Abstract
A review of the literature demonstrates that the Fox–Wright function is not only a mathematical puzzle, but its role is naturally to represent basic physical phenomena. Motivated by this fact, we studied a new representation of this function in terms of complex delta functions. This representation was useful to compute its Laplace transform with respect to the third parameter γ for which it also generalizes the one and two-parameter Mittag-Leffler functions. New identities involving the Fox–Wright function were discussed and used to simplify the results. Different fractional transforms were evaluated and the solution of a fractional kinetic equation was obtained by using its new representation. Several new properties of this function were discussed as a distribution.
Keywords:
Fox–Wright function; Mittag-Leffler function; fractional images; H-function; kinetic equation MSC:
44A20; 26A33
1. Introduction and Motivation
Developments in environmental sciences are significantly influenced by contemporary gas theories and astrophysics. Differential equations are important to model the evolution of stars such as the sun, playing a significant role in studies on global warming [1]. The entire inner structure of stars is formed of gases, which are described by three properties: mass, temperature and pressure. In actuality, a cloud needs a rather more powerful gravitational force, as compared to its internal pressure, in order to become a star. Nuclear fusion takes place in the cloud, producing light, as a protostar develops as the result. Equations of state, translucence, and nuclear energy production rate serve as the foundation for mathematical models and constructions. Nuclear processes in such stars are the source of energy. Therefore, the reaction rate for each type of generation and devastation describes the way in which the kinetic equation characterizes the change in the chemical composition of stars. In order to investigate this composition (t) using the production and destruction rate, Haubold and Mathai proposed the following basic kinetic equation [1]:
where is defined by By neglecting the spatial variation and inhomogeneity in (t) with respect to species concentration, the following are obtained: , and
Next, performing integration on this equation and ignoring subscript leads to the following:
By means of the Riemann–Liouville (R–L) fractional integral , we can obtain the following non-integer-order kinetic equation:
where is a constant. Following this, we have the following fractional kinetic equation [1,2,3] involving a general integrable function f(t):
A review of the literature reveals that there is no single equation addressing the integration with respect to the third parameter of a Fox–Wright function . More recently, Giusti et al. [4] beautifully described the key results and applications emerging from the three-parameter generalization of the Mittag-Leffler function in connection with a special Fox–Wright function. For example, this function is important to model and study anomalous relaxation in dielectrics [5], linear viscoelasticity [6], renewal processes [7], stochastic processes and diffusion [8]. Hence, there is a natural need for the fractional calculus of such functions.
Taking motivation from these facts, we present this research article, which is organized as follows. Basic preliminaries and required definitions are given in the next section, Section 2. Distributional representation of a Fox–Wright function and its application to the fractional kinetic equation are presented in Section 3. New fractional calculus formulae or identities involving a Fox–Wright function are given in Section 3.1 and Section 3.2. Existence of a new representation and its distributional properties are discussed in Section 4. Further applications of the new representation are discussed in Section 5. The conclusion and future directions are included in the last section, Section 6. Throughout the article, stands for the real portion of any complex number, whereas stands for complex numbers and denotes the reals. is a set of negative integers that contains 0, while is a set of positive reals.
2. Preliminaries
2.1. Special Functions and Fractional Integral Transforms
The gamma function is the generalization of a factorial and is considered as a basic special function, defined as [9]
This is a well-studied special function with tremendous applicability and representations; the basic Pochhammer symbols are defined by
Magnus Gustaf Mittag-Leffler [10] suggested a function defined by
which appears to be a natural replacement for the exponential function. The Mittag-Leffler function has numerous generalizations. For instance, the definition of the two-parameter Mittag-Leffler functions is
and three variables provide a definition of the generalized Mittag-Leffler function or, in fact, a special Fox–Wright function given by
which was also studied by Prabhakar [11]. Note that is an entire function [12] of order ρ = 1/ and type σ = 1. Furthermore, Fox–Wright and Fox–H functions [13] are related to the generalized Mittag-Leffler function as follows:
These relations are important in order to study the asymptotic behavior of the generalized Mittag-Leffler function. Here (and following), an -function [13] is defined by
where, is a suitable contour of Mellin–Barnes type, which splits up the poles of and . Considering in Equation (7), we obtain Meijer -function [13]:
However, an -function [13] has the following connection with a Fox–Wright function :
It is further related with the hypergeometric and other special functions [9] as follows:
The Kiryakova fractional transforms ((multiple) E–K integral operators), as defined in [14] (p. 8, Equation (18)), are
Order of integration is expressed by , and are taken as weights, while are accompanying parameters. Since becomes zero when , the upper limit as infinity is approached becomes meaningless in Equation (11). The corresponding Kiryakova’s fractional derivative ((multiple) E–K derivative operators) of Riemann–Liouville (R–L) form, having multi-order , is well defined by [14] (p. 9)
where , is a polynomial of variable of degree , given by
and the corresponding Kiryakova’s fractional derivative ((multiple) E–K derivative operators) in Caputo sense is expressed by [14] (p. 9, as well as the related references within)
The following action of Kiryakova’s fractional transform [14] (p. 9, Equation (27)) is significant for this research:
Relationship of the kernels of different fractional operators and the (multiple) E–K operators is listed in Table 1.
