Journal of Inequalities in Pure and Applied Mathematics FEKETE-SZEGÖ FUNCTIONAL FOR SOME SUBCLASS OF NON-BAZILEVIČ FUNCTIONS volume 7, issue 3, article 117, 2006. T.N. SHANMUGAM, M.P. JEYARAMAN AND S. SIVASUBRAMANIAN Department of Mathematics Anna University Chennai 600025 Tamilnadu, India EMail: shan@annauniv.edu Department of Mathematics Easwari Engineering College Chennai-600 089 Tamilnadu, India EMail: jeyaraman mp@yahoo.co.in EMail: sivasaisastha@rediffmail.com Received 18 November, 2005; accepted 24 March, 2006. Communicated by: A. Lupaş Abstract Contents JJ J II I Home Page Go Back Close c 2000 Victoria University ISSN (electronic): 1443-5756 108-06 Quit Abstract In this present investigation, the authors obtain a sharp Fekete-Szegö’s inequality for certain normalized analytic functions f(z) defined on the open unit disk α 1+α z z for which (1 + β) f(z) − βf 0 (z) f(z) , (β ∈ C, 0 < α < 1) lies in a region starlike with respect to 1 and is symmetric with respect to the real axis. Also, certain applications of our results for a class of functions defined by convolution are given. As a special case of this result, Fekete-Szegö’s inequality for a class of functions defined through fractional derivatives is also obtained. 2000 Mathematics Subject Classification: Primary 30C45. Key words: Analytic functions, Starlike functions, Subordination, Coefficient problem, Fekete-Szegö inequality. Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Fekete-Szegö Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Applications to Functions Defined by Fractional Derivatives 12 References Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page Contents JJ J II I Go Back Close Quit Page 2 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au 1. Introduction Let A denote the class of all analytic functions f (z) of the form (1.1) f (z) = z + ∞ X ak z k (z ∈ ∆ := {z ∈ C/ |z| < 1}) k=2 and S be the subclass of A consisting of univalent functions. Let φ(z) be an analytic function with positive real part on ∆ with φ(0) = 1, φ0 (0) > 0 which maps the unit disk ∆ onto a region starlike with respect to 1 and is symmetric with respect to the real axis. Let S ∗ (φ) be the class of functions f ∈ S for which zf 0 (z) ≺ φ(z), (z ∈ ∆) f (z) and C(φ) be the class of functions f ∈ S for which 00 1+ zf (z) ≺ φ(z), f 0 (z) (z ∈ ∆), where ≺ denotes the subordination between analytic functions. These classes were introduced and studied by Ma and Minda [3]. They have obtained the Fekete-Szegö inequality for the functions in the class C(φ). Since f ∈ C(φ) if and only if zf 0 ∈ S ∗ (φ), we get the Fekete-Szegö inequality for functions in the class S ∗ (φ). Recently, Shanmugam and Sivasubramanian [9] obtained FeketeSzegö inequalities for the class of functions f ∈ A such that zf 0 (z) + αz 2 f 00 (z) ≺ φ(z) (1 − α)f (z) + αzf 0 (z) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page Contents JJ J II I Go Back Close Quit Page 3 of 16 (0 ≤ α < 1). J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au Also, Ravichandran et al. [7] obtained the Fekete-Szegö inequality for the class of Bazilevič functions. For a brief history of the Fekete-Szegö problem for the class of starlike, convex and close-to-convex functions, see the recent paper by Srivastava et al. [11]. Obradovic [4] introduced a class of functions f ∈ A, such that, for 0 < α < 1, α z 0 < f (z) > 0, z ∈ ∆. f (z) He called this class of function as "Non-Bazilevič" type. Tuneski and Darus [14] obtained the Fekete-Szegö inequality for the non-Bazilevič class of functions. Using this non-Bazilevič class, Wang et al.