Meromorphically Starlike Functions at Infinity

Transcript

1 Punjab University Journal of Mathematics (ISSN ) Vol. 48(2)(2016) pp Meromorphically Starlike Functions at Infinity Imran Faisal Department of Mathematics, University of Education, Lahore, Attock,Campus, Attock city. (corresponding author) Shafiqa Jabeen Department of Mathematics, University of Education, Lahore, Attock, Campus, Attock city. Maslina Darus School of Mathematical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, Bangi Selangor D. Ehsan, Malaysia. Received: 26 April, 2016 / Accepted: 25 July, 2016 / Published online: 05 August 2016 Abstract. In this letter, we introduce meromorphically starlike functions by using a particular class of analytic functions that present authors applied in 2012 [cf.[1]]. These functions map the punctured unit disk into a starlike domain at infinity. Besides, we investigate coefficient estimate, radii of convexity and distortion theorems as well. Finally, we ascertain that these subclasses are closed under convex linear combination. AMS (MOS) Subject Classification Codes: 30C45 Key Words: Meromorphic, Coefficient inequalities, Convex linear combinations. 1. INTRODUCTION Let stand for the class of f(z) = 1 z + a n z n, (1. 1) that are analytic in U, where U = {z : 0 < z < 1}. 11

2 12 Imran Faisal, Maslina Darus and Shafiqa Jabeen represent all of f(z) such that f(z) = 1 z + a n z nq, q = 1 1, N, (1. 2) and are analytic in U. When goes to infinity then 1 1/ approaches to 1; hence =. also denote functions such as + f(z) = 1 z + a n z nq, q = 1 1, N, (1. 3) and analytic in U. In view of above definitions, We define a generalized subclass, denoted by (β, δ, γ) for f belonging to as follows. A function f defined by (1. 2 ) belongs to (β, δ, γ), if it get by analytic criterion s.t. zf (z) f(z) + 1 [ 2γ zf ] [ (z) f(z) + β zf ] (z) f(z) + 1 < δ, z U, (1. 4) for 0 β < 1, 0 < δ 1 and 1 2 < γ 1. When then 1 1/ 1; hence = implies (β, δ, γ) equal to (β, δ, γ). We consider + (β, δ, γ) equal to + (β, δ, γ), clearly + (β, δ, γ) is the generalized form of the class (β, δ, γ) that Aouf and Joshi introduced in 1998 [cf.[2]]. Similar work has been seen for different subclasses done by other author s[ see for example [3-5] ]. Using (1. 4 ), we obtain the characterization properties for the special family of meromorphic functions at infinity defined in (1. 2 ) as follow: Theorem 1.1. A f(z) + belongs to + (β, δ, γ), iff it satisfies the analytic criterion {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} a n 2δγ(1 β), (1. 5) where 0 β < 1, 0 < δ 1, 1 2 < γ 1 and q having the same constraints as given in (1. 2 ). Proof. Let f + (β, δ, γ), then by using (1. 4 ) and after some calculation, we have R (nq+1) a n z nq+1 2γ(1 β) (1 2γ)nq a n z nq+1 (1 2γβ) a n z nq+1 < δ, z U. Letting z 1 through positive values and after doing some mathematics, we obtain {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} a n 2δγ(1 β). Conversely, since we have zf (z) + f(z) δ 2γ(zf (z) + βf(z)) (zf (z) + f(z)) =

3 Meromorphically Starlike Functions at Infinity 13 (nq + 1)a n z nq δ 2γ(β 1) 1 z + (2γ 1)(nq)a n z nq + (2βγ 1)a n z nq, (nq + 1)a n z nq δ[ 2γ(β 1) 1 z + (2γ 1)(nq)a n z nq + (1 2βγ)a n z ], nq or { (nq + 1) a n r nq+1 δ 2γ(1 β) + 2βγ) a n r nq+1 }, (1 2γ)(nq) a n r nq+1 (1 = (nq + 1) + δ[(1 nq) + 2γ(nq β)] a n r nq+1 2δγ(1 β). (1. 6) Hence by taking limit when r 1, then we have (nq + 1) + δ[(1 nq) + 2γ(nq β)] a n 2δγ(1 β) 0. It implies that f(z) + (β, δ, γ). The solution given in (1. 5 ) is sharp for f(z) = 1 z + 2δγ(1 β)z nq, (1. 7) (nq + 1) + δ[(1 nq) + 2γ(nq β)] where 0 β < 1, 0 < δ 1, 1 2 < γ 1 and q having the same constraints as given in (1. 2 ). Theorem 1.2. If f(z) + belongs to + (β, δ, γ), then 1 z δγ(1 β) 1 (q+1)+δ(1 q)+2δγ(q β) z f(z) z + δγ(1 β) (q+1)+δ(1 q)+2δγ(q β) z Proof. Let f + (β, δ, γ), then by using our previous Theorem1.1,we have δγ(1 β) a n (q + 1) + δ(1 q) + 2δγ(q β), since this implies f(z) 1 z + z a n, 1 z + δγ(1 β) (q + 1) + δ(1 q) + 2δγ(q β) z. Similarly, one can prove easily that f(z) 1 z δγ(1 β) (q + 1) + δ(1 q) + 2δγ(q β) z.

