volume 7, issue 4, article 138, 2006.
Received 06 October, 2006;
accepted 15 November, 2006.
Communicated by:N.E. Cho
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Journal of Inequalities in Pure and Applied Mathematics
ON CERTAIN CLASSES OF MEROMORPHIC FUNCTIONS INVOLVING INTEGRAL OPERATORS
KHALIDA INAYAT NOOR
Mathematics Department
COMSATS Institute of Information Technology Islamabad, Pakistan.
EMail:khalidanoor@hotmail.com
c
2000Victoria University ISSN (electronic): 1443-5756 252-06
On Certain Classes of Meromorphic Functions Involving Integral Operators
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Abstract
We introduce and study some classes of meromorphic functions defined by us- ing a meromorphic analogue of Noor [also Choi-Saigo-Srivastava] operator for analytic functions. Several inclusion results and some other interesting proper- ties of these classes are investigated.
2000 Mathematics Subject Classification:30C45, 30C50.
Key words: Meromorphic functions, Functions with positive real part, Convolution, Integral operator, Functions with bounded boundary and bounded radius rotation, Quasi-convex and close-to-convex functions.
This research is supported by the Higher Education Commission, Pakistan, through research grant No: 1-28/HEC/HRD/2005/90.
Contents
1 Introduction . . . 3 2 Main Results . . . 8
References
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1. Introduction
LetMdenote the class of functions of the form f(z) = 1
z +
∞
X
n=0
anzn,
which are analytic inD={z : 0<|z|<1}.
LetPk(β) be the class of analytic functions p(z) defined in unit discE = D∪ {0},satisfying the propertiesp(0) = 1and
(1.1)
Z 2π
0
Rep(z)−β 1−β
dθ≤kπ,
wherez = reiθ, k ≥ 2and0 ≤ β < 1.Whenβ = 0,we obtain the classPk defined in [14] and for β = 0, k = 2, we have the classP of functions with positive real part.
Also, we can write (1.1) as
(1.2) p(z) = 1
2 Z 2π
0
1 + (1−2β)ze−it 1−ze−it dµ(t),
whereµ(t)is a function with bounded variation on[0,2π]such that (1.3)
Z 2π
0
dµ(t) = 2, and
Z 2π
0
|dµ(t)| ≤k.
From (1.1), we can write, forp∈Pk(β),
(1.4) p(z) =
k 4 + 1
2
p1(z)− k
4 − 1 2
p2(z),
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where p1, p2 ∈P2(β) =P(β), z∈E.
We define the functionλ(a, b, z)by λ(a, b, z) = 1
z +
∞
X
n=0
(a)n+1 (c)n+1
zn, z ∈D,
c 6= 0,−1,−2, . . . , a >0,where(a)nis the Pochhamer symbol (or the shifted factorial) defined by
(a)0 = 1, (a)n =a(a+ 1)· · ·(a+n−1), n >1.
We note that
λ(a, c, z) = 1
z2F1(1, a;c, z),
2F1(1, a;c, z)is Gauss hypergeometric function.
Letf ∈ M.Denote byL(a, c);˜ M −→ M,the operator defined by L(a, c)f(z) =˜ λ(a, c, z)? f(z), z∈D,
where the symbol ? stands for the Hadamard product (or convolution). The operator L(a, c)˜ was introduced and studied in [5]. This operator is closely related to the Carlson-Shaeffer operator [1] defined for the space of analytic and univalent functions inE,see [11,13].
We now introduce a function(λ(a, c, z))(−1) given by λ(a, c, z)?(λ(a, c, z))(−1) = 1
z(1−z)µ, (µ >0), z∈D.
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Analogous toL(a, c),˜ a linear operatorIµ(a, c)onMis defined as follows, see [2].
Iµ(a, c)f(z) = (λ(a, c, z))(−1)? f(z), (1.5)
(µ >0, a >0, c6= 0,−1,−2, . . . , z ∈D).
We note that
I2(2,1)f(z) =f(z), and I2(1,1)f(z) = zf0(z) + 2f(z).
It can easily be verified that
z(Iµ(a+ 1, c)f(z))0 =aIµ(a, c)f(z)−(a+ 1)Iµ(a+ 1, c)f(z), (1.6)
z(Iµ(a, c)f(z))0 =µIµ+1(a, c)f(z)−(µ+ 1)Iµ(a, c)f(z).
(1.7)
We note that the operator Iµ(a, c)is motivated essentially by the operators de- fined and studied in [2,11].
Now, using the operatorIµ(a, c),we define the following classes of mero- morphic functions forµ >0, 0≤η, β <1, α≥0, z∈D.
