1. Preliminaries
Geometric Function Theory (GFT) is a vibrant and evolving area of complex analysis that continues to draw significant research interest. A key focus within GFT is the study of holomorphic univalent functions and their diverse subclasses. In recent years, bi-univalent function classes have gained considerable attention due to their intricate geometric properties and complex coefficient problems. More recently, Te-univalent function classes have been introduced, with initial studies addressing their coefficient estimates. These Te-univalent classes provide fresh opportunities to explore functional inequalities, refine coefficient bounds, and develop operator-based generalizations. This work contributes to the growing literature by introducing and analyzing new subfamilies of Te-univalent functions connected to q–Bernoulli polynomials.
Let
. Consider the class
of holomorphic functions
in
of the form
and define
as the set of functions
from
that are one-to-one (univalent) in
. In [
1], Bieberbach conjectured that for every function
, the coefficients satisfy
. In the effort to prove this conjecture, many new subclasses of
were introduced, and numerous significant results were obtained. Researchers devoted considerable effort to this problem over several decades, until it was ultimately resolved by Louis de Branges, who proved the conjecture for all
in [
2]. Another central problem in GFT is the Fekete–Szegö functional (FSF), given by
, which is studied for functions
[
3]. There’s been a bunch of research around this, especially on functions in various subclasses of
. One notable group among them is the bi-univalent functions, labeled
. This concept was introduced by Lewin in [
4]. A function
belongs to
if both
and its inverse
=
are univalent in the unit disk
; that is,
=
and
=
are both univalent in
}. According to the well-known Koebe theorem (see [
5]), every function
has an inverse, which is given by
satisfying
and
. The class
is non-empty, as evidenced by several functions that belong to it, such as
and
. However, functions like
,
, and the Koebe function
although members of the class
, do not belong to
. For a concise overview and insight into some properties of the class
, see [
6,
7,
8,
9]. A renewed interest in the study of this class was sparked by the influential paper by Srivastava and his collaborators [
10]. Following this work, numerous researchers have explored various intriguing subclasses of
; see [
11,
12,
13,
14,
15] and the references therein.
For the function
, define the operator
:
by
Let
denote the family of all univalent functions in
that are represented by (3). Every function
has an inverse
, given by
, and
,
, where
If both and are univalent in , then the function given by (1) is said to be Te-univalent in associated with the operator . Let denote the class of all Te-univalent functions in associated with , and are represented by (1). When , we have and if for some j, we have , where is given by (2).
Example illustrating non-emptiness of :
Consider the normalized Koebe function , which is a classical univalent function in . For the operator defined by (3), if the coefficients are chosen sufficiently close to 1, then is a small perturbation of . By the stability of univalence under small perturbations and standard distortion theorems, remains univalent in . Moreover, since the inverse function of the Koebe function is known explicitly and univalent on a certain disk, and acts linearly on coefficients, the inverse also remains univalent in a sufficiently small radius around zero, ensuring .
The convolution (or Hadamard product) of the function
defined by (1) and the function
given by
is defined by
(see [
16,
17]).
For complex parameters
and
, the generalized hypergeometric
where
and
is the shift factorial (or Pochhammer symbol) defined in terms of the Gamma function
, by
Corresponding to the function
defined by
Dziok and Srivastava [
18] introduced the operator
defined via the convolution (or Hadamard product) as follows:
where
.
If
is given by (
1), then we have
where
For simplicity of notation, we denote
. Let
denote the family of all functions represented by (
5) that are univalent in
. Every function
has an inverse
, defined by
, and
,
, where
A function is said to be Te-univalent in associated with the operator , if both and are univalent in . Let denote the family of all functions , which are Te-univalent in associated with .
Remark 1.
(i). For and we have
(ii). If for some j, we have , where ψ is given by (2). Polynomials like Bernoulli, Fibonacci, bivariate Fibonacci, Gegenbauer, Horadam, Lucas-Balancing, and Lucas-Lehmer, along with their various generalizations, hold a fundamental place across numerous fields in applied sciences and mathematics. These areas include combinatorics, computer science, engineering, number theory, numerical analysis, and physics. Their rich algebraic and analytic features have sparked extensive research, especially within GFT, where they help in constructing and studying subclasses of regular functions.
Given their extensive applications, numerous extensions and generalizations of these polynomial families have emerged in the literature. Among them,
q-Bernoulli polynomials, a q-calculus-based generalization of classical Bernoulli polynomials, have gained notable attention recently (see [
19]). Developed through the framework of
q-calculus [
20], these polynomials offer a versatile foundation for defining new classes of analytic functions. Their structural qualities enable the derivation of sharper coefficient bounds and functional inequalities, providing deeper insights into bi-univalent functions and related classes within the unit disk.
