Bulletin of the. Iranian Mathematical Society

Transcript

1 ISSN: X Print ISSN: Online Bulletin of the Iranian Mathematical Society Vol , No 3, pp Title: A certain convolution approach for subclasses of univalent harmonic functions Authors: R M El-Ashwah and M K Aouf Published by Iranian Mathematical Society

2 Bull Iranian Math Soc Vol , No 3, pp Online ISSN: A CERTAIN CONVOLUTION APPROACH FOR SUBCLASSES OF UNIVALENT HARMONIC FUNCTIONS R M EL-ASHWAH AND M K AOUF Communicated by Hamid Reza Ebrahimi Vishki Abstract In the present paper we study convolution properties for subclasses of univalent harmonic functions in the open unit disc and obtain some basic properties such as coefficient characterization and extreme points Keywords: Analytic, harmonic, convolution MSC2010: Primary: 30C45 1 Introduction A continuous function f = u+iv is a complex valued harmonic function in a simply connected complex domain D C if both u and v are real harmonic in D It was shown by Clunie and Sheil-Small 5 that such harmonic function can be represented by f = h+g, where h and g are analytic in D Also, a necessary and sufficient condition for f to be locally univalent and sense preserving in D is that h z > g z, see also, 2, 9, 10 and 20 Denote by H the family of functions f = h + g, which are harmonic univalent and sense-preserving in the open unit disc U = z C : z < 1} with normalization f0 = h0 = f z0 1 = 0 and h, g are of the form: 11 hz = z + a n z n, gz = b n z n 0 b 1 < 1; z U Also denote by T H the subfamily of H consisting of harmonic functions f = h + g of the form: 12 hz = z a n z n, gz = b n z n a n, b n 0; 0 b 1 < 1; z U Article electronically published on June 15, 2015 Received: 14 November 2012, Accepted: 18 April 2014 Corresponding author c 2015 Iranian Mathematical Society 739

3 A certain convolution approach for subclasses of univalent harmonic functions 740 Let 13 ϕz = z + ϕ n z n ϕ n 0; n N = 1, 2, }, be a given analytic function in U, then ϕz + ϕz hz + gz = z + ϕ n a n z n + ϕ n b n z n 14 ϕ 1 = 1; 0 b 1 < 1; z U, where is the convolution operation Later on, variouse subclasses of H have been introduced and studied by several authors see e g 2, We shall show in this note that these subclasses are special cases of the general class H 0 ϕ, λ, σ, α giving in the following definition Definition 11 Let ϕz be given by 13 A harmonic function f = h+ḡ H where h and g are given by 11 belongs to the class H 0 ϕ, λ, σ, α if z h ϕ z σz g ϕ z Re 1 λ h ϕ z + σg ϕ z + λ z h ϕ z σz g ϕ z >α 15 0 α < 1; 0 λ 1; σ 1; z U, We note that i H 0 Hq s α 1, λ, 1, α = R s,q H α 1, λ, α where Hq s α 1 is the Dziok-Srivastava operator 6 see Murugusundaramoorthy et al 12; ii H 0 Hq s α 1, 0, 1, α = R s,q H α 1, α where Hq s α 1 is the Dziok-Srivastava operator 6 see Al-kharsani and Al-Khal 2; iii H 0 z 1 z, λ, 1, α = SH λ, α see Ozturk et al 14; iv H 0 z 1 z, 0, 1, α = SH α and H0 z 1 z, 0, 1, α = K 2 H α see Jahangiri 10; v H 0 z 1 z, 0, 1, 0 = SH and H0 z 1 z, 0, 1, 0 = K 2 H see Silverman 19 Now we denote by HP 0 ϕ, λ, σ, k, α a general class of various subclasses of harmonic parabolic starlike functions introduced and studied by several authors see e g 8, 16, 17 and 21 Definition 12 Let ϕz be given by 13 A harmonic function f = h+ḡ H belongs to the class HP 0 ϕ, λ, σ, k, α if } 1+ke Re iγ zh ϕ z σzg ϕ z 1 λh ϕz+σg ϕz+λzh ϕ z σzg ϕ z keiγ > α