We shall assume, unless stated otherwise, that a 6= 0,−1,−2, . . . , c 6=
0,−1,−2, . . .
Definition 1.1. A function f ∈ Mis said to belong to the class M Rk(η)for z ∈D,0≤η <1, k ≥2,if and only if
−zf0(z)
f(z) ∈Pk(η)
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andf ∈M Vk(η),for z ∈D, 0≤η <1, k ≥2,if and only if
−(zf0(z))0
f0(z) ∈Pk(η).
We callf ∈M Rk(η),a meromorphic function with bounded radius rotation of orderηandf ∈M Vka meromorphic function with bounded boundary rotation.
Definition 1.2. Letf ∈ M, 0≤η <1, k ≥2, z∈D.Then
f ∈M Rk(µ, η, a, c) if and only if Iµ(a, c)f ∈M Rk(η).
Also
f ∈M Vk(µ, η, a, c) if and only if Iµ(a, c)f ∈M Vk(η), z ∈D.
We note that, forz ∈D,
f ∈M Vk(µ, η, a, c) ⇐⇒ −zf0 ∈M Rk(µ, η, a, c).
Definition 1.3. Let α ≥ 0, f ∈ M, 0 ≤ η, β < 1, µ > 0andz ∈ D. Then f ∈ Bkα(µ, β, η, a, c), if and only if there exists a functiong ∈ M C(µ, η, a, c), such that
(1−α)(Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0 +α
−(z(Iµ(a, c)f(z))0)0 (Iµ(a, c)g(z))0
∈Pk(β).
In particular, for α = 0, k = a = µ = 2, andc = 1, we obtain the class of meromorphic close-to-convex functions, see [4]. For α = 1, k = µ = a = 2, c = 1,we have the class of meromorphic quasi-convex functions defined for z ∈ D.We note that the classC? of quasi-convex univalent functions, analytic inE,were first introduced and studied in [7]. See also [9,12].
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The following lemma will be required in our investigation.
Lemma 1.1 ([6]). Let u = u1 +iu2 and v = v1 +iv2 and let Φ(u, v) be a complex-valued function satisfying the conditions:
(i) Φ(u, v)is continuous in a domainD ⊂ C2, (ii) (1,0)∈ DandΦ(1,0)>0.
(iii) Re Φ(iu2, v1)≤0 whenever (iu2, v1)∈ Dandv1 ≤ −12(1 +u22).
Ifh(z) = 1+P∞
m=1cmzmis a function, analytic inE,such that(h(z), zh0(z))∈ Dand Re(h(z), zh0(z))>0forz ∈E,thenReh(z)>0inE.
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2. Main Results
Theorem 2.1.
M Rk(µ+ 1, η, a, c)⊂M Rk(µ, β, a, c)⊂M Rk(µ, γ, a+ 1, c).
Proof. We prove the first part of the result and the second part follows by using similar arguments. Let
f ∈M Rk(µ+ 1, η, a, c), z ∈D and set
H(z) = k
4 +1 2
h1(z)− k
4 − 1 2
h2(z)
=−
z(Iµ(a, c)f(z))0 Iµ(a, c)f(z)
(2.1) ,
whereH(z)is analytic inEwithH(0) = 1.
Simple computation together with (2.1) and (1.7) yields (2.2) −
z(Iµ+1(a, c)f(z))0 Iµ+1(a, c)f(z)
=
H(z) + zH0(z)
−H(z) +µ+ 1
∈Pk(η), z ∈E.
Let
Φµ(z) = 1 µ+ 1
"
1 z +
∞
X
k=0
zk
#
+ µ
µ+ 1
"
1 z +
∞
X
k=0
kzk
# ,
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then
(H(z)? zΦµ(z)) =H(z) + zH0(z)
−H(z) +µ+ 1
= k
4 + 1 2
(h1(z)? zΦµ(z))− k
4 − 1 2
(h2(z)? zΦµ(z))
= k
4 + 1
2 h1(z) + zh01(z)
−h1(z) +µ+ 1
− k
4 − 1
2 h2(z) + zh02(z)
−h2(z) +µ+ 1
. (2.3)
Sincef ∈M Rk(µ+ 1, η, a, c),it follows from (2.2) and (2.3) that
hi(z) + zh0i(z)
−hi(z) +µ+ 1
∈P(η), i= 1,2, z ∈E.
Lethi(z) = (1−β)pi(z) +β.Then
(1−β)pi(z) +
(1−β)zp0i(z)
−(1−β)pi(z)−β+µ+ 1
+ (β−η)
∈P, z ∈E.
We shall show thatpi ∈P, i= 1,2.
