On a certain family of meromorphic p valent functions
1. Mathematical Theory and Modeling www.iiste.org
ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online)
Vol.2, No.8, 2012
On a Certain Family of Meromorphic p- valent Functions
with Negative Coefficients
D.O. Makinde
Department of Mathematics,
Obafemi Awolowo University, Ile-Ife 220005, Nigeria.
dmakinde@oauife.edu.ng, domakinde.comp@gmail.com
Abstract
In this paper, we introduce and study a new subclass ܯ ሺߛ ,ߚ ,ߙ .ݓሻ of meromorphically p-valent functions with
negative coefficients. We first obtained necessary and sufficient conditions for a certain meromorphically
p-valent function to be in the classܯ ሺߛ ,ߚ ,ߙ .ݓሻ, we then investigated the convex combination of certain
meromorphic functions as well as the distortion theorems and convolution properties.
AMS mathematics Subject Classification: 30C45.
Key Words: Meromophic, p-valent, Distortion, Convolution.
1.Introduction
Let ܯ denote the class of functions ݂ሺݖሻ of the form
ஶ
ࢌሺࢠሻ ൌ ܽ ݖ ሺܽ 0, ܰ ∈ ൌ ሼ1,2,3, … ሽ ሺ1ሻ
ࢠ
ୀ
Which are analytic and univalent in the punctured unit disk
ܦൌ ሼ 0 ݀݊ܽ ܥ ∈ ݖ ∶ ݖ൏ ലݖല ൏ 1ሽ
an which have a simple pole at the origin ሺ ݖൌ 0ሻ with residue 1 there. Altintas et all [1] defines the function
ܯሺߚ ,ߙ ,ሻ as the function ݂ሺݖሻ ∈ ܯ satisfying the inequality:
Re ሼ݂ݖሺݖሻ െ ߙ ݖଶ ݂ ᇱ ሺݖሻሽ ߚ (2)
For some ߙሺߙ 1ሻ and ߚሺ0 ߚ ൏ 1ሻ, for all .ܦ ∈ ݖOther subclasses of the class ܯ were studied recently by
Cho et al [2, 3]. While Firas Ghanim and Maslina Darus [4] obtained various properties of the function of the
form
ଵ
݂ሺݖሻ ൌ ∑ஶ ܽ ݖ
ୀଵ
௭ି
With fixed second coefficient. Also, M.Kamali et al [5] studied various properties of meromorphic
P-valent function of the form:
ଵ
݂ሺݖሻ ൌ െ ∑ஶ ܽ ݖ ሺܽ 0; ܰ ∈ ൌ ሼ1,2, … ሽሻ
ୀ
௭
Let ܣ௪ ሺݖሻ denote the set of functions analytic in ܦgiven by
ଵିఈ
݂ሺݖሻ ൌ െ ∑ஶ ܽሺ௭ି௪ሻ ሺܽ 0; ܰ ∈ ൌ ሼ1,2, … ሽሻ
ୀ (3)
ሺ௭ି௪ሻ
Which have a pole of order at ሺ ݖൌ ݓሻ, ܦ ∈ ݖand ݓis an arbitrary fixed point in .ܦWe define the function
݂ሺݖሻ in ܣ௪ ሺݖሻ to be in the class ܯ ሺߛ ,ߚ ,ߙ .ݓሻ if it satisfies the inequality
ܴ݁ሼሺ ݖെ ݓሻ݂ሺݖሻ െ ߚሺ ݖെ ݓሻଶ ݂′ሺݖሻሽ ߛ (4)
and ݓis an arbitrary fixed point in .ܦWe define the function ݂ሺݖሻ in ܽ௪ ሺݖሻ to be in the class
For some ߚሺߚ ൏ 1ሻ, ߙሺ0 ߙ ൏ 1ሻand ߛሺ0 ߛ ൏ 1ሻ, for all .ܦ ∈ ݓ ,ݖ
The purpose of this paper is to investigate some properties of the functions belonging to the class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ.
