# BIII/. Malaysinn Math. Sc. Soc. (Second Series) 26 (2003) Coefficients of the Inverse of Strongly Starlike Functions. ROSlHAN M.

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1 BULLETIN of the MALAYSIAN MATHEMATICAL SCIENCES SOCIETY BIII/. Malaysinn Math. Sc. Soc. (Second Series) 26 (2003) Coefficients of the Inverse of Strongly Starlike Functions ROSlHAN M. ALI School of Mathematical Sciences, Universiti Sains Malaysia, USM Pulau Pinang, Malaysia Dedicated to the memory ofprofessor Mohamad Rashidi Md. Razali Abstract. For the class of strongly starlike functions, sharp bounds on the first four coefficients of the inverse functions are determined. A sharp estimate for the Fekete-Szegll coefficient functional is also obtained. These results were deduced from non-linear coefficient estimates of functions with positive real part. 1. Introduction An analytic function f in the open unit disk U = {z :1 z 1< I} is said to be strongly starlike of order a, 0 < a ~ 1, if f is normalized by f(o) = 0 = 1'(0) - I and satisfies I ~g zl'(z) I< tra f(z) 2 (z E U). The set of all such functions is denoted by SS (a). This class has been studied by several authors [1,2,5,7,9,10]. In [5] it was shown that an univalent function f belongs to SS (a) if and only if for every W E f(u) a certain lens-shaped region with vertices at the origin 0 and W is contained in f(u). If (1) is in the class SS (a), then the inverse of f admits an expansion (2)

2 64 R.M. Ali near w = 0. It is our purpose here to determine sharp bounds for the first four coefficients of IYn I, and to obtain a sharp estimate for the Fekete-Szego coefficient functional IY3 - ty Preliminary results Let us denote by P the class of normalized analytic functions p in the unit disk U with positive real part so that p(o) = 1and Re p(z) > 0, Z E U. It is clear that f E SS (a) ifand only if there exists a function pep so that zj'(z) / fez) = pa (z). By equating coefficients, each coefficient of fez) = z + a 2 z 2 + a 3 z can be expressed in terms of coefficients of a function p(z) =1 + c1z + C2Z 2 + C3z in the class P. For example, (3) Using representations (1) and (2) together with f(f-l (w» = w or we obtain the relationships Y2 = -a 2 2 Y3 = -a3 + 2a2 (4) 3 Y4 = -a4 + 5a2a3-5a2 Thus coefficient estimates for the class SS (a) and its inverses become non-linear coefficient problems for the class P. Our principal tool is given in the following lemma.

3 Coefficients of the Inverse of Strongly Starlike Functions 65 Lemma 1 [31. Afunction p(z) = 1 + I;~\ Ckzk belongs to P ifand only if for every sequence {zd of complex numbers which satisfy lim k _ Ho supizk 11/ k < 1. Lemma 2. If p(z) = 1 + I;~I Ckzk E P, then,05,j.j.5,2 elsewhere If j.j. < 0 or j.j. > 2, equality holds if and only if p(z) = (1 + a)/(1- a), I& I= 1. if 0 < j.j. < 2, then equality holds if and only if p(z) = (1 + a Z )/(1- a Z ), I& I= 1. For j.j. = 0, equality holds ifand only if l+a I-a p(z):= pz(z) =,1,--+(1-,1,)--,05, A. 5, 1,1&1 = 1. I-a l+a For j.j. = 2, equality holds ifand onlyif p is the reciprocalof P2' Remark. Ma and Minda [6] had earlier proved the above result. proof. We give a different Proof Choose the sequence {zk} ofcomplex numbers in Lemma 1 to be Zo = - f.j.c\ /2, z\ = 1, and zk = 0 if k > 1. This yields that is, (5)