Within GFT, recent work has increasingly targeted subclasses of regular functions, particularly those in specific
-subclasses, that are subordinate to famous polynomial sequences. Over the past several decades, many authors have explored coefficient problems and functionals such as the FSF, for elements of these families (see [
21,
22,
23,
24,
25,
26]). These studies not only contribute to the theoretical framework of GFT but also open avenues for practical applications across mathematical modeling and signal processing.
A deeper mathematical motivation for employing Bernoulli polynomials emerges from their pivotal role in summation theory, exemplified by the Euler-Maclaurin summation formula. This classical formula establishes a vital connection between discrete summation and integral calculus, utilizing Bernoulli numbers and polynomials as fundamental coefficients. The comprehensive study by Leinartas and Shishkina [
27] further elucidates this point by analyzing the Euler-Maclaurin formula in the context of summation over lattice points within simplexes, thereby revealing the natural and fundamental appearance of Bernoulli polynomials in analytic and combinatorial problems. This important foundational role extends to their
q-analogues, providing strong motivation for integrating
q-Bernoulli polynomials into the framework of geometric function theory and the study of subclasses of Te-univalent functions.
Here, we review the q-derivative operator, a basic q-calculus tool that is important in many domains, including operator theory, computer science, quantum physics, hypergeometric series, and other related disciplines. We list important definitions and ideas below, presuming that .
Definition 1. Let Φ be a function from the complex plane to itself, i.e., Φ: . Then the q-derivative of Φ, denoted by , is defined byand , provided exists. The q-bracket number, denoted by , is given by . One can easily verify that and . Also, we note that and . So, as .
For a function
defined by (
1), we derive that
. The
q-exponential function
is defined as
,
. We note that
=
=
. The
q-exponential function
is a unique function that satisfies the condition
The
q-derivative operator
serves as a cornerstone in analyzing various subfamilies of holomorphic functions, underscoring its strong connection with GFT. The notion of
q-analogues of integral and derivative operators, along with their early applications, was first introduced by Jackson in [
20], laying the foundation for modern
q-calculus. Building upon Jackson’s work, Ismail et al. extended the framework by proposing a
q-analogue of the class of
q-starlike functions in [
28]. Later, the framework was broadened through the introduction of fractional
q-calculus operators, as presented in [
29].
These developments spurred extensive research exploring the applications of
q-calculus in GFT. For instance, the authors of [
29] defined the
q-analogue of the Ruscheweyh differential operator, thereby expanding the analytical toolkit available for studying geometric function classes. In [
30,
31], the
q-calculus framework was applied to multivalent functions, while Zhang et al. [
32] investigated
q-starlike functions associated with generalized conic domains using the convolution (Hadamard product) approach. Furthermore, the work in [
33] explored an operator constructed via the
q-hypergeometric function, deepening insights into the interplay between
q-calculus and analytic function theory.
Such advancements demonstrate how the q-derivative operator bridges classical and modern analytical structures, providing a versatile foundation for defining novel subclasses, establishing coefficient estimates, and formulating functional inequalities within the unit disk.
The
q-Bernoulii polynomials (see [
34])
, with
,
, and
j a non-negative integer, satisfy the following linear homogeneous recurrence relation for
:
with initial polynomials
and clearly
The generating function of the
q-Bernoulii polynomials
,
, is given as follows (see [
19,
35]):
The introduction of the deformation parameter q through q-Bernoulli polynomials offers a powerful and flexible framework for extending classical results in GFT. By enabling interpolation between well-established function classes and facilitating the construction of novel ones, these polynomials play a central role in the study of subordination, superordination, and operator-related problems. Their importance stems from their ability to define q-analogues of differential and integral operators, which underpin key analytic techniques in GFT. The rich analytic structure and intrinsic connections of q-Bernoulli polynomials to special functions, together with the adjustable parameter q, allow for sharper coefficient estimates, refined growth bounds, and enhanced geometric characterizations of analytic functions. Moreover, they serve as a conceptual bridge linking classical analysis to emerging areas such as quantum calculus and mathematical physics, thereby broadening the scope and impact of univalent function theory.