57
2. Mathematical Theory and Modeling www.iiste.org
ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online)
Vol.2, No.8, 2012
2.Main Results
Theorem 1 Let the function ݂ሺݖሻ be in the class ܣ௪ ሺݖሻ. Then ܣ௪ ሺݖሻ belong to the class ܯ ሺߛ ,ߚ ,ߙ .ݓሻ if and
only if
∑ஶ ሺ1 െ ݊ߚሻܽ ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ
ୀଵ (5)
Proof: let ݂ሺݖሻ be as in (3), suppose that
݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,ߙ .ݓሻ
Then we have from (4) that
ଵିఈ ିሺଵିఈሻ
Reሼሺ ݖെ ݓሻ ቀሺ௭ି௪ሻ െ ∑ஶ ܽ ሺ ݖെ ݓሻ ቁ െ ߚሺ ݖെ ݓሻଶ ሾቀሺ௭ି௪ሻశభ െ ∑ஶ ݊ܽ ሺ ݖെ ݓሻିଵ ቁሿሽ
ୀ ୀ
=Reሼ1 െ ߙ െ ∑ஶ ܽ ሺ ݖെ ݓሻା ߚሺ1 െ ߙሻ ∑ஶ ߚ݊ܽ ሺ ݖെ ݓሻାଵ ሽ
ୀ ୀ
= Reሼሺ1 െ ߙሻሺ1 ߚሻ െ ∑ஶ ሺ1 െ ݊ߚሻܽ ሺ ݖെ ݓሻାሽ ߛ ሺܦ ∈ ݖሻ, w an arbitrary fixed point in D
ୀ
If we choose ሺ ݖെ ݓሻ to be real and let ሺ ݖെ ݓሻ → 1ି , we get
ஶ
ሺ1 െ ߙሻሺ1 ߚሻ െ ሺ1 െ ݊ߚሻܽ ߛሺߚ ൏ 1; ߛ ൏ 1, ߚ݊ ൏ 1ሻ
ୀ
Which is equivalent to (5)
Conversely, suppose that the inequality (5) holds. Then we have:
|ሺ ݖെ ݓሻ݂ሺݖሻ െ ߚሺ ݖെ ݓሻଶ ݂ ᇱ ሺݖሻ െ ሺ1െ∝ሻሺ1 ߚሻ|
=|-∑ஶ ሺ1 െ ݊ߚሻܽ ሺ ݖെ ݓሻା | ∑ஶ ሺ1 െ ݊ߚሻܽ |ሺ ݖെ ݓሻ|ା
ୀଵ ୀ
ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ,
ሺ ݓ ,ܦ ∈ ,ݖan arbitrary fixed point in ߚ ܦ൏ 1, ܱ ߙ ൏ 1,0 ߛ ൏ 1ሻ which implies that ݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,∝ ,ݓሻ.
This completes the prove of the Theorem 1.
Corollary 1: Let ݂ሺݖሻ be in ܣ௪ ሺݖሻ . If ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,∝ ,ݓሻ , then
ሺ1 െ ߙሻሺ1 ߚെ ߛሻ
ܽ , ݊ 1, ߚ ൏ 1, ݊ߚ ൏ 1
1 െ ݊ߚ
Theorem 2. Let the function ݂ሺݖሻ be in ܣ௪ ሺݖሻ and in the function ݃ሺݖሻ be defined by
ଵିఈ
ሺ௭ି௪ሻ
െ ∑ஶ ܾ ሺ ݖെ ݓሻ , ܾ 0
ୀଵ (6)
Be in the same class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ. Then the function ݄ሺݖሻ defined by
ஶ
1െߙ
݄ሺݖሻ ൌ ሺ1 െ ߣሻ݂ሺݖሻ ߣ݃ሺݖሻ ൌ െ ܿ ሺ ݖെ ݓሻ
ሺ ݖെ ݓሻ
ୀ
is also in the class ܯ ሺߛ ,ߚ ,∝ ,ݓሻ, where ܿ ൌ ሺ1 െ ߣሻܽ ߣܾ ; 0 ߣ 1
proof: Suppose that each of ݂ሺݖሻ, ݃ሺݖሻ is in the class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ. Then by (5) we have
ஶ ஶ
ሺ1 െ ݊ߚሻܿ ൌ ሺ1 െ ݊ߚሻሾሺ1 െ ߣሻܽ ܾ ሿ
ୀ ୀ
ஶ ஶ
ൌ ሺ1 െ ߣሻ ሺ1 െ ݊ߚሻܽ ߣ ሺ1 െ ݊ߚሻܾ
ୀ ୀଵ
ሺ1 െ ߣሻ(1 െ ߙሻ ሺ1 ߚሻ െ ߛ ߣሺ1 െ ߙሻሺ1 ߚሻ െ ߛ
ൌ(1 െ ߙሻሺ1 ߚሻ െ ߛ ሺ0 ߙ ൏ 1, ߚ ൏ 1, 0 ߛ ൏ 1, 0 ߣ 1ሻ
This complete the proof of Theorem 2.
