- Original research
- Open access
- Published:
Estimation of initial Maclaurin coefficients of certain subclasses of bounded bi-univalent functions
Journal of the Egyptian Mathematical Society volume 27, Article number: 16 (2019)
Abstract
In this paper, two bounded bi-univalent function subclasses were defined by using Salagean q-differential operator. The functions are defined in the open unit disc of complex plane. The main purpose is to determine some estimations on the initial Maclaurin coefficients for functions in these subclasses. Finally, the Fekete-Szegö inequalities for these are also obtained.
Introduction
Let \(\mathcal {A}\) denotes the class of analytic functions of the form
normalized by the conditions \(f(0)=f^{^{\prime }}(0)-1=0\), which are defined on the open unit disc \(U=\left \{ z\in \mathbb {C}:\left \vert z\right \vert <1\right \}\). Let \(\mathcal {S}\) be the subclass of \(\mathcal {A}\) consisting of all functions of the form (1) which are univalent in U. In the geometric function theory, there are two important subclasses of \(\mathcal {S}\), which are the well-known subclasses of starlike and convex functions, namely, \(\mathcal {S^{*}}\) and \(\mathcal {K}\), for which the inequalities Re{zf′(z)/f(z)}>0 and Re{1+zf′′(z)/f′(z)}>0(z∈U) are the sufficient conditions, respectively (see [1], Ch.8). An analytic function f is subordinate to an analytic function g, written f(z)≺g(z), provided there exist an analytic function w defined on U with w(0)=0 and |w(z)|<1 satisfying f(z)=g(w(z)) (see [2]). Ma and Minda [3] gave a unified representation of various subclasses of starlike and convex functions by introducing the classes \(\mathcal {S^{*}}(\varphi)\) and \(\mathcal {K}(\varphi)\) of functions \(f\in \mathcal {S}\) satisfying zf′(z)/f(z)≺φ(z) and 1+zf′′(z)/f′(z)≺φ(z)(z∈U), respectively, where φ is an analytic function with positive real part in the unit disc U, φ(0)=1, φ′(0)>0, and φ maps U onto a region starlike with respect to 1 and symmetric with respect to the real axis. The classes \(\mathcal {S^{*}}(\varphi)\) and \(\mathcal {K}(\varphi)\) include several well-known subclasses as special case. For example, when φ(z)=(1+Az)/(1+Bz)(1≤B<A≤1), the classes \(\mathcal {S^{*}}(\varphi)\) and \(\mathcal {K}(\varphi)\) are reduced to the subclasses \(\mathcal {S^{*}}[A,B]\) and \(\mathcal {K}[A,B]\), which were introduced by Janowski [4]. For 0≤β<1, the classes \(\mathcal {S^{*}}(\beta)= \mathcal {S^{*}}((1+(1-2\beta)z)/(1-z))\) and \(\mathcal {K}(\beta)= \mathcal {K}((1+(1-2\beta)z)/(1-z))\) are subclasses of starlike and convex functions of order β (see [1], Ch.9), \(\mathcal {S^{*}}:=\mathcal {S^{*}}(0)=\mathcal {S^{*}}((1+z)/(1-z))\) and \(\mathcal {K}:=\mathcal {K}(0)=\mathcal {K}((1+z)/(1-z))\). Moreover, the subclasses of strongly starlike and strongly convex functions of order α(0≤α<1) can be obtained by \(\mathcal {S^{*}_{\alpha }}:=\mathcal {S^{*}}({\left ((1+z)/(1-z)\right)}^{\alpha })\) and \(\mathcal {K_{\alpha }}:=\mathcal {K}({\left ((1+z)/(1-z)\right)}^{\alpha })\) (see [5]).