4 741 El-Ashwah and Aouf 16 0 α < 1; 0 λ 1; σ 1; k 0; γ real;z U We note that i HP 0 z + n m z n, λ, 1 m, 1, α = SH m + 1, m, λ, α see Sudharasan et al 21; ii HP 0 z 1 z, λ, 1, 0, α = SH, λ, α see Ozturk et al 14; iii HP 0 z 1 z, 0, 1, 1, α = G H α see Rosy et al 16 In this paper, we obtain necessary and sufficient convolution conditions for the classes H 0 ϕ, λ, σ, α and HP 0 ϕ, λ, σ, k, α, sufficient coefficient bounds for functions in these two classes, these sufficient coefficient conditions are shown to be also necessary for functions with negative coefficients Finally we determined growth estimates and extreme points for the class T H 0 ϕ, λ, σ, α = H 0 ϕ, λ, σ, α T H The class T H 0 ϕ, λ, σ, α includes as special cases several subclasses studied in 9 and 19 2 Main results We now derive the convolution characterization for functions in the class H 0 ϕ, λ, σ, α Theorem 21 Let f = h + ḡ H and ϕz be given by 13, then f H 0 ϕ, λ, σ, α if and only if z + 1 λx+2α 1 z 2 x+α λx+2α 1 z 1 λx+2α 1 z 2 21 α 1 α 21 α h ϕ 1 z 2 σg ϕ 1 z x = 1; z = 0 Proof A necessary and sufficient condition for f = h + ḡ to be in the class H 0 ϕ, λ, σ, α, with h, g of the form 11, is given by 15 Since z h ϕ z σz g ϕ z = 1 1 λ h ϕ z + σg ϕ z + λ z h ϕ z σz g ϕ z at z = 0, the condition 15 is equivalent to } 1 zh ϕ z σzg ϕ z 1 λh ϕz+σg ϕz+λzh ϕ z σzg ϕ z α x 1 x x = 1; x 1; 0 < z < 1

5 A certain convolution approach for subclasses of univalent harmonic functions 742 By a simple algebraic manipulation, 22 yields 0 x + 1 z h ϕ z σzg ϕ z αx λ h ϕ z + σg ϕ z +λ z h ϕ z σz g ϕ z x 1 1 λ h ϕ z + σg ϕ z +λ z h ϕ z σz g ϕ z 2z + 1 λ x + 2α 1 z 2 = h ϕ 1 z 2 σg ϕ 2x + α λx + 2α 1z 1 λ x + 2α 1 z 2 1 z 2 The latter condition together with 15 establishes the result 21 for all x = 1 Remark 22 Taking λ = 0 and b 1 = 0 in Theorem 21 we obtain the result obtained by Ali et al 3, Theorem 2; ii Taking λ = 0, σ = 1, α = 0 and ϕ = z, in Theorem 21 we obtain 1 z the result obtained by Ahuja et al 1, Theorem 26; z ii Taking λ = 0, σ = 1, α = 0 and ϕ = 2, in Theorem 21 we obtain 1 z the result obtained by Ahuja et al 1, Theorem 28 Necessary coefficient conditions for harmonic starlike functions were obtained in 5 see also 18 Using the convolution characterization, we now derive a sufficient condition for harmonic functions to belong to the class H 0 ϕ, λ, σ, α Theorem 23 Let f = h + ḡ H and ϕz be given by 13, then f H 0 ϕ, λ, σ, α if nλ α1 λ a n ϕ n + σ nλ + α1 λ b n ϕ n 1

6 743 El-Ashwah and Aouf Proof For h and g given by 11, 21 gives z + 1 λx+2α 1 z 2 x+α λx+2α 1 z 1 λx+2α 1 z 2 21 α 1 α 21 α h ϕ 1 z 2 σg ϕ 1 z 2 = z + } 1 λ x + 2α 1 n + n 1 a n ϕ n z n 2 σ z x + α λx + 2α 1 n n 1 1 nλ α1 λ a n ϕ n σ } 1 λ x + 2α 1 b n ϕ n z n 2 nλ + α1 λ b n ϕ n The last expression is non-negative by hypothesis, and hence by Theorem 21, it follows that f Hp 0 ϕ, λ, σ, α Remark 24 i Taking λ = 0 and b 1 = 0 in Theorem 23 we obtain the result obtained by Ali et al 3, Theorem 3; ii Taking λ = 0, σ = 1, α = 0 and ϕ = z, in Theorem 23 we obtain 1 z the result obtained by Ahuja et al 1, Corollary 27; z iii Taking λ = 0,, σ = 1, α = 0 and ϕ = 2, in Theorem 23 we 1 z obtain the result obtained by Ahuja et al 1, Corollary 29 Theorem 25 Let f = h + ḡ H and ϕz be given by 13, then f HP 0 ϕ, λ, σ, k, α if and only if h ϕ z + 1 λx+1ke iγ +x+2α 1 21 α z 2 1 z 2 x+1ke iγ +x+α λx+1ke iγ +x+2α 1 +x+2α 1 1 α z 1 λx+1keiγ 21 α z 2 σg ϕ 1 z 2 0 x = 1; z = 0 Proof A necessary and sufficient condition for f = h + ḡ to be in the class HP 0 ϕ, λ, σ, k, α, with h, g of the form 11, is given by 16 Since 1 + ke iγ z h ϕ z σz g ϕ z ke iγ = 1 1 λ h ϕ z + σg ϕ z + λ z h ϕ z σz g ϕ z at z = 0, the condition 16 is equivalent to } 1 1+ke iγ zh ϕ z σzg ϕ z 1 λh ϕz+σg ϕz+λzh ϕ z σzg ϕ z keiγ α x 1 x + 1