Table 1.
Significant special cases of (multiple) E–K operators [14,15,16,17,18].
2.2. Special Functions and Theory of Distributions
Generalized functions (also known as distributions) constitute continuous linear functionals on a specific set of test functions, whereas the distribution space is the inverse (or dual) to the space of test functions [19] (Volume I–V) and [20]. Gelfand and Shilov [19] provided a thorough study and explanation of such spaces. The most frequently mentioned test functions are compact support, denoted by —its dual space is . The convergent integral can be used to construct distributions that correspond to a locally integrable function and test function as follows:
These types of distributions are known as regular distributions. Singular functions are significant entities because they are included in the class of generalized functions (or distributions). As a result, their definition and other operations of calculus lead to functions. Furthermore, delta function is the most popular distribution, defined by
and This is one of the best instances of a singular distribution because it behaves as a continuous linear functional on a set of test functions and cannot be constructed from a locally integrable function. Assuming this is true, then
But, for , the above equation becomes
if we assume is a locally integrable function. However, in that case, the Lebesgue’s general theory of convergence demonstrates that Equation (20) converges to zero as , leading to an inconsistency.
An infinitely differentiable class of test functions that is closed under Fourier transformation includes rapidly decaying functions denoted by , with as their dual space. In actuality, the Fourier transforms of the functions of the abovementioned dual space are not its elements, but rather they belong to a different space which consists of complex functions. It is significant to note that Fourier transforms of belong to . Furthermore, an entire function does not vanish in only a particular interval , which provides the following inclusion:
And, for and ,
This involves the constants and determined by , and is the imaginary part of . Similarly, the derivatives of the delta function
act as a singular distribution. For example, the electromotive force in an electric circuit [20] (p. 164) is given by
Fourier transforms of commonly used trigonometric and hyperbolic functions such as , , , and are also delta functions (singular distributions) [19] (Volume 1). Fourier transform of the exponential function [19] (Volume 1, p. 169, Equation (8))
Is an element of such that, for [19] (p. 159, Equation (4)); see also [20] (p. 201, Equation (9)),
leads to the following expansion, given in [19] (p. 160, Volume I):
The convolution of delta function with a suitable function yields
and
For this research, we consider [21] (Equation (2.13))
Based on this, we developed many new and novel results. Prior to this, Chaudhry and Qadir [22] obtained the following representation of a gamma function:
This can also be obtained by putting in Equation (30), which is also modified and generalized by Tassaddiq [23,24,25] as follows:
For further similar studies of other special functions, the interested reader is referred to [26,27] and references therein.
Unless otherwise mentioned in this article, the values of the parameters shall be deemed normal, as specified in Section 2.
3. New Representation of a Fox–Wright Function with Application to the Fractional Kinetic Equation
This section contains the distributional representation of a Fox–Wright function as a series involving the complex delta function [19,20]. This is extremely useful in computing the Laplace transform of this function with respect to the third parameter , which results in the solution of the new integral equation involving it.
Theorem 1.
A Fox–Wright function has a new representation, given as
Proof.
This follows from the modification of (30), given as
The stated form can thus be obtained by inserting (34) in (30) and using (5). □
Corollary 1.
A Fox–Wright function has a new representation, which is given as
Proof.
By using (27) in (33),
Thus, the stated form is obtained. □
The gamma function findings can be retrieved in Equations (33) and (35) by using .
As a result, it is clear that the mathematical notions and facts concerning the delta function exist for a Fox–Wright function in relation to the new representation. This sheds new light on more new results in various directions. For example, by using [20] (p. 227) for one can obtain
Taking this provides
And, by using the relation (5),
Moreover, one can compute that
For fractional calculus, many researchers have made substantial contributions [28]. Prior research on many generic families of fractional kinetic equations has been performed in the literature [29]. In contrast to the many multi-parameter extensions of the Mittag-Leffler function and the Hurwitz–Lerch function, Srivastava examined far more general functions in [30,31]. In particular, kinetic equations of fractional order seem to have obtained interest, due to the recent uncovering of their relationship with the theory of continuous-time random walks [32]. These equations are being explored, with the goal of first determining and then interpreting certain physical effects known to regulate processes that include diffusion in porous media, anomalous propagation, and so on. A review of the literature revealed that no such equation incorporating the integration of the Fox–Wright function with regard to its third parameter has ever been created. The primary objective of this section is to pose and address this issue.
