On the (k,s)-Hilfer-Prabhakar Fractional Derivative With Applications to Mathematical Physics

In this paper we introduce the (k, s)-Hilfer-Prabhakar fractional derivative and discuss its properties. We find the generalized Laplace transform of this newly proposed operator. As an application, we develop the generalized fractional model of the free-electron laser equation, the generalized time-fractional heat equation, and the generalized fractional kinetic equation using the (k, s)-Hilfer-Prabhakar derivative.


INTRODUCTION
Fractional calculus is the area of mathematical analysis that deals with the study and application of integrals and derivatives of arbitrary order. In recent decades, fractional calculus has become of increasing significance due to its applications in many fields of science and engineering [1][2][3][4][5]. The first application of fractional calculus was given by Abel [6] and includes the solution to the tautocrone problem. Fractional calculus also has applications in biophysics, wave theory, polymers, quantum mechanics, continuum mechanics, field theory, Lie theory, group theory, spectroscopy, and other scientific areas [7][8][9]. Although this calculus has a long history, over the past few decades it has attracted greater attention because of the fascinating results obtained when it is used to model certain real-world problems [10][11][12][13]. What makes fractional calculus special is that there are numerous types of fractional operators, so any scientist modeling real-world phenomena can choose the operator that fits their purposes the best. Each classical fractional derivative is usually defined in terms of a specific integral. Among the most well-known concepts of fractional derivatives are the Riemann-Liouville, Caputo, Grünwald-Letnikov, and Hadamard derivatives [10,14,15], whose formulations involve single-kernel integrals and which are used to investigate, for example, memory effect problems [16].
The Riemann-Liouville fractional derivative is remarkable, but it has some drawbacks when used to model physical phenomena because of its improper physical conditions. Caputo's great contribution was to develop a concept of fractional derivative appropriate for physical conditions [17]. A number of other families of fractional operators have been established, such as the Liouville, Erdlyi-Kober, Hadamard, Grünwald-Letnikov, Hilfer, Hilfer-Prabhakar, and k-Hilfer-Prabhakar operators, to mention just a few [10,[18][19][20]. Because there are so many concepts of fractional operator, it has become necessary to define generic fractional operators, of which the classical ones are particular cases. One class of extensions of Riemann-Liouville fractional operators comprises the so-called k-Prabhakar integral operators, which can be found in [21]. Inspired by the definitions of k-Prabhakar integral operators and k-Hilfer-Prabhakar derivatives [20], the authors introduced the (k, s)-Hilfer fractional derivative, which unifies a large class of fractional operators [19,20]; [Samraiz et al., accepted].
In recent years, the generalization of integral and differential operators has become an important subject of research in fractional calculus [9,20,[22][23][24][25][26][27][28]. Different special functions, including the Gauss hypergeometric function, Mittag-Lefflerstyle functions, the Wright function, Meijer's G function, and Fox's H function, appear in the kernels of several generalizations of the integral operators. R. Hilfer introduced the Hilfer fractional derivative in [9], which is a generalization of the Riemann-Liouville and Caputo fractional derivatives.The Prabhakar integral and derivative operators are obtained from the Riemann-Liouville integral operator by extending its kernel to involve the three-parameter Mittag-Leffler function [19]. This paper is motivated by the rich applications of fractional differential equations (FDEs) in physics, economics, engineering, and many other branches of science [8,10,13,17]. Since no general method exists that can be used to analytically solve every FDE, one of the most pressing and challenging tasks is to develop suitable methods for finding analytical solutions to certain classes of FDEs [29][30][31]. Researchers have become interested in fractional interpretations of the classical integral transforms, i.e., Laplace and Fourier transforms [32][33][34], in the past few years. It can be shown that integral transformations such as the Laplace, Fourier, generalized Laplace, and ρ-Laplace transforms are useful methods for obtaining analytical solutions to some classes of FDEs. In this framework, we use a generalized Laplace transform to obtain analytical solutions to certain classes of FDEs that contain (k, s)-Hilfer-Prabhakar fractional derivatives. Given the wide range of fractional operators available in the literature, it can be difficult to choose the most suitable approach for a given problem. It is therefore essential to consider generalizations of classical fractional operators to aid in choosing an appropriate operator.
Diaz et al. [35] defined k-gamma and k-beta functions as follows.

THE (k,s)-HILFER-PRABHAKAR FRACTIONAL DERIVATIVE AND GENERALIZED LAPLACE TRANSFORMS
In this section we introduce a new family of operators called the (k, s)-Hilfer-Prabhakar fractional derivative. The generalized Laplace transforms of these operators are also studied in this section.
Note that if we choose ν = 0 in the above definition, we get (1.
2) corresponding to m = 1; and if we take ν = 1, we obtain (1. 3) corresponding to m = 1. and PROOF Using the estimates in Proposition 1.11, we get where C 1 and C 2 are the constants defined by (2.1) and (2.2).
In particular, and thus the result is proved.
PROOF. By using Definition 2.1, Theorem 1.12, and Theorem 1.10, we obtain which proves the result.

GENERALIZATION OF THE FREE-ELECTRON LASER EQUATION
The integrodifferential free-electron laser equation describes the unsaturated behavior of the free-electron laser. Several attempts have been made to solve the generalized fractional integrodifferential free-electron laser equation in recent years. In this section, we develop a generalized fractional model of the free-electron laser equation that involves the novel (k, s)-Hilfer-Prabhakar derivative. where θ ∈ (0, ∞), f ∈ L 1 [0, ∞), µ ∈ (0, 1), ν ∈ [0, 1], ω, λ ∈ R, ρ > 0, and γ , σ ≥ 0, is given by PROOF. By applying the generalized Laplace transform to both sides of (3.1) and using Theorems 2.5 and 1.12, we get which can also be written as Using the binomial expansion gives Applying the inverse Laplace transform, we obtain hence the result.

GENERALIZATION OF THE FRACTIONAL KINETIC DIFFERINTEGRAL EQUATION
Fractional differential equations are important tools for developing mathematical models of numerous phenomena in fields such as physics, dynamic systems, control systems, and engineering. In mathematical modeling, kinetic equations describe the continuity of the motion of a substance and are basic equations of mathematical physics and the natural sciences.

REMARK 5.2.
If we take s = 0, k = 1, ν = γ = σ = 0, µ → 0, a = 1, and b = −c p , then we get the following fractional kinetic equation given in [39]: where D p 0 + is the Riemann-Liouville fractional integral operator, defined as Here N(t) denotes the number density of a given species at time t, with N 0 = N(0) being the number density of that species at time t = 0, c is a constant, and f ∈ L 1 [0; ∞). COROLLARY 5.3. If we take s = 0 and ν = 0, then we get the following Cauchy problem given in [42]: where ω ∈ C, a, b ∈ R(a = 0), µ, ρ, q, k > 0, and γ , σ ≥ 0. The solution to the problem is N(t) = d

CONCLUSION
A new generalized fractional derivative operator, referred to as the (k, s)-Hilfer-Prabhakar fractional derivative, is developed in this article. The generalized Laplace transform of the proposed operator is also studied. Potential applications of the proposed operator are discussed, which concern fractional models of the free-electron laser equation, heat equation, and kinetic equation that involve the new operator. The results in this article suggest that this novel operator can be used to solve various types of problems arising in mathematical physics and other fields.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/ supplementary material.