4 14 Imran Faisal, Maslina Darus and Shafiqa Jabeen 2. RADII OF CONVEXITY Theorem 2.1. If f(z) + belongs to + (β, δ, γ), then f is meromorphically convex having order ρ in z < r(β, δ, γ, ρ), where [ ] 1 (nq + 1) + δ[(1 nq) + 2γ(nq β)](1 ρ) nq+1 r(β, δ, γ, ρ) = inf, 2δγ(1 β)nq(nq + 2 ρ) where 0 β < 1, 0 < δ 1, 1 2 < γ 1, 0 ρ < 1 and q having the same constraints as given in (1. 2 ). Proof. Suppose f + (β, δ, γ), then by Theorem1.1, we get (nq + 1) + δ[(1 nq) + 2γ(nq β)] a n 1. (2. 8) 2δγ(1 β) To prove our main result, it is enough to show that 2 + zf (z) f (z) (1 ρ), or f (z) + (zf (z)) f (z) (1 ρ). Constraints on parameters are considered same as given in the statement, so from above expression, we get nq(nq + 1) a n z nq 1 nq(nq + 1) a n z nq+1 1 z + nq a 2 n z nq 1 1. nq a n z nq+1 The expression is bounded above by (1 ρ) if Using (2. 8 ), above expression is true if impies that nq(nq + 2 ρ) z nq+1 (1 ρ) nq(nq + 2 ρ) a n z nq+1 1. (2. 9) (1 ρ) (nq + 1) + δ[(1 nq) + 2γ(nq β)], n N, 2δγ(1 β) [ ] 1 (nq + 1) + δ[(1 nq) + 2γ(nq β)](1 ρ) nq+1 z, n N, (2. 10) 2δγ(1 β)nq(nq + 2 ρ) and therefore [ ] 1 (nq + 1) + δ[(1 nq) + 2γ(nq β)](1 ρ) nq+1 r(β, δ, γ, ρ) = inf, n N. 2δγ(1 β)nq(nq + 2 ρ)

5 Meromorphically Starlike Functions at Infinity 15 Theorem 3.1. Let f 0 (z) = 1 z and f n (z) = 1 z + 3. EXTREME POINTS 2δγ(1 β)z nq (nq + 1) + δ[(1 nq) + 2γ(nq β)], n N, then f + (β, δ, γ) f(z) = λ nqf nq (z), where λ n 0 and Proof. Suppose f(z) = λ n f n (z), where λ n 0 and λ n = 1. Then f(z) = λ n f n (z) = λ 0 f 0 (z) + λ n f n (z) = 1 z + λ n 2δγ(1 β)z nq (nq + 1) + δ[(1 nq) + 2γ(nq β)]. λ n = 1. By using (2. 8 ) and above expression, we get λ n = 1 λ 0 1. Hence by Theorem 1.1, one can see easily that f + (β, δ, γ). Conversely, Suppose that f + (β, δ, γ), since 2δγ(1 β)z nq a n (nq + 1) + δ[(1 nq) + 2γ(nq β)], n N. we adjust (nq + 1) + δ[(1 nq) + 2γ(nq β)] λ n = a n, n N 2δγ(1 β) and λ 0 = 1 λ n. Then clearly f(z) = λ nf n (z), as required. 4. CONVEX LINEAR COMBINATION Theorem 4.1. f + (β, δ, γ) is closed under convex linear combination. Proof. Let f(z) and g(z) belong to + (β, δ, γ), where f(z) = 1 z + a n z nq, and g(z) = 1 z + b n z nq. We consider h(z) = µf(z) + (1 µ)g(z), (0 µ < 1), then by using Theorem 1.1, we have {(nq + 1) + δ [(1 nq) + 2γ(nq β)]} µa n + (1 µ)b n 2δγ(1 β). Next we discussed a special subclass of meromorphic functions denoted by + r, [β, δ, γ] contained all f(z) = 1 z + 2rδγ(1 β)z {(q + 1) + δ[(1 q) + 2γ(q β)]} + a n z nq, 0 r 1 (4. 11) Theorem 4.2. Let the functions defined by (4. 11 ) belong to + r, [β, δ, γ] iff {(nq + 1) + δ [(1 nq) + 2γ(nq β)]} a n 2δγ(1 β)(1 r). (4. 12) It is true for f n (z) = 1 z + r 2δγ(1 β) {(q+1)+δ[(1 q)+2γ(q β)]} z + 2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} znq, n 2.