Theorem 2.
Following non-integer order kinetic equation with respect to the third parameter of a Fox–Wright function,
leads to the following solution:
Proof.
Let us begin by applying the Laplace transform (see [1,2]) to both sides of (41):
in which
Then, by employing (37),
expresses the above Equation (46) as follows:
One can determine the result after a simple computation, stated as
Furthermore, let
and use
to calculate (the inverse Laplace transform) of (48), given by
Using (4) in (49), we can finally obtain (42). □
Remark 1.
It is remarkable that the solution approach is traditional [1,2], and the response rate is a function of the fractional parameter . Typically, it is described in terms of the Mittag-Leffler function, as seen in the preceding solution. As a result, the sum over the coefficients
in (42) is clearly defined and finite:
In the same way,
3.1. New Fractional Image Formulae Involving a Fox–Wright Function
Lemma 1.
Using the definition of a Fox–Wright function, we demonstrate that the following identity is accurate:
Proof.
Using Equation (37), we obtain the following:
then
The necessary result is, therefore, established from both of the aforementioned Equations (53) and (54). □
Remark 2.
It should be noted that a general result can be deduced from (52) as follows:
Theorem 3.
The Kriyakova’s fractional transform involving the Fox–Wright function is computedas
Proof.
Consider the following:
The summation and integration are then exchanged:
which, after using (16), gives
and, by making use of Equation (9) in Equation (59), gives the subsequent result
As a result, applying Remark 2 yields the appropriate simplified form. □
Important special cases of Equation (56) are listed in Table 2.
Table 2.
Formulae for fractional integrals containing a Fox–Wright function.
3.2. Generalized Fractional Derivatives Involving a Fox–Wright Function
We can derive the generalized fractional derivatives involving a Fox–Wright function by applying Theorem 1’s technique and the new representation of a Fox–Wright function. Here, we directly derive them using the general result [14] (Theorem 4), stated as
Applying Kriyakova’s fractional derivatives (Multiple E–K fractional derivatives) (61) on (37) and then using (52) yields generalized fractional derivatives involving a Fox–Wright function:
Further related cases of Equation (62) are listed in Table 3.
Table 3.
Formulae for fractional derivatives containing a Fox–Wright function.
4. Convergence of New Series Representation as a Distribution
The new series representation of a Fox–Wright function is established using a delta function, which is significant if it is correctly specified in terms of the distributional concept. As a result, it is exciting to show that the given representation is a distribution (generalized function) on space , as stated in the following theorem.
Theorem 4.
A Fox–Wright function acts as a distribution over the space .
Proof.
Consider the following combination, by taking
and :
After that, choose any sequence,
and, making use of ,
To study the convergence of new representations, consider the following:
where
Consequently, Equation (66) displays that is convergent as a product of functions that increase slowly and diminish quickly. The Abel theorem can also be used to confirm this. As a result, a Fox–Wright function behaves as a distribution over □
The following example is used to better understand the preceding subject in the sense of generalized functions [20] by using the shifting property of a delta function.
Example 1.
Let ; then,
For , this leads to
These results provide new perspectives on the existence of further similar results; for example, considering in Equation (68), one can derive the Laplace transform of .
4.1. Validity of the New Generalized Representation
The primary goal in this section is to validate the stability of the new identities achieved through novel representation. Taking in [21] (Equation (2.1)) and using (5), the Fourier transform representation of a Fox–Wright function is given as
The Fourier transform preserves the duality property; hence, for any function
The following result is obtained by applying this characteristic to Equation (70):
The aforementioned identity’s corresponding form is given as
This can also be accomplished as a special instance of the main identity (68) by putting . These particulars demonstrate that the findings of this novel representation are consistent with those produced by traditional methods. Additionally, by taking in (73), the following formula can be obtained:
This certifies the authenticity of a new representation, resulting in new identities that are unreachable through conventional procedures. However, individual instances of these unexpected consequences are consistent with previous findings. These identities are easily accomplished by employing the Fourier transform, and proving that this is a more efficient method for validating new representations.
4.2. New Properties of a Fox–Wright Function as a Distribution
Following the concepts and approach in [20] (pp. 199–207, Chapter 7), new distribution features are provided here. These properties hold for a Fox–Wright function due to its new formulation in terms of the delta function [19,20].
Theorem 5.