We form the functionalΦ(u, v)by takingu = pi(z), v = zp0i(z)withu = u1 +iu2, v = v1 +iv2. The first two conditions of Lemma1.1 can easily be verified. We proceed to verify the condition (iii).
Φ(u, v) = (1−β)u+ (1−β)v
−(1−β)u−β+µ+ 1 + (β−η),
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implies that
Re Φ(iu2, v1) = (β−η) + (1−β)(1 +µ−β)v1
(1 +µ−β)2+ (1−β)2u22. By takingv1 ≤ −12(1 +u22),we have
Re Φ(iu2, v1)≤ A+Bu22 2C , where
A= 2(β−η)(1 +µ−β)2−(1−β)(1 +µ−β), B = 2(β−η)(1−β)2−(1−β)(1 +µ−β), C = (1 +µ−β)2+ (1−β)2u22 >0.
We note thatRe Φ(iu2, v1)≤ 0if and only ifA ≤0andB ≤ 0.FromA ≤ 0, we obtain
(2.4) β = 1
4 h
(3 + 2µ+ 2η)−p
(3 + 2µ+ 2η)2−8 i
, andB ≤0gives us0≤β <1.
Now using Lemma1.1, we see that pi ∈ P forz ∈ E, i = 1,2and hence f ∈M Rk(µ, β, a, c)withβ given by (2.4).
In particular, we note that β = 1
4 h
(3 + 2µ)−p
4µ2+ 12µ+ 1i .
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Theorem 2.2.
M Vk(µ+ 1, η, a, c)⊂M Vk(µ, β, a, , c)⊂M Vk(µ, γ, a+ 1, c).
Proof.
f ∈M Vk(µ+ 1, η, a, c) ⇐⇒ −zf0 ∈M Rk(µ+ 1, η, a, c)
⇒ −zf0 ∈M Rk(µ, β, a, c)
⇐⇒f ∈M Vk(µ, β, a, c), whereβis given by (2.4).
The second part can be proved with similar arguments.
Theorem 2.3.
Bkα(µ+ 1, β1, η1, a, c)⊂ Bkα(µ, β2, η2, a, c)⊂ Bkα(µ, β3, η3, a+ 1, c), whereηi =ηi(βi, µ), i = 1,2,3are given in the proof.
Proof. We prove the first inclusion of this result and other part follows along similar lines. Let f ∈ Bkα(µ+ 1, β1, η1, a, c). Then, by Definition 1.3, there exists a functiong ∈M V2(µ+ 1, η1, a, c)such that
(2.5) (1−α)
(Iµ+1(a, c)f(z))0 (Iµ+1(a, c)g(z))0
+α
−(z(Iµ+1(a, c)f(z))0)0 (Iµ+1(a, c)g(z))0
∈Pk(β1).
Set
(2.6) p(z) = (1−α)
(Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0
+α
−(z(Iµ(a, c)f(z))0)0 (Iµ(a, c)g(z))0
,
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wherepis an analytic function inEwithp(0) = 1.
Now,g ∈ M V2(µ+ 1, η1, a, c)⊂M V2(µ, η2, a, c),whereη2 is given by the equation
(2.7) 2η22+ (3 + 2µ−2η1)η2−[2η1(1 +µ) + 1] = 0.
Therefore,
q(z) =
−(z(Iµ(a, c)g(z))0)0 (Iµ(a, c)g(z))0
∈P(η2), z ∈E.
By using (1.7), (2.5), (2.6) and (2.7), we have (2.8)
p(z) +α zp0(z)
−q(z) +µ+ 1
∈Pk(β1), q ∈P(η2), z ∈E.
With p(z) =
k 4 +1
2
[(1−β2)p1(z) +β2]− k
4 − 1 2
[(1−β2)p2(z) +β2], (2.8) can be written as
k 4 +1
2 (1−β2)p1(z) +α(1−β2)zp01(z)
−q(z) +µ+ 1 +β2
− k
4 − 1
2 (1−β2)p2(z) +α(1−β2)zp02(z)
−q(z) +µ+ 1 +β2
,
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where
(1−β2)pi(z) +α(1−β2)zp0i(z)
−q(z) +µ+ 1 +β2
∈P(β1), z ∈E, i= 1,2.
That is
(1−β2)pi(z) +α(1−β2)zp0i(z)
−q(z) +µ+ 1 + (β2−β1)
∈P, z ∈E, i= 1,2.
We form the functionalΨ(u, v)by taking u= u1+iu2 =pi, v =v1+iv2 = zp0i,and
Ψ(u, v) = (1−β2)u+α (1−β2)v
(−q1+iq2) +µ+ 1 + (β2 −β1), (q=q1+iq2).