58
3. Mathematical Theory and Modeling www.iiste.org
ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online)
Vol.2, No.8, 2012
3.Distortion Theorems
Theorem 3.: Let ݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,∝ ,ݓሻ, then
ଵିఈ ሺଵିఈሻሺଵାఉሻିఊ
| ݂ሺݖሻ| + | ݖെ |ݓ (7)
|௭ି௪| ଵିఉ
Proof : By Theorem 1,we have
ሺଵିఈሻሺଵାఉሻିఊ
∑ஶ ܽ
ୀ , ሺ0 ߙ ൏ 1, ߚ ൏ 1, 0 ߛ ൏ 1, ݊ ∈ ܰ, ݊ߚ ൏ 1 (8)
ଵିఉ
And
ሺଵିఈሻሺଵାఉሻିఊ
∑ஶ ݊ܽ ݊
ୀ , ሺ0 ߙ ൏ 1, ߚ ൏ 1, 0 ߛ ൏ 1, ݊ ∈ ܰ, ݊ߚ ൏ 1 (9)
ଵିఉ
Let ݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ, we have
1െߙ
∞
݂ሺݖሻ | ݖെ |ݓ ܽ
| ݖെ |ݓ
ୀଵ
1െߙ ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ
| ݖെ |ݓ
| ݖെ |ݓ 1 െ ݊ߚ
Which completes the proof of the Theorem 3.
Theorem 4: Let ݂ሺݖሻ ∈ ܣ௪ ሺݖሻ. If ݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ then
1െߙ ݊ሾሺ1 െ ߙሻሺ1 ߚሻ െ ߛሿ
ห݂ ᇱሺ௭ሻ ห | ݖെ |ݓିଵ
| ݖെ |ݓାଵ 1 െ ݊ߚ
01 ݊ ݏ݅ ݁ݎ݄݁ݐ
ଵିఈ
ห݂ ᇱሺ௭ሻ ห |௭ି௪|శభ+| ݖെ |ݓିଵ ∑ஶ ݊ܽ
ୀ
1െߙ ݊ሾሺ1 െ ߙሻሺ1 ߚሻ െ ߛሿ
| ݖെ |ݓିଵ
| ݖെ |ݓ ାଵ 1 െ ݊ߚ
This completes the proof of the Theorem 4.
4.Convolution Properties
4.
Let the convolution of two complex-valued meromorphic functions
ଵିఈ
݂ଵ ሺݖሻ ൌ +∑ஶ ܽଵ ሺ ݖെ ݓሻ
ୀଵ
ሺ௭ି௪ሻ
And
ଵିఈ
݂ଶ ሺݖሻ ൌ +∑ஶ ܽଶ ሺ ݖെ ݓሻ
ୀଵ
ሺ௭ି௪ሻ
Be defined by
ஶ
1െߙ
ܨሺݖሻ ൌ ൫݂ଵ ሺݖሻ ∗ ݂ଶ ሺݖሻ൯ ൌ ܽଵ ܽଶ ሺ ݖെ ݓሻ
ሺ ݖെ ݓሻ
ୀଵ
Theorem 5. Let the function ܨሺݖሻ be in the class ܣ௪ ሺݖሻ. Then ܨሺݖሻ belong to the class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ if and
only if
ஶ
ሺ1 െ ݊ߚሻܽଵ ܽଶ ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ
ୀ
Proof: supposed that
59
4. Mathematical Theory and Modeling www.iiste.org
ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online)
Vol.2, No.8, 2012
ܨሺݖሻ ∈ ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ
Then we have from (4) that
ଵିఈ ିሺଵିఈሻ
Re {ሺ ݖെ ݓሻ ሺ௭ି௪ሻ െ ∑ஶ ܽଵ ܽଶ ሺ ݖെ ݓሻ ሻ െ ߚሺ ݖെ ݓሻଶ ሾቀሺ௭ି௪ሻశభ െ ∑ஶ ܽଵ ܽଶ ሺ ݖെ ݓሻିଵ ቁሿሽ
ୀଵ ୀଵ
=Re ሼ1 െ ߙ െ ∑ஶ ܽଵ ܽଶ ሺ ݖെ ݓሻା ߚሺ1 െ ߙሻ ∑ஶ ߚܽଵ ܽଶ ሺ ݖെ ݓሻାଵ ሽ
ୀଵ ୀଵ
=Re {ሺ1 െ ߙሻሺ1 ߚሻ െ ∑ஶ ሺ1 െ ݊ߚሻܽଵ ܽଶ ሺ ݖെ ݓሻା ሽ ߛ ሺܦ ∈ ݖሻ, ݓan arbitrary fixed point in ܦ
ୀ
If we choose ሺ ݖെ ݓሻ to be real and let ሺ ݖെ ݓሻ → 1ି , we get
ሺ1 െ ߙሻሺ1 ߚሻ െ ∑ஶ ሺ1 െ ݊ߚሻܽଵ ܽଶ ߛ ሺߚ ൏ 1; 0 ߛ ൏ 1, ߚ݊ ൏ 1ሻ