A special Fox–Wright function has the following characteristics as a generalized function (distribution) for an arbitrary test function, in which , are arbitrary real or complex constants:
- (a)
- The combined effect of a Fox–Wright function and any distribution is:
- (b)
- A Fox–Wright function multiplied by an arbitrary constantgives the following:
- (c)
- An arbitrary complex constantis used to shift a Fox–Wright function:
- (d)
- A Fox–Wright function is transposed as:
- (e)
- The independent variable multiplied by a positive constant :
- (f)
- Differentiating a Fox–Wright function as a distribution:
- (g)
- A special Fox–Wright function’s distributional Fourier transform:
- (h)
- The Fourier transform’s duality property:
- (i)
- The Fourier transform and Parseval’s identity:
- (j)
- Differentiation characteristics of the Fourier transform:
- (k)
- A Fox–Wright function’s Taylor series:
- (l)
- A Fox–Wright function has the property of convolution:
- (m)
- If is a bounded support distribution, then:
Proof.
The approach of Theorem 4 and the attributes of the delta function can be used to achieve Results (a)–(e). Similarly, Equation (13) is used to show result (f):
It is a convergent sum of fast-decaying and slow-growing functions (as claimed and demonstrated in Theorem 4). The demonstration of outcomes (g)–(k) can also be obtained by using the delta function properties for the Fourier transform. As a result, the following is a confirmation of result (g):
Likewise, Parseval’s identity of Fourier transform is established, given as
The result (i) can be proven by considering
and so forth; this results in
Equation (14) allows for the following proof of outcome number (j), which is
This produces the desired outcome. Next, using Equation (16), which is also clarified by the following example [20] (p. 207), result (k) can be shown.
Example 2.
Let
then,
The definition of and can also be used to further compute the following identities:
Next, result (l) is proven by using the fact that Fourier and inverse Fourier transformations are continuous linear functional from to [20] (p. 203) and, therefore, in view of Equation (70). Hereafter, in light of Theorem 7.9.1, as presented and proven in [20] (p. 206), the proof of result (m) is finished. The example that follows helps to further illustrate this.
Example 3.
Consider the following distribution
of bounded support:
which, because of the novel representation, is an advantageous identity. □
Then, based on the information provided above, we obtain
5. Further Applications and Discussion
The convergent behavior of with slowly rising functions is the subject of the discussion above, but the sum may converge for a wide range of entities. As a result, the new series representation is convergent for every ; nevertheless, due to the definition of the delta function, this infinite series is well defined over a wider set of functions. Here is an additional debate that serves this goal. Every function is mapped to its value at zero by the linear Dirac delta function. Consequently, using (35), one can compute the following new identities for a real :
Example 4.
Let
then, if ,
Example 5.
Let ; then, for ,
where
denotes the imaginary part of the complex number. Similarly, if , then, when ,
It follows that, for the new representation of a special Fox–Wright function, the mathematical notions and facts related to delta functions actually exist. This sheds new light on more novel outcomes in various directions. In Section 3, a fractional kinetic equation using a Fox–Wright function was already solved using these identities, along with additional fractional formulae that were computed.
It should be noted that (52) and (56) are taken into account while evaluating the succeeding outcomes involving the products of a vast class of special functions:
Therefore, new integrals of products of special functions can be computed by using (11) and (33) along with the formulation of Dirac delta function:
Moreover, new integrals of special function products are computed using the Fox–H function relation with other special functions, such as those specified in Equations (7)–(10) for the G-function, Fox–Wright function, and Mittag-Leffler function. As an example,
6. Conclusions
The various E–K operators of the generalized fractional calculus were used to obtain the new fractional transformations of a Fox–Wright function. As a result, analogous new images were obtained as special instances of the various other famous fractional transforms. This was only conceivable because the distributional representation was used to study the Laplace transform of a Fox–Wright function, whereas was used to design and solve a new fractional kinetic equation with respect to the parameter . As corollaries, specific examples involving the original Mittag-Leffler function were presented. A freshly derived representation of the generalized Mittag-Leffler function and its Laplace transform was critical in achieving the goal of this research. It is possible to conclude that this discovery is significant in terms of expanding the applicability of the Fox–Wright function beyond its initial scope.
Author Contributions
Each author equally contributed to writing and finalizing the article. Conceptualization, A.T. and R.S.; Methodology, A.T., R.S. and D.K.A.; Software, R.M.K.; Validation, R.S.; Formal analysis, D.K.A.; Resources, R.M.K.; Data curation, D.K.A.; Writing—original draft, D.K.A.; Writing—review & editing, A.T. and R.M.K.; Visualization, R.M.K.; Supervision, R.S.; Project administration, A.T. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The study did not report any data.
Acknowledgments
Asifa Tassaddiq would like to thank the Deanship of Scientific Research at Majmaah University for supporting this work under Project Number No. ICR-2023-501. The authors are also thankful to the worthy reviewers and editors for their useful and valuable suggestions for improving this paper, which led to a better presentation.
Conflicts of Interest
The authors declare no conflict of interest.
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