For
,
both analytic in
, the function
is said to be subordinate to
if there is a Schwarz function
, analytic in
, such that
and
, with the relation
. This subordination is denoted by
or
. In the special case where
, we have
In recent years, the study of Te-univalent function classes has witnessed significant advancements, with particular emphasis on generalized subclasses characterized by differential operators and subordination principles [
36,
37]. Among these, the class
has attracted considerable attention due to its remarkable ability to unify and extend numerous known subclasses through its parametric flexibility. Notably, recent investigations by Saravanan [
38] and Swamy et al. [
39] have focused on structured subclasses of
that are subordinate to specific polynomial sequences. This subordination approach substantially enriches the theory of analytic functions by providing sharper coefficient estimates, refined growth theorems, and precise distortion bounds. Such studies delve deeply into the geometric properties of these functions and open promising new directions for the analytical characterization of complex-valued functions.
Motivated by these developments, the present work introduces a novel subclass of , denoted by , consisting of functions subordinate to the q-Bernoulli polynomials. For this subclass, we establish rigorous bounds on the initial coefficients and investigate the Fekete–Szegö functional, advancing the existing theoretical framework. Through these contributions, this paper not only extends the landscape of Te-univalent function theory but also furnishes new analytical tools that leverage the intrinsic algebraic structure of q-Bernoulli polynomials.
Throughout this paper, we employ the GF
, with
,
,
j a non-negative integer and
, as defined in (
11) and the function
as defined in (
7). Unless otherwise specified, the parameter
is assumed to lie in
, and the indices satisfy
.
Definition 2. Let , , , and . If satisfiesandthen we say that . Remark 2. This definition introduces, for the first time in Te-univalent functions theory, a formulation involving a linear combination of two expressions where each term’s denominator is itself a linear combination of two distinct expressions. This innovative construction enables the blending of function classes that were traditionally studied independently, as exemplified in Cases 4 and 5. By unifying these previously separate classes within a single framework, this approach opens novel perspectives and is poised to stimulate renewed interest and further exploration among researchers in GFT. The resulting interplay enriches the analytical structure and broadens the scope for developing new subclasses and deriving more comprehensive coefficient estimates and geometric properties.
For particular choices of , , and , the class produces the following subfamilies of :
1. Let
. Then
,
,
, and
, is a family of functions
satisfying
and
2. Let
. Then
,
,
, and
, represents the set of functions
that meet
and
3. Let
. Then
,
,
, and
, is a family of functions
satisfying
and
4. Let
. Then
,
,
, and
is a family of functions
satisfying
and
5. Let
. Then
,
,
, and
is a family of functions
satisfying
and
In
Section 2, we derive estimates for the coefficients
,
, and the functional
for functions in the class
. In
Section 3, interesting outcomes from the results obtained for the defined class are presented, highlighting their connections to previously published findings.
3. Particular Cases
Using the above definition, we can obtain a number of subfamilies of associated with q-Bernoulli polynomials for specific parameters, such as , and . Consequently, the associated results are derived from the findings presented in the publication. We go over a few of these in this section.
Specializing Theorem 1 to the case , we obtain the following result:
Corollary 2. Let , , and . If is an element of the class , , thenandwhere If we take in Theorem 1, we arrive at the following corollary.
Corollary 3. Let , , and . If is an element of the class , , thenandwhere When is set to 1 in Theorem 1, the following corollary follows.
Corollary 4. Let , , and . If is a member of the class , thenandwhere Specializing Theorem 1 to , we obtain the following result:
Corollary 5. Let , , and . If is an element of the class , thenandwhere The important cases from the family corresponding to and are obtained as follows.
Case 1. Let
. Then
, and
is a family of functions
satisfying
where
.
Case 2. Let
. Then
,
, and
is a family of functions
satisfying
where
.
The initial coefficient estimates and FSF, for functions belonging to the families and are presented in the following two corollaries.
Corollary 6. Let , and . If is an element of the class , thenand for where Corollary 7. Let , and . If is an element of the class , thenandwhere Specializing Theorem 1 to , we obtain the following result:
Corollary 8. Let , , and . If is an element of the class , thenandwhereand V = . The important cases from the family corresponding to and are obtained as follows.
Case 3. Let
. Then
,
, and
is a family of functions
satisfying
where
.
Case 4. Let
. Then
,
, and
is a family of functions
satisfying
and
The initial coefficient estimates and the functional , for functions belonging to the families and are established in the following two corollaries.
Corollary 9. Let , and . If is an element of the class , thenand where Corollary 10. Let , and . If is an element of the class , thenandwhere Remark 4. When and , Corollaries 9, 10, and 4 reduce to Corollaries 3.1, 3.2, and 3.3, respectively, in the paper entitled “Certain subfamilies of Te-univalent functions subordinate to Bernoulli polynomials” by Swamy, Frasin, and Lupas, which has been submitted for publication.