Vol. 21, No. 2 (2015), pp. 77–82.
A CERTAIN SUBCLASS OF MULTIVALENT ANALYTIC
FUNCTIONS WITH NEGATIVE COEFFICIENTS FOR
OPERATOR ON HILBERT SPACE
Abbas Kareem Wanas
Department of Mathematics
College of Computer Science and Mathematics University of Al-Qadisiya Diwaniya - Iraq
Abstract.By making use of the operators on Hilbert space, we introduce and study
a subclassAkp(α, β, δ, T) of multivalent analytic functions with negative coefficients.
Also we obtain some geometric properties.
Key words: Hilbert space, analytic function, convex set, extreme points.
Abstrak. Dengan menggunakan operator-operator di ruang Hilbert, kami memperkenalkan dan mempelajari suatu subclass dari fungsi analitik multivalen Akp(α, β, δ, T) dengan koefisien negarif. Kami juga mendapatkan beberapa sifat
geometri mereka.
Kata kunci: Ruang Hilbert, fungsi analitik, himpunan konveks, titik-titik ekstrim.
1. Introduction
LetAp be the class of functionsf of the form:
f(z) =zp+ ∞ X
n=1
an+pz
n+p(p∈N={1,2, ...}), (1)
which are analytic and p-valent in the open unit diskU ={z ∈C:|z|<1}. Let
kp denote the subclass ofAp consisting of functions of the form:
f(z) =zp− ∞ X
n=1
an+pz n+p (a
n+p≥0, p∈N={1,2, ...}), (2)
2000 Mathematics Subject Classification: 30C45, 30C50. Received: 06-06-2015, revised: 06-04-2015, accepted: 14-04-2015
Definition 1.1. A function f ∈ kp is said to be in the class Akp(α, β, δ) if it
satisfies
f′
(z)−pzp−1 α(f′(
z)−β) +p−β
<0,
where0≤α <1,0≤β < p,0< δ≤1 danz∈U.
LetH be a Hilbert space on the complex field. Let T be a linear operator onH. For a complex analytic functionf on the unit diskU, we denotedf(T), the operator onH defined by the usual Riesz-Dunford integral [2]
f(T) = 1 2πi
Z
c
f(z)(zI−T)−1dz,
where I is the identity operator on H, c is a positively oriented simple closed rectifiable contour lying inU and containing the spectrumσ(T) ofT in its interior domain [3]. Also f(T) can be defined by the series
f(T) = ∞ X
n=0
f(n)(0)
n! T n,
which converges in the norm topology [4].
Definition 1.2. Let H be a Hilbert space and T be an operator on H such that
T 6=∅ and kTk <1. Let α, β be real numbers such that 0 ≤ α <1, 0 ≤β < p,
0 < δ ≤1. An analytic functionf on the unit disk is said to belong to the class Akp(α, β, δ, T)if it satisfy the inequality
kf′
(T)−pTp−1k
< δkα(f′
(T)−β) +p−βk,
where∅ denote the zero operator onH.
The operator on Hilbert space were consider recently be Xiaopei [8], Joshi [6], Chrakim et al. [1], Ghanim and Darus [5], and Selvaraj et al. [7].
2. Main Results
Theorem 2.1. Letf ∈kpbe defined by (2). Thenf ∈ Akp(α, β, δ, T)for allT 6=∅
if and only if
∞ X
n=1
(n+p)(1 +δα)an+p≤δ(p−β)(1 +α). (3)
where0≤α <1,0≤β < p,0< δ≤1. The result is sharp for the function f given by
f(z) =zp−δ(p−β)(1 +α) (n+p)(1 +δα)z
Proof. Suppose that the inequality (3) holds. Then, we have
Upon clearing denominator in (5) and letting r→1, we obtain
∞
which completes the proof.
and
pkTkp−1−δ(p−β)(1 +α)
1 +δα kTk
p≤ kf′
(T)k ≤pkTkp−1+δ(p−β)(1 +α)
1 +δα kTk
p
The result is sharp for the function f given by
f(z) =zp−δ(p−β)(1 +α) (p+ 1)(1 +δα)z
p+1.
Proof. According to the Theorem 2.1, we get
∞ X
n=1
an+p≤
δ(p−β)(1 +α) (p+ 1)(1 +δα).