4 66 R.M. Ali If f.j < 0 or f.j > 2, the expression on the right of inequality (5) is bounded above by 4(f.J - 1)2. rotations. Equality holds if and only if Ic I 1= 2, i.e., p(z) =(1 + z)/(l - z) or its If 0 < f.j < 2, then the right expression of inequality (5) is bounded above by 4. In this case, equality holds if and only if Ic. I= 0 and Ic 2 1= 2, i.e., p(z) = (1 + Z2)/(1- Z2) or its rotations. Equality holds when f.j = 0 if and only if Ic 2 1 = 2, i.e., [8, p. 41] 1+&z I-a p(z):= P2(z) =A.--+(I-A.)--, 0 ~ A. ~ 1, lel= 1. I-a 1+&z Finally, when f.j = 2, then 1c 2 - c I 2 1 = 2 ifand only if p is the reciprocal of P2' Another interesting application of Lemma 1 occurs by choosing the sequence {zt> to be Zo = &1 2 - f3c2' Zt = -}'Ct, Z2 = 1, and Z/c = 0 if k > 2. In this case, we find that IC 3 - (13 + Y)CI C2 + & where v'= 8(13-1) + 13(8 - y). 13(13-1). ~ 4 + 4y(y -1) Ic I (28 - y)c/ - (213-1)c 2 1 -I c 2 - }'C/ 1 2 = 4 + 4y<r - 1) I CI (13-1)IC 2 - ~ CI 21 _ (8 - f3y)2 I C 1 4 (6) 13(13-1) I Lemma 3. Let p(z) = 1+I:=I Ck zk E P. If 0 ~ 13 ~ 1 and 13(213-1) ::; 8 ~ 13, then 2 Proof If 13 = 0, then 8 = 0 and the result follows since Ic 3 1 ~ 2. If 13 = 1, then 8 =1 and the inequality follows from a result 0[[4]. (6) that We may assume that 0 < 13 < 1 so that 13(13-1) < O. With y = 13, we fmd from

5 Coefficients of the Inverse of Strongly Starlike Functions I C3-2fJC1C2+ &)31 2 :5: 4 + 4fJ(fJ -1) Ic l l 2 + 4fJ(fJ -1)IC2- fc) _ (15 - fj2)2 I C 1 4 fj(fj - l 1) :5: 4 + bx + CX 2 := hex) with X = I C E [0,4], b = 4fJ(fJ - 1), and C = - (15 - fj2)2 / fj(fj - 1). Since C ~ 0, it follows that hex) :5: h(o) provided h(o) - h(4) ~ 0, i.e., b + 4c :5: 0. This condition is equivalent to /321 :5: /3(1 - /3), which completes the proof. With 15 = fj in Lemma 3, we obtain an extension of Libera and Zlotkiewicz [4] result that IC 3-2c\c2 +c 1 3 1:5: 2. Corollary 1. If p(z) = 1 + 1:;=1 Ckzk EP, and 0 :5: fj :5: 1, then When /3 = 0, equality holds ifand only if 3 1+ e-2trik/3 z p(z) := P3(Z) = 1: Ak -2trik/3' (1&1 = 1) k=1 1 - &e z Ak ~ 0, with A) + A2 + A3 = 1. If fj = 1, equality holds if and only if P is the reciprocal of P3' If < fj < 1, equality holds if and only if p(z) = (1 + &z)/l - &z), I&1=1, or p(z) = (1 + &Z3)!(1- &Z3), 1&1 = 1. Proof We only need to find the extremal functions. If /3 = 0, then equality holds if and only if 1c 3 1 = 2, i.e., p is the function P3 [8, p. 41]. If /3 = 1, then equality holds ifand only if P is the reciprocal of P3' When 0 < fj < 1, we deduce from (6) that IC) -2f3c\c2 +f3c :5: 4+4fJ(fJ-1)lc) fJ(fJ- 1)IC2 _~CI212 - fj(fj - 1) I C :5: 4 + 4fJ(fJ - 1) 1c) fj(fj - 1) I C\ 1 4 :5: 4.

6 68 RM.Ali Equality occurs in the last inequality if and only if either IC 1 1= or IC l 1= 2. If Icll= 0, then Iczl= 0, i.e., p(z) = (1 + EZ 3 )/(l- EZ 3 ), lei = 1. p(z) = (1 + EZ)/(l- EZ), IeI= 1. "CO Lemma 4. If p(z) = I + L.k=1 ckzk E P, then If Icll= 2, then Ic3 - (JJ + l)clcz + JlC1 3 \ ~ max {2, 21 2JJ -II} = { ~12JJ -II : ~ls~:he~: Proof For 0 ~ JJ ~ I, the inequality follows from Lemma 3 with 8 = JJ, and 2P = JJ + 1. For the second estimate, choose p = JJ, r = I, and 8 = JJ in (6). Since JJ(JJ - 1) > 0, we conclude from (5) and (6) that 3. Coefficient bounds Theoreml. Let f E SS (a) and f-l(w) = w+rzwz +r3w Then Irzl ~ 2a, with equality ifand only if zf'(z) = (I+EZ)a, lei = 1. fez) 1- EZ (7) Further For a > 1/5, extremal functions are given by (7). and only if zf'(z) _ (I + EZ 2 )a Ie I= 1, fez) - 1- EZ z ' If < a < 1/5, equality holds if (8)