7 A certain convolution approach for subclasses of univalent harmonic functions x = 1; x 1; z 0 By a simple algebraic manipulation similar to that used in Theorem 21, we obtain the desired result Proceeding similarly as in Theorem 23, the following sufficient condition for the class HP 0 ϕ, λ, σ, k, α is easily derived Theorem 26 Let f = h + g H and ϕz be given by 13, then f HP 0 ϕ, λ, σ, k, α if nk + 1 λ k + α 1 λ k + α a n ϕ n + σ nk + 1 λ k + α + 1 λ k + α b n ϕ n 1 Theorem 27 Let ϕz be given by 13 and f = h + g where h, g are given by 12 Then f T H 0 ϕ, λ, σ, α if and only if nλ α1 λ nλ + α1 λ 24 a n ϕ n + σ b n ϕ n 1 Proof If f Re 1 αz belongs to the class T H 0 ϕ, λ, σ, α, then 15 is equivalent to n1 αλ α1 λa n ϕ n z n σ n1 αλ+α1 λb n ϕ n z n > 0, z 1 λ+nλa n ϕ n z n +σ 1 λ nλb n ϕ nz n for z U Letting z 1 through real values yields condition 24 Conversely, for h, g given by 12, we have z + 1 λx+2α 1 z 2 x+α λx+2α 1 z 1 λx+2α 1 z 2 21 α 1 α 21 α h ϕ 1 z 2 σg ϕ 1 z 2 z 1 nλ α1 λ a n ϕ n σ nλ + α1 λ b n ϕ n which is non-negative by hypothesis, this completes the proof of Theorem 27 Remark 28 i Taking σ = 1 and ϕz = z + Γ n 1 α 1 z n, where α 1 n 1 α q n 1 Γ n 1 α 1 = β 1 n 1 β s n 1 1 n 1 25 q s + 1, α i Ci = 1, 2,, q and β j C\Z 0 j = 1, 2,,

8 745 El-Ashwah and Aouf in Theorem 27 we obtain the result obtained by Murugusundaramoorthy et al 13 Corollary 29 Let ϕz = z + 2 α1+λ 2+α1 3λ If f T H0 ϕ, λ, σ, α satisfies 2 α1+λ 2+α1 3λ for z = r < 1, fz 1 + b 1 r + 2 α1 + λϕ 2 and fz 1 b 1 r 2 α1 + λϕ 2 The function fz = z + b 1 z + 2 α1 + λϕ 2 ϕ n z n with ϕ n ϕ 2 n 2, and σ 1+α1 λ 1 α b 1 < 1, then α1 λ 1 + α1 λ 1 + α1 λ and its rotations show that the bounds are sharp 2 α1 + λ 2 + α1 3λ b 1 r 2 2 α1 + λ 2 + α1 3λ b1 r 2 2 α1 + λ 2 + α1 3λ b 1 z 2 The following covering result follows from the left side inequality in Corollary 29 Corollary 210 Let f T H 0 ϕ, λ, σ, α, the 1 + α1 λ w : w < α1 + λϕ α1 3λ ϕ 2 1 α } b 1 fu Theorem 211 Let h 1 z = z, h n z = z n1 αλ α1 λϕ n z n, and g n z = 1 α z + σn1 αλ+α1 λϕ n z n n 1 A function f T H 0 ϕ, λ, σ, α if and only if f can expressed in the form f z = λ n h n + γ n g n, where λ n 0, γ n 0, λ 1 = 1 λ n γ n In particular, the extreme points of the class T H 0 ϕ, λ, σ, α are h n } and g n }, respectively Proof Let f z = λ n h n + γ n g n = z λ n 1 α n1 αλ α1 λϕ n z n + γ n 1 α n1 αλ+α1 λϕ n z n

