SUBCLASSES OF P-VALENT STARLIKE FUNCTIONS DEFINED BY USING CERTAIN FRACTIONAL DERIVATIVE OPERATOR

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1 Sutra: International Journal of Mathematical Science Education Technomathematics Research Foundation Vol. 4 No. 1, pp , 2011 SUBCLASSES OF P-VALENT STARLIKE FUNCTIONS DEFINED BY USING CERTAIN FRACTIONAL DERIVATIVE OPERATOR SOMIA MUFTAH AMSHERI Department of Mathematics, SCIM, University of Bradford, Bradford, BD7 1DP, U.K. somia_amsheri@yahoo.com VALENTINA ZHARKOVA Department of Mathematics, SCIM, University of Bradford, Bradford, BD7 1DP, U.K. v.v.zharkova@bradford.ac.uk In this paper we introduce two subclasses of p-valent starlike functions in the open unit disk by using fractional derivative operator. We obtain coefficient inequalities distortion theorems for functions belonging to these subclasses. Further results include distortion theorems (involving the generalized fractional derivative operator). The radii of convexity for functions belonging to these subclasses are also studied. Keywords: p-valent functions; starlike functions; convex functions; fractional derivative operators. 1. Introduction Fractional calculus operators have recently found interesting applications in the theory of analytic functions. The classical definition of fractional calculus its other generalizations have fruitfully been applied in obtaining, the characterization properties, coefficient estimates distortion inequalities for various subclasses of analytic functions. For numerous references on the subject, one may refer to the works by Srivastava Owa (1989) Srivastava Owa (1992). See also Owa (1978); Srivastava Owa (1987); Raina Srivastava (1996) Raina Nahar (2002). Let denote the class of functions defined by 1.1 which are analytic p-valent in the unit disk : 1. Also denote, the subclass of consisting of functions of the form 0,

2 18 Somia Muftah Amsheri Valentina Zharkova Two interesting subclasses,,,, of univalent starlike functions with negative coefficients in the open unit disk were introduced by Srivastava Owa (1991a). In fact, these classes become the subclasses of the class, which was introduced by Gupta (1984) when the function is univalent with negative coefficients. Using the results of Srivastava Owa (1991a), Srivastava Owa (1991b) have obtained several distortion theorems involving fractional derivatives fractional integrals of functions belonging to the classes,,,,. Recently, Aouf Hossen (2006) have generalized the results of Srivastava Owa (1991a) to the case of certain p-valent functions with negative coefficients. With these points in view, by making use of a certain fractional derivative operator, we introduce new subclasses,,,,,,,,,, (defined below) of p- valent starlike functions with negative coefficients. This paper is organized as follows: Section 2 gives preliminary details definitions of p-valent starlike functions, p-valent convex functions fractional derivative operators. In Section 3 we describe coefficient inequalities for the functions belonging to the subclasses,,,,,,,,,,. Section 4 considers the distortion properties. Its further distortion properties (involving the generalized fractional derivative operator) are discussed in section 5. Finally, in section 6 we determine the radii of convexity for functions belonging to these subclasses of p-valent starlike functions. 2. Preliminaries And Definitions A function is called p-valent starlike of order if satisfies the conditions Re 2.1 Re for 0,,. We denote by, the class of all p-valent starlike functions of order. Also a function is called p-valent convex of order if satisfies the following conditions: Re Re

3 Subclasses of p-valent starlike functions 19 for 0,,. We denote by, the class of all p-valent convex functions of order. We note that,, 2.5 for 0. The class, was introduced by Patil Thakare (1983), the class, was introduced by Owa (1985). Also, we denote by,, the classes obtained by taking intersections, respectively, of the classes,, with ; that is,,,, The classes,, were introduced by Owa (1985). In particular, the classes 1, 1, when 1, were studied by Silverman (1975). Let ₂₁, ; ; be the Gauss hypergeometric function defined, for by; see Srivastava Karlsson (1985). ₂₁, ; ; 2.6! where is the Pochhammer symbol defined, in terms of the Gamma function, by Γ Γ 1, , 0, 1, 2, 2.7 Now we recall the following definitions of fractional derivative operators, adopted for working in classes of analytic functions in complex plane which were used by Owa (1978); Srivastava Owa (1987); Srivastava Owa (1991b); Raina Nahar (2002); Raina Srivastava (1996); see also, Srivastava Owa (1989) Srivastava Owa (1992) as follows. Definition 2.1. The fractional derivative of order is defined, for a function, by 1 Γ1 2.8