The first two conditions of Lemma1.1are clearly satisfied. We verify (iii), with v1 ≤ −12(1 +u22)as follows
Re Ψ(iu2, v1)
= (β2−β1) + Re
α(1−β2)v1{(−q1+µ+ 1) +iq2} (−q+µ+ 1)2+q22
≤ 2(β−2−β1)| −q+µ+ 1|2−α(1−β2)(−q1+µ+ 1)(1 +u22) 2| −q+µ+ 1|2
= A+Bu22
2C , C =| −q+µ+ 1|2 >0
≤0, if A≤0 and B ≤0,
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where
A= 2(β2−β1)| −q+µ+ 1|2−α(1−β2)(−q1+µ+ 1), B =−α(1−β2)(−q1+µ+ 1)≤0.
FromA≤0,we get
(2.9) β2 = 2β1| −q+µ+ 1|2+αRe(−q(z) +µ+ 1) 2| −q+µ+ 1|2+αRe(−q(z) +µ+ 1) .
Hence, using Lemma 1.1, it follows that p(z), defined by (2.6), belongs to Pk(β2) and thus f ∈ Bkα(µ, β2, η2, a, c), z ∈ D. This completes the proof of the first part. The second part of this result can be obtained by using similar arguments and the relation (1.6).
Theorem 2.4.
Bkα(µ, β, η, a, c)⊂ Bk0(µ, γ, η, a, c) (i)
Bαk1(µ, β, η, a, c)⊂ Bkα2(µ, β, η, a, c), for 0≤α2 < α1. (ii)
Proof. (i). Let
h(z) = (Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0, h(z)is analytic inEandh(0) = 1.Then
(2.10) (1−α)
(Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0
+α
−(z(Iµ(a, c)f(z))0)0 (Iµ(a, c)g(z))0
=h(z) +α zh0(z)
−h0(z),
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where
h0(z) = −(z(Iµ(a, c)g(z))0)0
(Iµ(a, c)g(z))0 ∈P(η).
Since f ∈ Bkα(µ, β, η, a, c),it follows that
h(z) +α zh0(z)
−h0(z)
∈Pk(β), h0 ∈P(η), for z ∈E.
Let
h(z) = k
4 +1 2
h1(z)− k
4 − 1 2
h2(z).
Then (2.10) implies that
hi(z) +α zh0i(z)
−h0(z)
∈P(β), z ∈E, i= 1,2,
and from use of similar arguments, together with Lemma 1.1, it follows that hi ∈P(γ), i= 1,2,where
γ = 2β|h0|2+αReh0 2|h0|2+αReh0 .
Therefore h ∈ Pk(γ), and f ∈ Bk0(µ, γ, η, a, c), z ∈ D. In particular, it can be shown that hi ∈ P(β), i = 1,2. Consequently h ∈ Pk(β) and f ∈ Bk0(µ, β, η, a, c)inD.
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Forα2 = 0,we have (i). Therefore, we letα2 >0and f ∈ Bkα1(µ, β, η, a, c).
There exist two functionsH1, H2 ∈Pk(β)such that H1(z) = (1−α1)
(Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0
+α1
−(z(Iµ(a, c)f(z))0)0 (Iµ(a, c)g(z))0
H2(z) = (Iµ(a, c)f(z))0
(Iµ(a, c)g(z))0, g ∈M V2(µ, η, a, c).
Now
(2.11) (1−α2)
(Iµ(a, c)f(z))0 (Iµ(a, c)g(z))0
+α2
−(z(Iµ(a, c)f(z))0)0 (Iµ(a, c)g(z))0
= α2 α1
H1(z) +
1− α2 α1
H2(z).
Since the class Pk(β)is a convex set [10], it follows that the right hand side of (2.11) belongs toPk(β)and this shows that f ∈ Bkα2(µ, β, η, a, c)forz ∈ D.
This completes the proof.
Letf ∈ M, b > 0and let the integral operatorFbbe defined by (2.12) Fb(f) = Fb(f)(z) = b
zb+1 Z z
0
tbf(t)dt.
From (2.12), we note that
(2.13) z(Iµ(a, c)Fb(f)(z))0 =bIµ(a, c)f(z)−(b+ 1)Iµ(a, c)Fb(f)(z).
Using (2.12), (2.13) with similar techniques used earlier, we can prove the fol- lowing:
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Theorem 2.5. Letf ∈M Rk(µ, β, a, c),orM Vk(µ, β, a, c),orBkα(µ, β, η, a, c), forz ∈D.ThenFb(f)defined by (2.12) is also in the same class forz ∈D.
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On Certain Classes of Meromorphic Functions Involving Integral Operators
Khalida Inayat Noor
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