[15] studied many subordination results for the class N (α, β, A, B) defined as N (α, β, A, B) ( := f ∈ A : (1 + β) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page z f (z) α − βf 0 (z) z f (z) 1+α ≺ 1 + Az 1 + Bz ) , where β ∈ C, −1 ≤ B ≤ 1, A 6= B, 0 < α < 1. In the present paper, we obtain the Fekete-Szegö inequality for functions in a more general class Nα,β (φ) of functions which we define below. Also we give applications of our results to certain functions defined through convolution (or λ Hadamard product) and in particular we consider a class Nα,β (φ) of functions defined by fractional derivatives. The aim of this paper is to give a generalization the Fekete-Szegö inequalities for some subclass of Non-Bazilevič functions . Contents JJ J II I Go Back Close Quit Page 4 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au Definition 1.1. Let φ(z) be an univalent starlike function with respect to 1 which maps the unit disk ∆ onto a region in the right half plane which is symmetric with respect to the real axis, φ(0) = 1 and φ0 (0) > 0. A function f ∈ A is in the class Nα,β (φ) if α 1+α z z 0 (1 + β) − βf (z) ≺ φ(z), (β ∈ C, 0 < α < 1). f (z) f (z) g For fixed g ∈ A, we define the class Nα,β (φ) to be the class of functions f ∈ A for which (f ∗ g) ∈ Nα,β (φ). Remark 1. Nα,−1 1+z is the class of Non-Bazilevič functions introduced by 1−z Obradovic [4]. 1+(1−2γ)z Remark 2. Nα,−1 , 0 ≤ γ < 1 is the class of Non-Bazilevič 1−z functions of order γ introduced and studied by Tuneski and Darus [14]. √ 2 1+ z 2 Remark 3. We call Nα,β 1 + π2 log 1−√z the class of "Non-Bazilevič parabolic starlike functions". To prove our main result, we need the following: Lemma 1.1 ([3]). If p1 (z) = 1 + c1 z + c2 z 2 + · · · is an analytic function with a positive real part in ∆, then −4v + 2 if v ≤ 0, 2 if 0 ≤ v ≤ 1, |c2 − vc21 | ≤ 4v − 2 if v ≥ 1. Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page Contents JJ J II I Go Back Close Quit Page 5 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au When v < 0 or v > 1, the equality holds if and only if p1 (z) is (1 + z)/(1 − z) or one of its rotations. If 0 < v < 1, then the equality holds if and only if p1 (z) is (1 + z 2 )/(1 − z 2 ) or one of its rotations. If v = 0, the equality holds if and only if 1+z 1 1 1−z 1 1 + λ + − λ (0 ≤ λ ≤ 1) p1 (z) = 2 2 1−z 2 2 1+z or one of its rotations. If v = 1, the equality holds if and only if p1 is the reciprocal of one of the functions such that equality holds in the case of v = 0. Also the above upper bound is sharp, and it can be improved as follows when 0 < v < 1: |c2 − vc21 | + v|c1 |2 ≤ 2 (0 < v ≤ 1/2) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian and |c2 − vc21 | + (1 − v)|c1 |2 ≤ 2 (1/2 < v ≤ 1). Title Page Contents JJ J II I Go Back Close Quit Page 6 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au 2. Fekete-Szegö Problem Our main result is the following: Theorem 2.1. Let φ(z) = 1 + B1 z + B2 z 2 + B3 z 3 + · · · . If f given by (1.1) belongs to Nα,β (φ), then µB12 (1+α) B2 2 − (α+2β) − 2(α+β)2 + 2(α+β)2 B1 if µ ≤ σ1 ; B1 2 − (α+2β) if σ1 ≤ µ ≤ σ2 ; |a3 − µa2 | ≤ µB12 B2 (1+α) 2 − 2(α+β) if µ ≥ σ2 , + 2 B1 (α+2β) 2(α+β)2 Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian where, (1 + α)(2β + α)B12 − 2(B2 − B1 )(β + α)2 , 2(2β + α)B12 (1 + α)(2β + α)B12 − 2(B2 + B1 )(β + α)2 σ2 := . 