4 20 Somia Muftah Amsheri Valentina Zharkova where 0 1 is analytic function in a simply- connected region of the z-plane containing the origin, the multiplicity of is removed by requiring log ( to be real when 0. Definition 2.2. Let 01,,. Then, in terms of the familiar Gauss s hypergeometric function ₂F₁, the generalized fractional derivative operator,,, is,,, ₂₁,1;1;1 2.9 where f(z) is analytic function in a simply- connected region of the z-plane containing the origin with the order, 0, where 0, 1, the multiplicity of is removed by requiring log ( to be real when 0. Notice that,,,, Srivastava Owa (1987) have established various distortion theorems for the fractional calculus of functions belonging to the classes,,. We now define the following classes of p-valent starlike functions based on fractional derivative operator. Definition 2.3. The function is said to be in the class,,,,, if Μ,,, Μ,,, , 1 ; max, 1 ; 0; 0; 01; For the function 0, 2.12 belonging to,. Dented by,,, the modification of the fractional derivative operator which is defined in terms of,,, as follows: with Μ,,,,,,,,,, Γ1Γ1 Γ1Γ

5 Subclasses of p-valent starlike functions 21 Further, if satisfies the condition (2.11) for,, we say that,,,,,. The above-defined classes,,,,,,,,,, are of special interest they contain many well-known classes of analytic functions. In particular, in view of (2.13), we find that Μ,,, 2.15 Thus, for 0, we have,,,,,,,,,,,,,,,, where,,,,,, are precisely the subclasses of p-valent starlike functions which were studied by Aouf Hossen (2006). Furthermore, for 0 1, we obtain,, 1,,,,,,, 1,,,,, where,,,, are the subclasses of starlike functions which were studied by Srivastava Owa (1991a) as well as by Srivastava Owa (1991b). In order to prove our results we shall need the following lemmas for the classes,, due to Owa (1985): Lemma 2.4. Let the function defined by (2.12). Then is in the class, if only if 2.16 Lemma 2.5. Let the function defined by (2.12). Then is in the class, if only if 2.17 We mention to the following known result which shall be used in the sequel (Raina Nahar (2002); see also Raina Srivastava (1996)). Lemma 2.6. Let,,, such that 0 0,1 then

6 22 Somia Muftah Amsheri Valentina Zharkova,,, Coefficient Inequalities Theorem 3.1. Let the function be defined by (1.2). If belongs to the class,,,, then,,,,, where ,,,,,,, ,, is given by (2.14). Proof. Applying Lemma 2.6, we have from (1.2) (2.13) that 3.2 Μ,,, Since,,,,,, there exist a function belonging to the class, such that Μ, Μ,,,,,, It follows from (3.3) that,,, Re 2,,, 2 Choosing values of on the real axis so that through real axis, we have or, equivalently,,,,,,, 3.4 is real, letting 1 2,,, 2

7 Subclasses of p-valent starlike functions 23,,, Note that, by using Lemma 2.4,, implies Making use of (3.6) in (3.5), we complete the proof of Theorem Corollary 3.2. Let the function be defined by (1.2) be in the class,,,,,. Then ,,, 1 where,,, is given by (3.2). The result (3.7) is sharp for a function of the form: with respect to 2 12,,, 1 3.8, Remark 1. Letting 1, 0, 0 in Corollary 3.2, we obtain a result was proved by [Gupta (1984), Theorem 3]. In a similar manner, Lemma 2.5 can be used to prove the following theorem: Theorem 3.3. Let the function be defined by (1.2) be in the class,,,,,. Then,,, where,,, is given by (3.2). Corollary 3.4. Let the function be defined by (1.2) be in the class,,,,,. Then 2 12,,, where,,, is given by (3.2). The result (3.11) is sharp for a function of the form:

8 24 Somia Muftah Amsheri Valentina Zharkova with respect to 2 12,,, , Distortion Properties Next, we state prove results concerning distortion properties of belonging to the classes,,,,,,,,,,. Theorem 4.1. Let,, such that 0, 1, Also, let defined by (1.2) be in the class,,,,,. Then,,,,,, 4.2,,,,,, 4.3 p1,,,,,, p 1,,,,,, for, provided that 0,0 01, where 1121 γ,,,,, 11 1 γ1 The estimates for are sharp Proof. We observe that the function,,, defined by (3.2) satisfy the inequality,,,,,,,, provided that 1. Thereby, showing that,,, is non-decreasing. Thus under conditions stated in (4.1), we have ,,,,,,, 4.7 For,,,,,, (3.5) implies,,,

