1. In [3], we have introduced the following operator concerning the functions of form (1.2):
Definition 2.2 (see [3]). Let , , and , , . We say that is in the class if
Theorem 2.3 (see [3]). Let , , and . The function of the form (1.2) is in the class if and only if
Remark 2.4. Using the condition (1.3), we can to prove that
β, , , and .
Definition 2.5 (see [4]). Let , , and , , . We say that is in the class if
Theorem 2.6 (see [4]). Let , , and . The function of the form (1.2) is in the class if and only if
Remark 2.7. Using the condition (2.2), we can prove that
β, , , and .
Let be the class of functions of the form
which are analytic in the open unit disk . For any and , we define
which was called -neighborhood of . So, for , we observe that
The concept of neighborhoods was first introduced by Goodman in [5] and then generalized by Ruscheweyh in [6].
We propose to investigate the -neighborhoods of the subclasses and of the class of normalized analytic functions in with negative coefficients, where is the subclass of -starlike functions with negative coefficients of order and type introduced in [3] and is the subclass of -convex functions with negative coefficients of order and type studied in [4].
3. Main Results
We start by considering the linear operator (2.1) and conclude the study with several general inclusion relations associated with the neighborhoods for some subclasses of starlike and convex functions with negative coefficients.
Using the inequality (1.3) from Definition 2.5 and the inequality (2.1) from Definitionββ2.2, we obtain the subclasses , and and from Theorem 2.3, we derive the corresponding results.
Theorem 3.1. Let
where , , , and ; then
Proof. For and making use of the condition (2.2), we obtain , so that
On the other hand, we also find from (2.2) and (3.3) that
Thus,
which in view of definition (2.10), proves Theorem 3.1.
Remark 3.2. If and , we obtain that for , .
In a similar way, applying (2.5) instead of (2.2), we can prove the following.
Theorem 3.3. Let
where , , , and ; then
Consequently, we determine the neighborhood for each of the classes and , which we define as follows. A function defined by (2.8) is said to be in the class if there exists a function such that
Analogously, a function defined by (2.8) is said to be in the class if there exists a function such the inequality (3.8) holds.
Remark 3.4. If , we obtain that for , .
Further, we consider the inclusion relations just studied and generalize them by taking into account the relation (2.9).
Theorem 3.5. If and
then
, , , and .
Proof. Let . Making use of (2.9), we find that
which readily implies the coefficients of inequality
Furthermore, since , we have
so that
provided that is given precisely by (3.9), which evidently completes our proof of Theorem 3.5.
Example 3.6. For a given , , , we consider with , , , (for , ), where is given by (3.9). Then we have that
where . Therefore, .
Example 3.8. For a given , , , we consider with , , , (for , ), where is given by (3.16). Then we have that
where . Therefore, .
Acknowledgment
This work was partially supported by the strategic project POSDRU 107/1.5/S/77265, inside POSDRU Romania 2007-2013 cofinanced by the European Social Fund-Investing in People.
References
H. Silverman, βUnivalent functions with negative coefficients,β Proceedings of the American Mathematical Society, vol. 51, pp. 109β116, 1975.
G. S. SΔlΔgean, βSubclasses of univalent functions,β in Complex AnalysisβFifth Romanian-Finnish Seminar, Part 1 (Bucharest, 1981), vol. 1013 of Lecture Notes in Mathematics, pp. 362β372, Springer, Berlin, Germany, 1983.
M. Acu, I. Dorca, and S. Owa, βOn some starlike functions with negative coefficients,β Acta Universitatis Apulensis, Special Issue, Alba Iulia. In press.