Báo cáo hóa học: "Research Article Univalence of Certain Linear Operators Defined by Hypergeometric Function" pdf

Hindawi Publishing Corporation Journal of Inequalities and Applications Volume 2009, Article ID 807943, 12 pages doi:10.1155/2009/807943 Research Article Univalence of Certain Linear Operators Defined by Hypergeometric Function R Aghalary and A Ebadian Department of Mathematics, Faculty of Science, Urmia University, Urmia, Iran Correspondence should be addressed to A Ebadian, a.ebadian@urmia.ac.ir Received 11 January 2009; Accepted 22 April 2009 Recommended by Vijay Gupta The main object of the present paper is to investigate univalence and starlikeness of certain integral operators, which are defined here by means of hypergeometric functions Relevant connections of the results presented here with those obtained in earlier works are also pointed out Copyright q 2009 R Aghalary and A Ebadian This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited Introduction and Preliminaries Let H denote the class of all analytic functions f in the unit disk D n ≥ 0, a positive integer, let An ∞ f ∈H:f z z an k zn k , {z ∈ C : |z| < 1} For 1.1 k with A1 : A, where A is referred to as the normalized analytic functions in the unit disc A function f ∈ A is called starlike in D if f D is starlike with respect to the origin The class of all starlike functions is denoted by S∗ : S∗ For α < 1, we define S∗ α f ∈ A : Re zf z f z > α, z ∈ D , 1.2 and it is called the class of all starlike functions of order α Clearly, S∗ α ⊆ S∗ for < α < For functions fj z , given by ∞ fj z k ak,j zk , j 1, , 1.3 Journal of Inequalities and Applications we define the Hadamard product or convolution of f1 z and f2 z by f1 ∗ f2 z : ∞ ak,1 ak,2 zk : f2 ∗ f1 z 1.4 k An interesting subclass of S the class of all analytic univalent functions is denoted by U α, μ, λ and is defined by f ∈A: U α, μ, λ 1−α μ z f z z f z α μ f z − < λ, z ∈ D , 1.5 where < α ≤ 1, ≤ μ < αn, and λ > The special case of this class has been studied by Ponnusamy and Vasundhra and Obradovi´ et al c For a,b,c ∈ C and c / 0,-1,-2, ., the Gussian hypergeometric series F a,b;c;z is defined as ∞ F a, b; c; z a b c n n n n zn , n! z ∈ D, 1.6 where a n a a a · · · a n − and a It is well-known that F a, b; c; z is analytic in D As a special case of the Euler integral representation for the hypergeometric function, we have F 1, b; c; z Γc Γ b Γ c−b 1 b−1 t 1−t − tz c−b−1 dt, z ∈ D, Re c > Re b > 1.7 Now by letting φ a; c; z : F 1, a; c; z , 1.8 it is easily seen that zφ a; c 1; z cφ a; c; z − cφ a; c 1; z 1.9 For f ∈ A, Owa and Srivastava introduced the operator Ωλ : A → A defined by Ωλ f z Γ 2−λ λ d z Γ 1−λ dz z f t z−t λ dt, λ / 2, 3, , 1.10 Journal of Inequalities and Applications which is extensions involving fractional derivatives and fractional integrals Using definition of φ a; c; z : F 1, a; c; z we may write zφ 2; − λ; z ∗ f z Ωλ f z 1.11 This operator has been studied by Srivastava et al and Srivastava and Mishra ∞ k Also for λ < 1, Re α > 0, and f z z k ak z , let us define the function F by 1−λ α F z : λz ∞ 1−λ z t 1/α −2 f tz dt 1.12 ak k−1 α k zk This operator has been investigated by many authors such as Trimble , and Obradovi´ et al c If we take ψ m, γ, z ∞ 1−m k k−1 γ zk , 1.13 then we can rewrite operator F defined by 1.11 as z ψ λ, α, z ∗ F z f z z 1.14 From the definition of ψ m, γ, z it is easy to check that 1 γ ψ m, γ, z γ zψ m, γ, z For f ∈ U α, μ, λ with z/f z μ 1−m z 1−z 1.15 ∗φ a; c 1; z / for all z ∈ D we define the transform G by G z z/f z where a, c ∈ C and c / 0, −1, −2, Also for f ∈ U α, μ, λ with z/f z transform H by H z where m < and γ / 0; Re γ ≥ 1/μ z z μ μ ∗ φ a; c 1.16 ∗ ψ m, γ, z / for all z ∈ D we define the z/f z , 1; z μ ∗ ψ m, γ, z 1/μ , 1.17 Journal of Inequalities and Applications In this investigation we aim to find conditions on α, μ, λ such that f ∈ U α, μ, λ implies that the function f to be starlike Also we find conditions on α, μ, λ, m, γ, a, c for each f ∈ U α, μ, λ ; the transforms G and H belong to U α, μ, λ and S∗ For proving our results we need the following lemmas Lemma 1.1 cf Hallenbeck and Ruscheweyh