2(2β + α)B12 σ1 := The result is sharp. Proof. For f ∈ Nα,β (φ), let α 1+α z z 0 (2.1) p(z) := (1 + β) − βf (z) = 1 + b1 z + b2 z 2 + · · · . f (z) f (z) Title Page Contents JJ J II I Go Back Close Quit Page 7 of 16 From (2.1), we obtain −(α + β)a2 = b1 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au (2β + α) α+1 2 a2 − a3 2 = b2 . Since φ(z) is univalent and p ≺ φ, the function p1 (z) = 1 + φ−1 (p(z)) = 1 + c1 z + c2 z 2 + · · · 1 − φ−1 (p(z)) is analytic and has a positive real part in ∆. Also we have p1 (z) − 1 (2.2) p(z) = φ p1 (z) + 1 Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian and from this equation (2.2), we obtain 1 b1 = B1 c1 2 Title Page Contents and 1 1 b2 = B1 c2 − c21 2 2 1 + B2 c21 . 4 Therefore we have JJ J II I Go Back a3 − µa22 B1 c2 − vc21 =− 2(2β + α) Close Quit where 1 B2 (2β + α)(α + 1 − 2µ) v := 1− + B1 . 2 B1 2(β + α)2 Page 8 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au Our result now follows by an application of Lemma 1.1. To show that the φn bounds are sharp, we define the functions Kα,β (n = 2, 3, . . .) by (1 + β) !α z n − β K φα,β φn Kα,β (z) 0 !1+α z (z) = φ(z n−1 ), φn Kα,β (z) φn φn 0 Kα,β (0) = 0 = [Kα,β ] (0) − 1 λ and the function Fα,β and Gλα,β (0 < α < 1) by (1 + β) !α z λ Fα,β (z) λ 0 − β[Fα,β ] (z) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions !1+α z λ Fα,β (z) = φ(z n−1 ), T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian λ λ [Fα,β ](0) = 0 = [Fα,β ]]0 (0) − 1 Title Page and (1 + β) z Gλα,β !α − β[Gλα,β ]0 (z) z Contents !1+α Gλα,β (z) = φ(z n−1 ), [Gλα,β ](0) = 0 = [Gλα,β ]0 (0) − 1. JJ J II I Go Back φn λ Clearly, the functions Kα,β , [Fα,β ] and [Gλα,β ] ∈ Nα,β (φ) φ φ2 Also we write Kα,β := Kα,β . φ If µ < σ1 or µ > σ2 , then the equality holds if and only if f is Kα,β or one φ3 of its rotations. When σ1 < µ < σ2 , the equality holds if and only if f is Kα,β λ or one of its rotations. If µ = σ1 then the equality holds if and only if f is Fα,β Close Quit Page 9 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au or one of its rotations. If µ = σ2 then the equality holds if and only if f is Gλα,β or one of its rotations. √ 2 1+ z 2 √ Corollary 2.2. Let φ(z) = 1 + π2 log 1− z . If f given by (1.1) belongs to Nα,β (φ), then (1+α) 8 8µ 8 − π4 (α+β) if µ ≤ σ1 − 3π2 (α+2β) 2 + (α+β)2 π 4 4 − π2 (α+2β) if σ1 ≤ µ ≤ σ2 |a3 − µa22 | ≤ (1+α) 8 8µ 8 + π4 (α+β) if µ ≥ σ2 2 − (α+β)2 π 4 3π 2 (α+2β) where, Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian (1 + α)(2β + α) π164 − 2 3π8 2 − 2(2β + α) π164 4 π2 (1 + α)(2β + α) π164 − 2 3π8 2 + σ2 := 2(2β + α) π164 4 π2 σ1 := (β + α)2 Title Page (β + α)2 Contents . The result is sharp. JJ J II I Corollary 2.3. For β = −1, φ(z) = 1+(1−2γ)z , 0 ≤ γ < 1 in Theorem 2.1, 1−z we get the results obtained by Tuneski and Darus [14]. Go Back Remark 4. If σ1 ≤ µ ≤ σ2 , then, in view of Lemma 1.1, Theorem 2.1 can be improved. Let σ3 be given by Quit (1 + α)(2β + α)B12 − 2B2 (β + α)2 . σ3 := 2(2β + α)B12 Close Page 10 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au If σ1 ≤ µ ≤ σ3 , then |a3 − µa22 | (β + α)2 2 (α + 1 − 2µ)(2β + α) − |a2 |2 B1 − B2 + B1 (2β + α)B12 2(β + α)2 B1 ≤− . (2β + α) If σ3 ≤ µ ≤ σ2 , then (β + α)2 2 (α + 1 − 2µ)(2β + α) 2 |a3 − µa2 | − B1 + B2 − B1 |a2 |2 (2β + α)B12 2(β + α)2 B1 ≤− . (2β + α) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page Contents JJ J II I Go Back Close Quit Page 11 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au 3. Applications to Functions Defined by Fractional Derivatives λ In order to introduce the class Nα,β (φ), we need the following: Definition 3.1 (see [5, 6]; see also [12, 13]). Let f be analytic in a simply connected region of the z-plane containing the origin. The fractional derivative of f of order λ is defined by Z z d f (ζ) 1 λ Dz f (z) := dζ (0 ≤ λ < 1) Γ(1 − λ) dz 0 (z − ζ)λ where the multiplicity of (z − ζ)λ is removed by requiring that log(z − ζ) is