7 Coefficients of the Inverse of Strongly Starlike Functions 69 while if a = 1/ S, equality holds ifand only if Moreover, zf'(z) = P2(Z)-a = (2 1 + a +(1-2) I_a)-a, = 1,0::; 2::; 1. fez) I-a l+a { 2a "3 ' Ir 41::; 2a ( ) - 62a 2 + 1, 9 For a ~ 1/.[31, extremal functions are given by (7), while for 0 < a ::; 1/.[31, equality holds ifand only if zf'(z) _ (1 + a 3 )a = 1. fez) a 3 ' Proof The following relations are obtained from (3) and (4): (9) The bound on Ir21 follows immediately from the well-known inequality Ic\ I ::; 2. Lemma 2 with Jl = 1 + Sa yields the bound on Ir 31 and the description of the extremal functions. For the fourth coefficient, we shall apply Lemma 3 with 2fJ = 1 + Sa and o = (31a 2 + lsa + 2)/6. The conditions on fj and 0 are satisfied if a ::; 1/.[31. Thus Ir 41 ::; 2a/3, with equality ifand only if zf'(z) / fez) = [(1 + a 3 )/(1 _a 3 )] a. For 1/.[31 < a ::; 1/ S, Corollary 1 yields I I I 1 + Sa 31 31a ( ) E::; c3-(1+sa)clc2+-2-ci + 6 Icll ::;"362a2+1.

8 70 R.M. Ali f.j It remains to determine the estimate for I / 5 < a :\$; 1. Appealing to Lemma 4 with = 5a, and because 31a 2-15a + 2 > 0 in (0, I], we conclude that 3\ 3la2- lsa + 2 I 13 I E I :\$; C I 3 - (I + Sa)c,c2 + Sac, + 6 c,:\$; 2(lOa - 1) + ~(3Ia2-15a + 2) = ~(62a2 + I). 3 3 I (5 _ 4t)a2, t:\$; S -l/a 4 21 S-l/a S+l/a y 3 - ty2 :\$; a, 4 :\$; t :\$; 4 ( 4t _ 5)a 2 t > S + l/a, - 4 If 5-~1 a < t < 5+~1 a, equality holds ifand only if f is given by (8). If t < 5-~1 a or t > 5+~1 a, equality holds ifand only if f is given by (7). If t = 5+~1 a, equality holds if and only if j~~i zj'(z) _ ()-a f(z) - P2 Z = P2 (z)a, while if t = 5-~1 a, then equality holds if and only if Proof From (9), we obtain 2 a [ 1+ (S - 4t)a 2] Y3 -ty2 =-- C 2 - C,. 2 2 The result now follows from Lemma 2. Remark. An equivalent result for the Fekete-Szego coefficient functional over the class SS * (a) was also given by Ma and Minda [S]. Acknowledgement. This research was supported by a Universiti Sains Malaysia Fundamental Research Grant. The author is greatly indebted to Professor R.W. Barnard for his helpful comments in the preparation ofthis paper.

9 Coefficients of the Inverse of Strongly Starlike Functions 71 References 1. D.A. Brannan, J. Clunie and W.E. Kirwan, Coefficient estimates for a class of starlike functions, Canad. 1. Math. 22 (1970), D.A. Brannan and W.E. Kirwan, On some classes of bounded univalent functions, 1. London Math. Soc. 1 (1969), C.R. Leverenz, Hermitian forms in function theory, Trans. Amer. Math. Soc. 286 (1984), R.J. Libera and E.J. Zlotkiewicz, Early coefficients of the inverse ofa regular convex function, Proc. Amer. Math. Soc. 85 (1982), W. Ma and D. Minda, An internal geometric characterization of strongly starlike functions, Ann. Univ. Mariae Curie-Sklodowska Sect. A 45 (1991), W. Ma and D. Minda, A unified treatment of some special classes of univalent functions, Proc. Can! on Complex Analysis. Tianjin (1992), M. Nunokawa and S. Owa, On certain conditions for starlikeness, Southeast Asian Bull. Math. 25 (2001), Ch. Pommerenke, Univalentfunctions. Vandenhoeck and Ruprecht, Gottingen J. Stankiewicz, Some remarks concerning starlike functions, Bull. Acad. Polan. Sci. Ser. Sci. Math. Astronom. Phys. 18 (1970), J. Stankiewicz, On a family ofstarlike functions, Ann. Univ. Mariae Curie-Sklodowska Sect. A ( ),

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