http://jipam.vu.edu.au/
Volume 7, Issue 2, Article 50, 2006
COEFFICIENT INEQUALITY FOR A FUNCTION WHOSE DERIVATIVE HAS A POSITIVE REAL PART
AINI JANTENG, SUZEINI ABDUL HALIM, AND MASLINA DARUS INSTITUTE OFMATHEMATICALSCIENCES
UNIVERSITIMALAYA, 50603 KUALALUMPUR, MALAYSIA
aini_jg@ums.edu.my
INSTITUTE OFMATHEMATICALSCIENCES
UNIVERSITIMALAYA
50603 KUALALUMPUR, MALAYSIA
suzeini@um.edu.my SCHOOL OFMATHEMATICALSCIENCES
FACULTY OFSCIENCES ANDTECHNOLOGY
UNIVERSITIKEBANGSAANMALAYSIA
43600 BANGI, SELANGOR, MALAYSIA
maslina@pkrisc.cc.ukm.my
Received 07 March, 2005; accepted 09 March, 2006 Communicated by A. Sofo
ABSTRACT. LetRdenote the subclass of normalised analytic univalent functionsf defined by f(z) =z+P∞
n=2anznand satisfy
Re{f0(z)}>0
wherez ∈ D = {z : |z| < 1}. The object of the present paper is to introduce the functional
|a2a4−a23|. Forf ∈ R, we give sharp upper bound for|a2a4−a23|.
Key words and phrases: Fekete-Szegö functional, Hankel determinant, Convex and starlike functions, Positive real functions.
2000 Mathematics Subject Classification. Primary 30C45.
1. INTRODUCTION
LetAdenote the class of normalised analytic functionsf of the form
(1.1) f(z) =
∞
X
n=0
anzn,
ISSN (electronic): 1443-5756
c 2006 Victoria University. All rights reserved.
068-05
where z ∈ D = {z : |z| < 1}. In [9], Noonan and Thomas stated that the qth Hankel determinant off is defined forq≥1by
Hq(n) =
an an+1 · · · an+q+1 an+1 an+2 · · · an+q+2
... ... ... ... an+q−1 an+q · · · an+2q−2
.
Now, letSdenote the subclass ofAconsisting of functionsf of the form
(1.2) f(z) = z+
∞
X
n=2
anzn which are univalent inD.
A classical theorem of Fekete and Szegö [1] considered the Hankel determinant off ∈ Sfor q= 2andn= 1,
H2(1) =
a1 a2 a2 a3
.
They made an early study for the estimates of|a3 −µa22| whena1 = 1and µreal. The well- known result due to them states that iff ∈ S, then
|a3−µa22| ≤
4µ−3, if µ≥1, 1 + 2 exp
−2µ 1−µ
, if 0≤µ≤1, 3−4µ, if µ≤0.
Hummel [3, 4] proved the conjecture of V. Singh that |a3−a22| ≤ 13 for the classC of convex functions. Keogh and Merkes [5] obtained sharp estimates for|a3−µa22|whenf is close-to- convex, starlike and convex inD.
Here, we consider the Hankel determinant off ∈ S forq= 2andn= 2, H2(2) =
a2 a3 a3 a4
.
Now, we are working on the functional|a2a4 −a23|. In this earlier work, we find a sharp upper bound for the functional|a2a4−a23|forf ∈ R. The subclassRis defined as the following.
Definition 1.1. Letf be given by (1.2). Thenf ∈ Rif it satisfies the inequality
(1.3) Re{f0(z)}>0, (z ∈ D).
The subclassRwas studied systematically by MacGregor [8] who indeed referred to numer- ous earlier investigations involving functions whose derivative has a positive real part.
We first state some preliminary lemmas which shall be used in our proof.
2. PRELIMINARYRESULTS
LetP be the family of all functionspanalytic inDfor whichRe{p(z)}>0and (2.1) p(z) = 1 +c1z+c2z2 +· · ·
forz ∈ D.
Lemma 2.1 ([10]). Ifp∈ P then|ck| ≤2for eachk.
Lemma 2.2 ([2]). The power series forp(z)given in (2.1) converges inDto a function inP if and only if the Toeplitz determinants
(2.2) Dn =
2 c1 c2 · · · cn
c−1 2 c1 · · · cn−1 ... ... ... ... ... c−n c−n+1 c−n+2 · · · 2
, n= 1,2,3, . . .
andc−k= ¯ck, are all nonnegative. They are strictly positive except forp(z) = Pm
k=1ρkp0(eitkz), ρk > 0, tk real andtk 6= tj for k 6= j; in this caseDn > 0 forn < m−1andDn = 0for n≥m.
This necessary and sufficient condition is due to Carathéodory and Toeplitz and can be found in [2].