The Koebe one quarter theorem ensures that the image of U under every univalent function \(f\in \mathcal {S}\) contains a disk of radius \(\frac {1}{4}\) (see [6]). Thus, every univalent function f has an inverse f−1 satisfying
A function \(f\in \mathcal {S}\) is said to be bi-univalent in U if both f and f−1 are univalent in U. Let Σ denotes the subclass of \(\mathcal {S}\), consisting of all bi-univalent functions defined on the unit disc U. Since f∈Σ has the Maclaurin series expansion given by (1), a simple calculation shows that its inverse g=f−1 has the series expansion
Examples of functions in the class Σ are
and so on. However, the familiar Koebe function is not a member of Σ. Other common examples of functions in \(\mathcal {S}\) such as
are also not members of Σ (see [7]). Several authors have introduced and investigated subclasses of bi-univalent functions and obtained bounds for the initial coefficients (see [7–11]). The research into Σ was started by Lewin ([9]). It focused on problems connected with coefficients. Many papers concerning bi-univalent functions have been published recently. A function f∈Σ is in the class \(\mathcal {S}^{*}_{\Sigma }(\beta)\) of bi-starlike function of order β(0≤β<1), or \(\mathcal {K}_{\Sigma }(\beta)\) of bi-convex function of order β if both f and f−1 are respectively starlike or convex functions of order β. For 0≤α<1, the function f∈Σ is strongly bi-starlike function of order α if both the functions f and f−1 are strongly starlike functions of order α. The class of all such functions is denoted by \(\mathcal {S}^{*}_{\Sigma,\alpha }\). These classes were introduced by Brannan and Taha [8], they obtained estimates on the initial coefficients a2 and a3 for functions in these classes. We owe the revival of these topics to Srivastava et al. ([7]). The investigations in this direction have also been carried out, among others, by Ali et al. [12], Frasin and Aouf [13]. Hamidi and Jahangiri (e.g., [14]) have revealed the importance of the Faber polynomials in general studies on the coefficients of bi-univalent functions. In fact, little is known about exact bounds of the initial coefficients of f∈Σ. For the most general families of functions given by (1), we know that |a2|<1.51 for bi-univalent functions ([9]), |a2|≤2 for bi-starlike functions (Kedzierawski [15]), and |a2|<1 for bi-convex functions ([8]). Only the last estimate is sharp, equality holds only for f(z)=z/(1−z) and its rotations.
In this study, we are concerned with a different type of classes of bi-univalent functions, which are of the bounded type. A bounded function classes was firstly introduced and discussed by Singh [16]. Singh and Singh [17] introduced a bounded starlike and convex function classes \(\mathcal {S}^{*}_{M}\) and \(\mathcal {K}_{M}\), respectively, these were followed by the subclasses \(\mathcal {S}^{*}_{M}(\alpha)\) and \(\mathcal {K}_{M}(\alpha)\) represented a bounded starlike and convex function of order α, respectively.
The q-difference operator, which was introduced by Jackson [18], and may go back to Heine [19], is defined by
and
For the function f(z) denoted by (1), we have
where
For function f∈Σ given by (1) and \(n\in \mathbb {N},0\leq q<1\), Salagean q-differential operator \(D_{q}^{n}\), introduced by Govindaraj and Sivasubramanian [20] (see also [21]), defined by
For the functions f(z) and g(w) denoted by (1) and (3), we have
In this present work, we introduce two bounded subclasses of Σ associated with Salagean q-differential operator and obtain the initial Maclaurin coefficients |a2| and |a3| for these function classes. Also, we give bounds for the Fekete-Szegö functional \(|a_{3} -\mu a^{2}_{2}|\) for each subclass.
Estimations of |a 2| and |a 3|
The main results in this section is to define two bounded subclasses of the class Σ, then some estimations of the first two Maclaurin coefficients of functions belonging to those subclasses were calculated.
Definition 1
For \(0\leq \lambda <1,b\in \mathbb {C}^{\ast }\) and \(M>\frac {1}{2},\) let \(\mathcal {S}_{\Sigma,q}^{n}(\lambda,b,M) \) be the subclass of Σ consisting of functions of the form (1) and satisfying the following condition
and
where z,w∈U, and g=f−1∈Σ is given by (3). Also, let \(\mathcal {C}_{\Sigma,q}^{n}(\lambda,b,M)\)be the subclass of Σ consisting of functions of the form (1) and satisfying the following condition
and
where z,w∈U, and g=f−1∈Σ is given by (3).
It is clear that
Lemma 1
Let \(m=1-\frac {1}{M}\left (M>\frac {1}{2}\right)\), f defined by (1) and g=f−1, then we have
and also,
where
Lemma 2
(see [22]) If \(h\in \mathcal {P}\), then |cn|≤2 for each \(n\in \mathbb {N}\), where \(\mathcal {P}\) is the family of all functions h which is analytic in U for which Re{h(z)}>0, where h(z)=1+c1z+c2z2+... for z∈U.
Remark 1
In Definitions 1, 2 and for special choices of the parameters λ,b,M, also, taking q→1−, then we can obtain the following subclasses:
which were introduced by Murugusundaramoorthy et al. [23].
which was introduced by Srivastava et al. [24].
which are the classes of bi-starlike and bi-convex functions introduced by Brannan and Taha [8].