Let h z be analytic and convex univalent in the unit disk D with h Also let g z b1 z ··· 1.18 z ∈ U; c / , 1.19 b2 z2 be analytic in D If zg z ≺h z c g z then g z ≺ψ z c zc z tc−1 h t dt ≺ h z z ∈ D; Re c ≥ 0; c / 1.20 and ψ z is the best dominant of 1.20 Lemma 1.2 cf Ruscheweyh and Stankiewicz If f andg are analytic and F and G are convex functions such that f ≺ F, g ≺ G, then f ∗ g ≺ F ∗ G Lemma 1.3 cf Ruscheweyh and Sheil-Small Let F and G be univalent convex functions in D Then the Hadamard product F ∗ G is also univalent convex in D Main Results We follow the method of proof adopted in 1, 10 Theorem 2.1 Let n be positive integer with n ≥ Also let n /2n < α ≤ and n 1−α < μ < αn If f z z an zn · · · belongs to U α, μ, λ , Then f ∈ S∗ γ whenever < λ ≤ λ α, μ, n, γ , where ⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ λ α, μ, n, γ : αn − μ 2α − γ − μ2 2α − γ − αn − μ ⎪ ⎪ ⎪ αn − μ − γ ⎪ ⎪ ⎪ , ⎩ n μγ − μ , 0≤γ ≤ μ−n 1−α , μ n 2.1 μ−n 1−α < γ < μ1 n Proof Let us define p z z f z μ 2.2 Journal of Inequalities and Applications Since f ∈ U α, μ, λ , we have 1−α z f z μ α μ z f z p z − f z α zf z μ αn − μ an zn 1 λω z , ··· where ω z is an analytic function with |ω z | < and ω ω Schwarz lemma, we have |ω z | ≤ |z|n By 2.3 , it is easy to check that 1− p z 1−α zf z α f z μλ α 2.3 ··· ω n−1 0 By ω tz dt, μ/α 0t 2.4 λω z 1 − μλ/α ω tz / tμ/α dt Therefore zf z −γ f z 1−γ α − μλ α − − αγ / − γ α α − μλ ω tz /tμ/α 1 ω tz /tμ/α α/ − γ dt 1 2.5 λω z dt We need to show that f ∈ S∗ γ To this, according to a well-known result and 2.5 it suffices to show that α− −αγ / 1− γ α − μλ α α−μλ 1 ω tz /t μ/α ω tz /t μ/α dt α/ 1−γ λω z dt / − iT, T ∈ R, 2.6 which is equivalent to ⎡ λ⎣ ··· ω z μ αγ − α /α − i − γ T α 1−γ 1 ω tz /tμ/α ⎤ dt iT ⎦ / − 1, T ∈ R Suppose that Bn denote the class of all Schwarz functions ω such that ω 0, and let ω n−1 M sup z∈D,ω∈Bn ,T ∈R ω z μ αγ − α /α − i − γ T α 1−γ iT ω tz /tμ/α dt , 2.7 ω 2.8 Journal of Inequalities and Applications then, f ∈ S∗ γ if λM ≤ This observation shows that it suffices to find M First we notice that ⎧ ⎪ ⎨1 M ≤ sup ⎩ T ∈R ⎪ μ/ n − μ /α ⎫ ⎪ − γ T2 ⎬ ⎪ ⎭ − α /α2 αγ α 1−γ T2 2.9 Define φ : 0, ∞ → R by αn − μ φ x μ 1−α αγ αn − μ α − γ √ − γ α2 x x 2.10 Differentiating φ with respect to x, we get μ αn − μ α3 − γ φ x 3√ x/2 2 αn − μ α2 − γ αn − μ α − γ αn − μ − 1−α αγ μ 2 − γ α2 x x 1−α αγ αn − μ α2 − γ √ − γ α2 x /2 2.11 x x Case Let < γ < μ − n − α /μ n Then we see that φ has its only critical point in the positive real line at x0 μ2 2α − γ − 1 1−γ 2 α αn − μ 2 − αγ 1−γ 2.12 Furthermore, we can see that φ x > for ≤ x < x0 and φ x < for x > x0 Hence φ x attains its maximum value at x0 and φ x ≤ φ x0 αn − μ αn − μ μ2 2α − γ − 2α − γ − αn − μ μ2 2α − γ − 2.13 Case Let γ > μ − n − α /μ n , then it is easy to see that φ x < 0, and so φ x attains its maximum value at and φ x ≤φ n μγ − μ , αn − μ − γ ∀x ≥ Now the required conclusion follows from 2.13 and 2.14 2.14 Journal of Inequalities and Applications By putting γ in Theorem 2.1 we obtain the following result Corollary 2.2 Let n be the positive integer with n ≥ Also let n /2n < α ≤ and n − α < μ < αn If f z z an zn · · · belongs to U α, μ, λ , then f ∈ S∗ whenever < λ ≤ αn − √ μ 2α − 1/ αn − μ μ2 2α − Remark 2.3 Taking α 1, μ in Theorem 2.1 and Corollary 2.2 we get results of 10 We follow the method ofproof adopted in 11 Theorem 2.4 Let n ≥ 2, a / 0, c ∈ C with Re c ≥ / c and the function ϕ z b1 z b2 z2 · · · n · · · ∈ U α, μ, λ and φ a; c; z defined z an z with bn / be univalent convex in D If f z by 1.8 satisfy the conditions μ z f z ∗ φ a; c 1; z / ∀z ∈ D, 2.15 φ a; c; z ≺ ϕ z , then the transform G defined by 1.16 has the following: G ∈ U α, μ, λ|bn ||c|/|c n| , ∗ G ∈ S whenever 0 and c ≥ max{2, a}, then φ a; c; z is univalent convex function in D So if we take ϕ z φ a; c; z in the Theorem 2.4, we obtain the following Corollary 2.5 For n ≥ 2, c, a > 0, and c ≥ max{2, a}, let the function f z U α, μ, λ and φ a; c; z defined by 1.8 satisfy the condition z f z μ ∗ φ a; c 1; z / ∀z ∈ D z an zn ··· ∈ 2.31 Then the transform G defined by 1.16 has the following: G ∈ U α, μ, λ| a n |c/| c n | c n ; ∗ G ∈ S whenever 0