Hence
kf(T)k ≥ kTkp− ∞ X
n=1
an+pkTk n+p
≥ kTkp− kTkp+1 ∞ X
n=1
an+p
≥ kTkp−δ(p−β)(1 +α) (p+ 1)(1 +δα)kTk
p+1.
Also,
kf(T)k ≤ kTkp+ ∞ X
n=1
an+pkTk n+p
≤ kTkp+δ(p−β)(1 +α) (p+ 1)(1 +δα)kTk
p+1
In view of Theorem 2.1, we have
∞ X
n=1
(n+p)an+p≤
δ(p−β)(1 +α) 1 +δα .
Thus
kf′
(T)k ≥pkTkp−1−
∞ X
n=1
(n+p)an+pkTk n+p−1
≥pkTkp−1− kTkp ∞ X
n=1
(n+p)an+p
≥pkTkp−1−δ(p−β)(1 +α)
1 +δα kTk
and
kf′
(T)k ≤pkTkp−1+kTkp ∞ X
n=1
(n+p)an+p
≤pkTkp−1+δ(p−β)(1 +α)
1 +δα kTk
p
Therefore the proof is complete.
Theorem 2.4. Let f0(z) =zp and
fn(z) =zp−δ(p−β)(1 +α) (n+p)(1 +δα)z
n+p, n≥1.
Thenf ∈ Akp(α, β, δ, T)if and only if it can be expressed in the form
f(z) = ∞ X
n=0
λnfn(z), (6)
whereλn≥0 and P∞
n=0λn = 1.
Proof. Assume thatf can be expressed by (6). Then, we have
f(z) = ∞ X
n=0
λnfn(z) =z p−
∞ X
n=0
δ(p−β)(1 +α) (n+p)(1 +δα)λnz
n+p. (7)
Thus
∞ X
n=0
(n+p)(1 +δα)
δ(p−β)(1 +α)
δ(p−β)(1 +α) (n+p)(1 +δα)λn=
∞ X
n=0
λn= 1−λ0≤1, (8)
and so f ∈ Akp(α, β, δ, T).
Conversely, suppose that f given by (2) in in the class Akp(α, β, δ, T). Then by Corollary 2.2, we have
an+p≤
δ(p−β)(1 +α) (n+p)(1 +δα). Setting
λn =
(n+p)(1 +δα)
δ(p−β)(1 +α)an, n≥1,
andλ0= 1−P∞
n=1λn. Then
f(z) = ∞ X
n=0
λnfn(z),
This completes the proof of the theorem.
Proof. Letf1 andf2 be the arbitrary elements ofAkp(α, β, δ, T). Then for every
t(0≤t≤1), we show that (1−t)f1+tf2∈ Akp(α, β, δ, T). Thus, we have
(1−t)f1+tf2=zp− ∞ X
n=1
((1−t)an+p+tbn+p)z n+p
.
Hence,
∞ X
n=1
(n+p)(1 +δα) ((1−t)an+p+tbn+p)
= (1−t) ∞ X
n=1
(n+p)(1 +δα)an+p+t ∞ X
n=1
(n+p)(1 +δα)bn+p
≤(1−t)δ(p−β)(1 +α) +tδ(p−β)(1 +α).
This completes the proof.
References
[1] Chrakim, Y., Lee, J.S., and Lee, S.H., ”A certain subclass of analytic functions with negative coefficients for operators on Hilbert space”,Math. Japonica,47(1) (1998), 155-124. [2] Dunford, N., and Schwarz, J.T., Linear Operator, Part I, General Theory, New York
-London, Inter Science, 1958.
[3] Fan, K., ”Analytic functions of a proper contraction”,Math. Z.,160(1978), 275-290. [4] Fan, K., ”Julia’s lemma for operators”,Math. Ann.,239(1979), 241-245.
[5] Ghanim, F., and Darus, M., ”On new subclass of analytic p-valent function with negative coefficients for operators on Hilbert space”,Int. Math. Forum,3:2(2008), 69-77.
[6] Joshi, S.B., ”On a class of analytic functions with negative coefficients for operators on Hilbert Space”,J. Appr. Theory and Appl., (1998), 107-112.
[7] Selvaraj, C., Pamela, A.J., and Thirucheran, M., ”On a subclass of multivalent analytic func-tions with negative coefficients for contraction operators on Hilbert space”,Int. J. Contemp. Math. Sci.,4:9(2009), 447-456.