6 16 Imran Faisal, Maslina Darus and Shafiqa Jabeen Proof. By using Theorem1.1, we have {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} a n 2δγ(1 β), this implies that {(q + 1) + δ[(1 q) + 2γ(q β)]} a 1 + {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} a n 2δγ(1 β), {(q+1)+δ[(1 q)+2γ(q β)]} replacing a 1 by, 0 r 1, and after doing some mathematics, we proved inequality given in (4. 12 ). Theorem 4.3. If the functions defined by (4. 11 ) belong to + r, [β, δ, γ] then f is convex in 0 < z < ρ(β, δ, γ, r) where ρ(β, δ, γ, r) is the largest value for which 3rδγ(1 β) {(q+1)+δ[(1 q)+2γ(q β)]} ρ2 + Sharp function for the result is given by f n (z) = 1 z + Proof. Here we need to get {(q+1)+δ[(1 q)+2γ(q β)]} z + nq(nq+2)2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} ρnq+1 1, n. zf (z) f (z) + 2 1, for the functions defined by (4. 11 ). Therefore, we have whenever zf (z) f (z) + 2 3rδγ(1 β) {(q + 1) + δ[(1 q) + 2γ(q β)]} ρ2 + Since f + r, [β, δ, γ], hence we may take a n = 2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} znq. 2rδγρ 2 {(q+1)+δ[(1 q)+2γ(q β)]} + nq(nq+1) a n ρ nq+1 1 {(q+1)+δ[(1 q)+2γ(q β)]} ρ2 nq a nρ nq+1 < 1, nq(nq + 2) a n ρ nq+1 < 1. 2λ n δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]}, λ n 1. For each fixed ρ, choose an integer n = n(ρ) for which maximal. Then nq(nq + 2) a n ρ nq+1 nq(nq+2)ρ nq+1 {(nq+1)+β[(1 nq)+2γ(nq β)]} is nq(nq+2)2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} ρnq+1, Now find the value ρ 0 = ρ(β, δ, γ, r) and the corresponding n(ρ 0 ) so that 3rδγ(1 β) nq(nq + 2)2δγ(1 β)(1 r) {(q + 1) + δ[(1 q) + 2γ(q β)]} ρ2 0+ {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} ρnq+1 0 = 1. For this value function f is convex in defined range of z.

7 Meromorphically Starlike Functions at Infinity 17 Theorem 4.4. Let and f n (z) = 1 z + f i (z) = 1 z + 2rδγ(1 β) {(q + 1) + δ[(1 q) + 2γ(q β)]} z, {(q+1)+δ[(1 q)+2γ(q β)]} z + then f(z) belongs to + r, [β, δ, γ] f(z) = 1. 2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} znq, n (4. 13), λ n f n (z), with λ n 0 and λ n = Proof. Suppose that f(z) = λ nf n (z), then after some calculation and using hypothesis, we get f n (z) = 1 z + by using Theorem 4.2, we have {(q+1)+δ[(1 q)+2γ(q β)]} z + 2δγ(1 β)(1 r)λ n {(nq+1)+δ[(1 nq)+2γ(nq β)]} znq (4., 14) 2δγ(1 β)(1 r) {(nq+1)+δ[(1 nq)+2γ(nq β)]} λ n {(nq+1)+δ[(1 nq)+2γ(nq β)]} 2δγ(1 β)(1 r) = therefore f(z) + r, [β, δ, γ]. Conversely, we suppose that λ n = 1 λ 1 1, f(z) = 1 z + 2rδγ(1 β) {(q + 1) + δ[(1 q) + 2γ(q β)]} z + a n z nq, belonging to class, discussed for (4. 11 ), and setting as λ n = a n 2δγ(1 β)(1 r) {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} {(nq + 1) + δ[(1 nq) + 2γ(nq β)]} a n, n 2δγ(1 β)(1 r), and λ 1 = 1 λ n. Then clearly f(z) = λ nf n (z), as required. Other work related to analytic functions and its properties can be found in [6-8]. 5. ACKNOWLEDGMENTS The work presented here is fully supported by UKM-AP We would like to thank the referee for giving us some valuable suggestions to improve the presentation of the article.

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