9 For,, Lemma 2.4 yields So that (4.8) reduces to Consequently, 1 Subclasses of p-valent starlike functions γ 11 1 γ1,,,,, On using (4.11), (4.12) (4.10), we easily arrive at the desired results (4.2) (4.3). Furthermore, we note from (3.5) that,,, 1 12 which in view of (4.9), becomes γ 111γ ,,,,, 4.14 Thus, we have On using (4.15), (4.16) (4.14), we arrive at the desired results (4.4) (4.5). Finally, we can prove that the estimates for are sharp by taking the function

10 26 Somia Muftah Amsheri Valentina Zharkova γ 11 1 γ with respect to 1, This completes the proof of Theorem 4.1. Corollary 4.2. Let the function be defined by (1.2) be in the class,,,,,. Then the unit disk is mapped onto a domain that contains the disk, where 1121 γ γ1 The result is sharp with the extremal function defined by (4.17). Remark 2. Letting 1, 0, 0 in Theorem 4.1, we obtain a result was proved by [Gupta (1984), Theorem 4]. Theorem 4.3. Under the conditions stated in (4.1), let the function defined by (1.2) be in the class,,,,,. Then,,,,, z,,,,,,, p 1,,,,,, p 1,,,,,, for, provided that 0,0 01, where ,,,,, The estimates for are sharp Proof. By using Lemma 2.5, we have since,, the assertions (4.20), (4.21), (4.22) (4.23) of Theorem 4.3 follow if we apply (4.25) to (3.5). The estimates for are attained by the function 4.26

11 Subclasses of p-valent starlike functions 27 with respect to 1 1, This completes the proof of Theorem 4.3. Corollary 4.4. Let the function be defined by (1.2) be in the class,,,,,. Then the unit disk is mapped onto a domain that contains the disk, where The result is sharp with the extremal function defined by (4.26). 5. Further Distortion Properties We next prove two further distortion theorems involving generalized fractional derivative operator,,,. Theorem 5.1. Let 0; 1;, 1,. Also let the function be defined by (1.2) be in the class,,,,,.then,,,,, 21 γ 1 z 1 γ1,,, 1 γ 12 z,, 1 γ1 for,,, is given by (2.14). The results (5.1) (5.2) are sharp Proof. Consider the function Μ,,, defined by (2.13). With the aid of (4.7) (4.14) we find that Μ,,,,,, 21 γ 1 γ1 5.3

12 28 Somia Muftah Amsheri Valentina Zharkova Μ,,,,,, 21 γ 1 γ1 which yields the inequality (5.1) (5.2) of Theorem 5.1. Finally, by taking the function defined by,,,,, The results (5.1) (5.2) are easily seen to be sharp. 1γ 12 z 1γ Corollary 5.2. Let the function defined by (1.2) be in the class,,,,,. Then,,, is included in a disk with its centre at the origin radius given by 1 1 γ 12,, 1 γ1 5.6 Similarly we can establish the following result: Theorem 5.3. Let 0; 1;, 1,, let the function be defined by (1.2) be in the class,,,,,. Then,,,,,,,,,, 1 z 1 z for,,, is given by (2.14). The results (5.7) (5.8) are sharp. Corollary 5.4. Let the function defined by (1.2) be in the class,,,,,. Then,,, is included in a disk with its centre at the origin radius given by,, Remark 3. Letting 1, using the relationship (2.10) in Theorem 5.1, Corollary 5.2, Theorem 5.3, Corollary 5.4, we obtain the results which were proved