Theorem 1
Let f given by (1) be in the subclass \(\mathcal {S}_{\Sigma,q}^{n}(\lambda,b,M)\). Then
and
where
Proof
Let \(f\in \mathcal {S}_{\Sigma,q}^{n}(\lambda,b,M)\) and g=f−1. Then, it satisfy the conditions (13). By the definition, there exist two analytic functions u,v:U→U with u(0)=v(0)=0 and |u(z)|<1,|v(w)|<1 for all z,w∈U satisfying
and
Now, define the two functions p(z) and q(z) by
It is equivalent to
Then p(z) and q(z) are analytic in U with p(0)=1=q(0). In view of Janowski [4], Since u,v:U→U, the functions p(z),q(z)∈P(M) and have a positive real part in U where P(M) is the class of all function ψ(z)=1+δ1z+δ2z2+... which are analytic in U and satisfy the condition
Therefore, in view of the Lemma 1, we have
By substituting from (7), (8), (17), and (18) into (15)and (16), we obtain
which yields the following relations
and
By adding (23) to (25) then use (27), we obtain
applying Lemma 2 to the coefficients p2 and q2, we conclude
By subtracting (25) from (23), we have
by substituting from (26) and (27) into (29), we conclude
Finally, by applying Lemma 2 to the coefficients p1,p2,q1 and q2, we conclude
The proof is completed. □
For n=0,b=1−β,m=1, and q→1−, we obtain the bounds corresponding to the class MΣ(β,λ) given by Murugusundaramoorthy et al. [23].
Corollary 1
Let f given by (1) be a function in the class MΣ(β,λ), then
and
Additionally, put λ=0, we obtain bounds of the class of bi-starlike function of order β donated by \(\mathcal {S}_{\Sigma }(\beta)\).
Corollary 2
[8] Let f given by (1) be in the class \(\mathcal {S}_{\Sigma }(\beta)\), then
and
Theorem 2
Let f given by (1) be in the subclass \(\mathcal {C}_{\Sigma,q}^{n}(\lambda,b,M)\). Then
and
where
Proof
Let \(f\in \mathcal {C}_{\Sigma,q}^{n}(\lambda,b,M) \) and g=f−1. Then, it satisfy the conditions (14). By the definition, there exist two analytic functions u,v:U→U with u(0)=v(0)=0 and |u(z)|<1,|v(w)|<1 for all z,w∈U satisfying
and
Now, define the two functions r(z) and s(z) by
It is equivalent to
Then r(z) and s(z) are analytic in U with p(0)=1=q(0). Since u,v:U→U, the functions r(z) and s(z) have a positive real part in U. Therefore, in view of the Lemma 2, we have
By following the same steps in proving Theorem 1, we can complete the proof of this theorem. □
For n=0,b=1−β,m=1,λ=0 and q→1−, we obtain the bounds corresponding to the class \(\mathcal {K}_{\Sigma }(\beta)\) given by Brannan and Taha [8].
Corollary 3
Let f given by (1) be in the class \(\mathcal {K}_{\Sigma }(\beta)\), then
and
Fekete-Szegö inequalities
Fekete and Szegö [25] introduced the generalized functional \(|a_{3} -\mu a_{2}^{2}|\) where μ is some real number. In this section, we obtain the Fekete-Szegö inequality for the functions belonging to the classes \(\mathcal {S}_{\Sigma,q}^{n}(\lambda,b,M)\) and \(\mathcal {C}_{\Sigma,q}^{n}(\lambda,b,M)\). Before establishing our results, we need the following Lemma introduced by Zaprawa [11].
Lemma 3
Let \(k,l\in \mathbb {R}\) and \(p_{1},p_{2}\in \mathbb {C}\). If |p1|,|p2|<R, then
Theorem 3
Let f given by (1) be in the class \(\mathcal {S}_{\Sigma,q}^{n}(\lambda,b,M)\) and \(\mu \in \mathbb {R}.\) Then
where
Proof
Using (29), we can write
where
Therefore, by applying Lemma 2 to the coefficients p2 and q2 which obtain
Thus, by applying Lemma 3 into (37), we conclude
which completes the proof. □
For n=0,b=1−β,m=1 and q→1−, we obtain bounds of the Fekete-Sezgö inequality of the class MΣ(β,λ) given by Zaprawa [11].
Corollary 4
Let f given by (1) be in the class MΣ(β,λ), then
Additionally, put λ=0, we obtain Fekete-Sezgö inequality of the class \(\mathcal {S}_{\Sigma }(\beta)\).
Corollary 5
Let f given by (1) be in the class \(\mathcal {S}_{\Sigma }(\beta)\), then
Theorem 4
Let f given by (1) be in the class \(\mathcal {C}_{\Sigma,q}^{n}(\lambda,b,M)\) and \(\mu \in \mathbb {R}.\) Then,
where
Proof
Just as we derived Theorem 3, we can deduce Theorem 4, so we choose to omit the proof □
Corollary 6
Let f given by (1) be in the class \(\mathcal {K}_{\Sigma }(\beta)\), then
References
Goodman, A. W.: Univalent functions, Vol. I, Mariner, Tampa Florida (1983).