3. MAINRESULT
Theorem 3.1. Letf ∈R. Then
|a2a4−a23| ≤ 4 9. The result obtained is sharp.
Proof. We refer to the method by Libera and Zlotkiewicz [6, 7]. Sincef ∈ R, it follows from (1.3) that
(3.1) f0(z) = p(z)
for somez ∈ D. Equating coefficients in (3.1) yields
(3.2)
2a2 =c1
3a3 =c2 4a4 =c3
.
From (3.2), it can be easily established that
|a2a4−a23|=
c1c3 8 − c22
9 . We make use of Lemma 2.2 to obtain the proper bound on
c1c3
8 − c922
. We may assume without restriction thatc1 >0. We begin by rewriting (2.2) for the casesn= 2andn= 3.
D2 =
2 c1 c2 c1 2 c1
¯
c2 c1 2
= 8 + 2 Re{c21c2} −2|c2|2−4c21 ≥0,
which is equivalent to
(3.3) 2c2 =c21+x(4−c21)
for somex,|x| ≤1. ThenD3 ≥0is equivalent to
|(4c3−4c1c2+c31)(4−c21) +c1(2c2−c21)2| ≤2 4−c212
−2
2c2−c21
2; and this, with (3.3), provides the relation
(3.4) 4c3 =c31+ 2(4−c21)c1x−c1(4−c21)x2+ 2(4−c21)(1− |x|2)z, for some value ofz, |z| ≤1.
Suppose, now, thatc1 =candc∈[0,2]. Using (3.3) along with (3.4) we get
c1c3
8 −c22 9
=
c4
288 + c2(4−c2)x
144 − (4−c2)(32 +c2)x2
288 + c(4−c2)(1− |x|2)z 16
and an application of the triangle inequality shows that
c1c3 8 −c22
9 (3.5)
≤ c4
288 +c(4−c2)
16 + c2(4−c2)ρ
144 + (c−2)(c−16)(4−c2)ρ2 288
=F(ρ)
with ρ = |x| ≤ 1. We assume that the upper bound for (3.5) attains at the interior point of ρ∈[0,1]andc∈[0,2], then
F0(ρ) = c2(4−c2)
144 +(c−2)(c−16)(4−c2)ρ
144 .
We note that F0(ρ) > 0 and consequently F is increasing and M axρ F(ρ) = F(1), which contadicts our assumption of having the maximum value at the interior point ofρ∈[0,1]. Now let
G(c) =F(1) = c4
288 + c(4−c2)
16 +c2(4−c2)
144 +(c−2)(c−16)(4−c2)
288 ,
then
G0(c) = −c(5 +c2) 36 = 0 impliesc= 0which is a contradiction. Observe that
G00(c) = −5−3c2 36 <0.
Thus any maximum points ofGmust be on the boundary ofc∈[0,2]. However,G(c)≥G(2) and thusGhas maximum value atc = 0. The upper bound for (3.5) corresponds toρ = 1and c= 0, in which case
c1c3 8 − c22
9
≤ 4 9. Equality is attained for functions inRgiven by
f0(z) = 1 +z2 1−z2.
This concludes the proof of our theorem.
REFERENCES
[1] M. FEKETE AND G. SZEGÖ, Eine Bemerkung uber ungerade schlichte funktionen, J. London Math. Soc., 8 (1933), 85–89.
[2] U. GRENANDERANDG. SZEGÖ, Toeplitz Forms and their Application, Univ. of California Press, Berkeley and Los Angeles, (1958)
[3] J. HUMMEL, The coefficient regions of starlike functions, Pacific J. Math., 7 (1957), 1381–1389.
[4] J. HUMMEL, Extremal problems in the class of starlike functions, Proc. Amer. Math. Soc., 11 (1960), 741–749.
[5] F.R. KEOGHANDE.P. MERKES, A coefficient inequality for certain classes of analytic functions, Proc. Amer. Math. Soc., 20 (1969), 8–12.
[6] R.J. LIBERAANDE.J. ZLOTKIEWICZ, Early coefficients of the inverse of a regular convex func- tion, Proc. Amer. Math. Soc., 85(2) (1982), 225–230.
[7] R.J. LIBERA AND E.J. ZLOTKIEWICZ, Coefficient bounds for the inverse of a function with derivative inP, Proc. Amer. Math. Soc., 87(2) (1983), 251–289.
[8] T.H. MACGREGOR, Functions whose derivative has a positive real part, Trans. Amer. Math. Soc., 104 (1962), 532–537.
[9] J.W. NOONANANDD.K. THOMAS, On the second Hankel determinant of areally meanp-valent functions, Trans. Amer. Math. Soc., 223(2) (1976), 337–346.
[10] CH. POMMERENKE, Univalent Functions, Vandenhoeck and Ruprecht, Göttingen, (1975)