real for z − ζ > 0. Using the above Definition 3.1 and its known extensions involving fractional derivatives and fractional integrals, Owa and Srivastava [5] introduced the operator Ωλ : A → A defined by (Ωλ f )(z) = Γ(2 − λ)z λ Dzλ f (z), (λ 6= 2, 3, 4, . . .). λ The class Nα,β (φ) consists of functions f ∈ A for which Ωλ f ∈ Nα,β (φ). g λ Note that Nα,β (φ) is the special case of the class Nα,β (φ) when (3.1) g(z) = z + ∞ X Γ(n + 1)Γ(2 − λ) n=2 Γ(n + 1 − λ) Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page Contents JJ J II I Go Back Close zn. P P∞ n n Let g(z) = z + ∞ n=2 gn z (gn >P0). Since f (z) = z + n=2 an z ∈ g n Nα,β (φ) if and only if (f ∗ g) = z + ∞ n=2 gn an z ∈ Nα,β (φ), we obtain the Quit Page 12 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au g coefficient estimate for functions in the class Nα,β (φ), from the corresponding estimate for functions in the class Nα,β (φ). Applying Theorem 2.1 for the function (f ∗ g)(z) = z + g2 a2 z 2 + g3 a3 z 3 + · · · , we get the following Theorem 3.1 after an obvious change of the parameter µ: Theorem 3.1. Let the function φ be given by φ(z) = 1 + B1 z + B2 z 2 + B3 z 3 + g · · · . If f given by (1.1) belongs to Nα,β (φ), then n o µg3 B12 (1+α) B2 1 2 − + B if µ ≤ σ1 ; − g (α+2β) 2(α+β)2 1 g22 2(α+β)2 3 B1 − g13 (α+2β) if σ1 ≤ µ ≤ σ2 ; |a3 − µa22 | ≤ n o µg3 B12 (1+α) B2 2 1 + 2(α+β) if µ ≥ σ2 , 2 g 2 − 2(α+β)2 B1 g3 (α+2β) 2 where Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian Title Page σ1 := σ2 := α)B12 2 g3 (1 + α)(2β + − 2(B2 − B1 )(β + α) 2 g2 2(2β + α)B12 g3 (1 + α)(2β + α)B12 − 2(B2 + B1 )(β + α)2 . g22 2(2β + α)B12 The result is sharp. Since (Ωλ f )(z) = z + Contents JJ J II I Go Back Close ∞ X Γ(n + 1)Γ(2 − λ) n=2 Γ(n + 1 − λ) Quit an z n , Page 13 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au we have g2 := (3.2) Γ(3)Γ(2 − λ) 2 = Γ(3 − λ) 2−λ and g3 := (3.3) Γ(4)Γ(2 − λ) 6 = . Γ(4 − λ) (2 − λ)(3 − λ) For g2 and g3 given by (3.2) and (3.3), Theorem 3.1 reduces to the following: Theorem 3.2. Let the function φ be given by φ(z) = 1 + B1 z + B2 z 2 + B3 z 3 + g · · · . If f given by (1.1) belongs to Nα,β (φ), then |a3 − µa22 | n o µg3 B12 (1+α) (2−λ)(3−λ) B2 2 + − − B 6 (α+2β) 2(α+β)2 1 g22 2(α+β)2 B1 − (2−λ)(3−λ) ≤ 6 (α+2β) n o µg3 B12 (1+α) B2 2 (2−λ)(3−λ) + − B 2 2 1 6 (α+2β) 2(α+β) 2(α+β)2 g 2 T.N. Shanmugam, M.P. Jeyaraman and S. Sivasubramanian if µ ≤ σ1 ; if σ1 ≤ µ ≤ σ2 ; if µ ≥ σ2 , Title Page where σ1 := σ2 := Fekete-Szegö Functional for some Subclass of Non-Bazilevič Functions Contents JJ J II I 2(3 − λ) (1 + α)(2β + α)B12 − 2(B2 − B1 )(β + α)2 , 3(2 − λ) 2(2β + α)B12 Go Back α)B12 Quit 2 2(3 − λ) (1 + α)(2β + − 2(B2 + B1 )(β + α) . 3(2 − λ) 2(2β + α)B12 Close Page 14 of 16 The result is sharp. J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au References [1] B.C. CARLSON AND D.B. SHAFFER, Starlike and prestarlike hypergeometric functions, SIAM J. Math. Anal., 15 (1984), 737–745. [2] A.W. GOODMAN, Uniformly convex functions, Ann. Polon. Math., 56 (1991), 87–92. [3] W. MA AND D. MINDA, A unified treatment of some special classes of univalent functions, in: Proceedings of the Conference on Complex Analysis, Z. Li, F. Ren, L. Yang, and S. Zhang (Eds.), Int. Press (1994), 157– 169. [4] M. OBRADOVIC, A class of univalent functions, Hokkaido Math. J., 27(2) (1998), 329–335. [5] S. 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LIAO, On certain generalized class of nonBazilevič functions, Acta Mathematica Academia Paedagogicae Nyiregyhaziensis, 21 (2005), 147–154. Close Quit Page 16 of 16 J. Ineq. Pure and Appl. Math. 7(3) Art. 117, 2006 http://jipam.vu.edu.au