Miller, S. S., Mocanu, P. T.: Differential subordinations: theory and applications. Marcel Dekker Inc., New York (2000).
Ma, WC, Minda, D: A unified treatment of some special classes of univalent functions. In: Proceedings of the Conference on Complex Analysis, pp. 157–169 (1992).
Janowski, W.: Extremal problems for a family of functions with positive real part and for some related families. Annales Polonici Mathematici. 23, 159–177 (1970).
Brannan, D. A., Kirwan, W. E.: On some classes of bounded univalent functions. J. Lond. Math. Soc. 2(1), 431–443 (1964).
Duren, P. L.: Univalent Functions, New York, Berlin. Springer-Verlag, Heidelberg and Tokyo (1983).
Srivastava, H. M., Mishra, A. K., Gochhayat, P.: Certain subclasses of analytic and bi-univalent functions. Appl. Math. Lett. 23(10), 1188–1192 (2010).
Brannan, D. A., Taha, T. S.: On some classes of bi-univalent functions, studia universitatis babes-bolyai mathematica. 31(2), 70–77 (1986).
Lewin, M.: On a coefficient problem for bi-univalent functions. Proc. Am. Math. Soc. 18, 63–68 (1967).
Li, X. -F., Wang, A. -P.: Two new subclasses of bi-univalent functions. Int. Math. Forum. 7(30), 1495–1504 (2012).
Zaprawa, P.: Estimates of Initial Coefficients for Bi-Univalent Functions. Abst. Appl. Anal., 6 (2014). Article ID 357480.
Ali, R. M., Lee, S. K., Ravichandran, V., Supramaniam, S.: Coefficient estimates for bi-univalent Ma-Minda starlike and convex functions. Appl. Math. Lett. 25(3), 344–351 (2012).
Frasin, B. A., Aouf, M. K.: New subclasses of bi-univalent functions. Appl. Math. Lett. 24(9), 1569–1573 (2011).
Hamidi, S. G., Jahangiri, J. M.: Faber polynomial Coefficient estimates for analytic bi-close-to-convex functions. Comptes Rendus Mathematique. 352(1), 17–20 (2014).
Kedzierawski, A. W.: Some remarks on bi-univalent functions, Annales Universitatis Mariae Curie-Sklodowska. Sect. A–Math. 39, 77–81 (1985).
Singh, R.: On a class of starlike functions. Compos. Math. 19(1), 78–82 (1968).
Singh, R., Singh, V.: On a class of bounded starlike functions. Indian J. Pure Appl. Math. 5, 733–754 (1974).
Jackson, F. H.: On q-functions and a certain difference operator. Trans. Royal Soc. Edinb. 45, 253–281 (1909).
Heine, E.: Handbuch der Kugelfunctionen. Theorie und Anwendungen. 1 (1878).
Govindaraj, M., Sivasubramanian, S.: On a class of analytic function related to conic domains involving q-calculus. Anal. Math. 43(3), 475–487 (2017).
El-Qadeem, A. H., Mamon, M. A.: Comprehensive subclasses of multivalent functions with negative coefficients defined by using a q-difference operator. Trans. Razmadze Math. Inst. 172(3), 510–526 (2018).
Pommerenke, C.: On univalent functions. Math. Ann. 236, 199–208 (1978).
Murugusundaramoorthy, G., Magesh, N., Prameela, V.: Coefficient bounds for certain subclasses of bi-univalent function. Abstr. Appl. Anal., 3 (2013). Article ID 573017.
Srivastava, H. M., Gaboury, S., Ghanim, F.: Coefficient estimates for some general subclasses of analytic and bi-univalent functions. Afrika Matematika. 28(5), 693–706 (2017).
Fekete, M., Szegö, G.: Eine Bemerkung über ungerade schlichte Functionen. J. Lond. Math. Soc. 8, 85–89 (1933).
Acknowledgements
Not applicable.
Funding
Not applicable.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Author information
Authors and Affiliations
Contributions
AHE collected the data regarding the previous articles about subclasses of bi-univalent functions, then choosing the bounded functions to investigate. MAM performed the calculations and was a major contributor in writing the manuscript. Both authors read and approved the final manuscript.
Corresponding author
Ethics declarations
Competing interests
The authors declare that they have no competing interests.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
About this article
Cite this article
El-Qadeem, A., Mamon, M. Estimation of initial Maclaurin coefficients of certain subclasses of bounded bi-univalent functions. J Egypt Math Soc 27, 16 (2019). https://doi.org/10.1186/s42787-019-0015-z
Received:
Accepted:
Published:
DOI: https://doi.org/10.1186/s42787-019-0015-z
Keywords
- Analytic function
- Starlike function
- Convex function
- Subordination
- Bi-univalent function
- Fekete-Szegö inequalities
- Bounded function