9 A certain convolution approach for subclasses of univalent harmonic functions 746 Since nλ α1 λ λ n ϕ n nλ α1 λ ϕ n nλ + α1 λ + σ γ n ϕ n σ nλ + α1 λ ϕ n = λ n + γ n = 1 λ 1 1, it follows from Theorem 27 that f T H 0 ϕ, λ, σ, α Conversely, if f T H 0 ϕ, λ, σ, α, then a n 1 α n1 αλ+α1 λϕ n Set λ n = n1 αλ α1 λ 1 α γ n, then This proves Theorem 211 a n ϕ n, γ n = n1 αλ+α1 λσ 1 α 1 α n1 αλ α1 λϕ n b n ϕ n, λ 1 = 1 λ n h n + γ n g n = z a n z n + b n z n = fz 3 Applications and b n λ n We can derive new results by taking ϕz as follows: 1- ϕz = z + m z n see 4, 7 and 15 with p = 1, where 1+l+γn 1 1+l l > 1, γ 0 and m Z = 0, ±1, ±2, } 2- ϕz = z + Γ n 1 α 1 z n see 6, where Γ n 1 α 1 is given by 25 Acknowledgments The authors thank the referees for their valuable suggestions which led to the improvement of this paper References 1 O P Ahuja, J M Jahangiri and H Silverman, Convolutions for special classes of harmonic univalent functions, Appl Math Lett , no 6, H A Al-kharsani and R A Al-Khal, Univalent harmonic functions, Inequal Pure Appl Math , no 2, 8 pages 3 R M Ali, B A Stephen and K G Subramanian, Subclasses of harmonic mapping defined by convolution, Appl Math Letters , no 10, A Catas, On certain classes of p-valent functions defined by multiplier transformations, in Proceedings of the International Symposium on Geometric Function Theory and Applications: GFTA 2007 Proceedings İstanbul, Turkey; August 2007 S Owa

10 747 El-Ashwah and Aouf and Y Polatoģlu, Editors, pp , TC İstanbul Kűltűr University Publications, Vol 91, TC İstanbul Kűltűr University, İstanbul, Turkey, J Clunie and T Sheil-Small, Harmonic univalent functions, Ann Acad Sci Fenn Ser A I Math J Dziok and H M Srivastava, Classes of analytic functions associated with the generalized hypergeometric function, Appl Math Comput , no 1, R M El-Ashwah and M K Aouf, Some properties of new integral operator, Acta Univ Apulensis Math Inform , F M Sakar and H Guney, On harmonic uniformly starlike functions defined by an integral operator, Acta Univ Apulensis, J M Jahangiri, Coefficient bounds and univalence criteria for harmonic functions with negative coefficients, Ann Univ Marie-Curie Sklodowska Sect A , no 2, J M Jahangiri, Harmonic functions starlike in the unit disk, J Math Anal Appl , no 2, J M Jahangiri, G Murugusungdaramoorthy and K Vijaya, Salagean-type harmonic univalent functions, Southwest J Pure Appl Math , no 2, J M Jahangiri, G Murugusungdaramoorthy and K Vijaya, Starlikeness of harmonic functions defined by Ruscheweyh derivatives, J Indian Acad Math , no 1, G Murugusundaramoorthy, K vijaya and B A Frasin, A subclass of harmonic functions with negative coefficients defined by Dziok-Srivastava operator, Tamkang J Math , no 4, M Ozturk, S Yalcin and M Yamankaradenz, Convex subclass of harmonic starlike functions, Appl Math Comput , no 2, J K Prajapat, Subordination and superordination preserving properties for generalized multiplier transformation operator, Math Comput Modelling , no 3-4, T Rosy, B Adolf Stephen, K G Subramanian and J M Jahangiri, Goodman-Ronningtype harmonic univalent functions, Kyungpook Math J , no 1, B Saker and S S Eker, On Salagean-type harmonic functions, General Math , no 2-3, T Sheil-Small, Constants for planer harmonic mappings, J London Math Soc , no 2, H Silverman, Harmonic univalent functions with negative coefficients, J Math Anal Appl , no 1, H Silverman and E M Silvia, Subclasses of harmonic univalent functions, New Zealand J Math , no 2, T V Sudharsan, R Ezhilarasi, B A Stephen and K G Subramnian, A new subclass of Salagean-type harmonic univalent functions, Chamchuri J Math , no 1, R M El-Ashwah Department of Mathematics, Faculty of Science, Damietta University, New Damietta 34517, Egypt address: r elashwah@yahoocom M K Aouf Department of Mathematics, Faculty of Science, Mansoura University, Mansoura 35516, Egypt address: mkaouf127@yahoocom