13 Subclasses of p-valent starlike functions 29 by [Srivastava Owa (1991b), Theorem 5, Corollary 3, Theorem 6, Corollary 4, respectively]. 6. Convexity Of Functions In,,,,, And,,,,, In view of Lemma 2.4, we know that the function defined by (1.2) is p-valent starlike in the unit disk if only if for,,,,,, we find from (3.5) (4.9) that 1,,,,, where,,,,, is defined by (4.6). Furthermore,,,,,,, we have 1,,,,, 6.3 where,,,,, is defined by (4.24). Thus we observe that,,,,,,,,,, are subclasses of p-valent starlike functions. Naturally, therefore, we are interested in finding the radii of convexity for functions in,,,,,,,,,,. We first state: Theorem 6.1. Let the function defined by (1.2) be in the class,,,,,. Then is p-valent convex in the disk, where inf 1,,,,,,,,,, is given by (4.6). The result is sharp. 6.4 Proof. It suffices to prove Indeed we have n

14 30 Somia Muftah Amsheri Valentina Zharkova Hence (6.5) is true if that is, if with the aid of (4.14), (6.8) is true if Solving (6.9) for, we get 1,,,,, 1,,,,, Finally, since is an increasing function for integers 1,, we have (6.5) for, where is given by (6.4). In order to complete the proof of Theorem 6.1, we note that the result is sharp for the function,,,,, of the form 1,,,,, 6.11 for some integers 1. Similarly we can prove the next theorem. Theorem 6.2. Let the function defined by (1.2) be in the class,,,,,. Then is p-valent convex in the disk, where inf 1,,,,, 6.12,,,,, is given by (4.24). The result is sharp for the function,,,,, of the form 1,,,,, for some integers

15 Subclasses of p-valent starlike functions Conclusion We have studied new classes,,,,,,,,,, of p-valent functions with negative coefficients defined by a certain fractional derivative operator in the unit disk. Many of the known results follow as particular cases from our results; see for example, Aouf Hossen (2006); Gupta (1984); Srivastava Owa (1991a) Srivastava Owa(1991b). We obtained the sufficient conditions for the function to be in,,,,,,,,,,. In addition, we derived a number of distortion theorems of functions belonging to these classes as well as distortion theorems for a certain fractional derivative operator of functions in the classes,,,,,,,,,,. At the end of this paper we have determined the radii of convexity for functions belonging to these classes Acknowledgments The authors thank the referee for some useful suggestions for improvement of the article. References Aouf, M. K. ; Hossen, H. M. (2006): Certain subclasses of p-valent starlike functions, Proc. Pakistan Acad. Sci. 43(2), pp Gupta, V. P. (1984): convex class of starlike functions, Yokohama Math. J. 32, pp Owa, S. (1978): On the distortion theorems. I, Kyungpook Math. J. 18, pp Owa, S. (1985): On certain classes of p-valent functions with negative coefficients, Bull. Belg. Math. Soc. Simon stevin, 59, pp Raina, R. K. ; Nahar, T. S. (2002): On certain subclasses of p-valent functions defined in terms of certain fractional derivative operators, Glasnik Matematicki, 37(1), pp Raina, R. K. ; Srivastava, H. M. (1996): A certain subclass of analytic functions associated with operators of fractional calculus, Comput. Math. Appl. 32, pp Partil, D. A. ; Thakare, N. K. (1983): On convex hulls extreme points of p-valent starlike convex classes with applications, Bull. Math. Soc. Sci. Math. R. S. Roumanie (N. S.), 27, pp Silverman, H. (1975): Univalent functions with negative coefficients, Proc. Amer. Math. Soc. 51, pp Srivastava, H. M. ; Karlsson, P. W. (1985): Multiple Gaussian hypergeometric series, Halsted Press (Ellis Horwood Limited, Chichester), Wiley, New York/Chichester/Brisbane/Toronto. Srivastava, H. M. ; Owa, S. (1987): Some applications of fractional calculus operators to certain classes of analytic multivalent functions, J. Math. Anal. Appl. 122, pp Srivastava, H. M. ; Owa, S. (Eds) (1989): Univalent functions, fractional calculus, their applications, Halsted Press/Ellis Horwood Limited/John Wiley Sons, New York/Chichester/Brisbane/Toronto. Srivastava, H. M. ; Owa, S. (1991a): Certain subclasses of starlike functions - I, J. Math. Anal. Appl. 161, pp

16 32 Somia Muftah Amsheri Valentina Zharkova Srivastava, H. M. ; Owa, S. (1991b): Certain subclasses of starlike functions - II, J. Math. Anal. Appl. 161, pp Srivastava, H. M. ; Owa, S. (Eds) (1992): Current topics in analytic function theory, World Scientific Publishing Company, Singapore, New Jersey London Hong Kong, 1992.