ୀ
which is equivalent to(5)
Conversely,suppose that the inequality (5) holds. Then we have:
|ሺ ݖെ ݓሻ ݂ሺݖሻ- ߚሺ ݖെ ݓሻଶ ݂ ᇱ ሺݖሻ െ ሺ1െ∝ሻሺ1 ߚሻ|
=|-∑ஶ ሺ1 െ ݊ߚሻܽଵ ܽଶ ሺ ݖെ ݓሻା | ∑ஶ ሺ1 െ ݊ߚሻܽଵ ܽଶ |ሺ ݖെ ݓሻ|ା
ୀଵ ୀ
ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ,
ሺܦ ∈ ݖሻ, ݓan arbitrary fixed point in ߚ ܦ൏ 1,0 ߙ ൏ 1,0 ߛ ൏ 1ሻ which implies that
݂ሺݖሻ ∈ ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ.
This completes the prove of Theorem 5.
Theorem 6. Let the function ܨሺݖሻ be inܣ௪ ሺݖሻ and the function ܩሺݖሻ defined by
ሺଵିఈሻ
+∑∞ b୬ଵ ܾଶ ሺ ݖെ ݓሻ , ܾଵ ܾଶ 0
ୀଵ
ሺ௭ି௪ሻ
Be in the same class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ.Then the function ܪሺݖሻ define by
ሺ1 െ ߙሻ
∞
ܪሺݖሻ ൌ ሺ1 െ ߣሻܨሺݖሻ ߣܩሺݖሻ ൌ ܿ ሺ ݖെ ݓሻ
ሺ ݖെ ݓሻ
ୀଵ
Is also in the class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ ,where ܿ ൌ ሺ1 െ ߣሻܽଵ ܽଶ ߣܾଵ ܾଶ ; 0 ߣ 1
proof: suppose that each of ܨሺݖሻ, ܩሺݖሻ is in the class ܯ ሺߛ ,ߚ ,ߙ ,ݓሻ. Then by 5 we have
∑∞ ሺ1 െ ݊ߚሻܿ = ∑∞ ሺ1 െ ݊ߚሻ ሺ1 െ ߣሻܽଵ ܽଶ ߣܾଵ ܾଶ
ୀଵ ୀଵ
∞ ∞
ൌ ሺ1 െ ߣሻ ሺ1 െ ݊ߚሻ ܽଵ ܽଶ ߣ ሺ1 െ ݊ߚሻb୬ଵ ܾଶ
ୀଵ ୀଵ
ሺ1 െ ߣሻ ሺ1 െ ߙሻሺ1 ߚሻ െ ߛ + ߣሺ1 െ ߙሻሺ1 ߚሻ െ ߛ
ൌ ሺ1 െ ߙሻሺ1 ߚሻ െ ߛሺ0 ߙ ൏ 1, ߚ ൏ 1, 0 ߛ ൏ 1, 0 ߣ 1ሻ
This completes the proof of Theorem6.
References
[1] O., Altintas H. Irmak and H.M. Strivastava A family of meromorphically univalent functions with positive
coefficient. DMS-689-IR 1994.
[2] N.E. Cho, S.Owa, S.H. Lee and O. Altintas, Generalization class of certain meromorphic univalent
functions eith positive coefficient, Kyungypook Math, J.(1989) 29, 133-139.
[3] N.E. Cho, , S.H. Lee, S.Owa, A class of meromorphic univalent functions wit
h positive coefficients, Kobe J. Math. (1987) 4, 43-50
[4] F. Ghanim and M. Darus On a certain subclass of meromorphic univalent functions wih fixed second positive
coefficients, surveys in mathematics and applications (2010) 5 49-60.
[5] M. Kamali, K. Suchithra, B.A. Stephen and A. Gangadharan, On certain subclass of meromorphic p –valent
functions with negative coefficients. General Mathematics (2011) 19(1) 109-122
60
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