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 [1–5]. 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 [7–9]. 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 [10–13]. 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, 18–20]. 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, 22–28]. Different special functions, including the Gauss hypergeometric function, Mittag-Leffler-style 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 [29–31]. Researchers have become interested in fractional interpretations of the classical integral transforms, i.e., Laplace and Fourier transforms [32–34], 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.

Definition 1.1. The k-gamma function is a generalization of the classical Γ function given by

where (θ)_n,k = θ(θ + k)(θ + 2k) ⋯ (θ + (n − 1)k) for n ≥ 1 is called the Pochhammer k symbol. The integral representation is

Clearly, Γ(θ)=limk→1Γk(θ) and Γk(θ)=kθk-1Γ(θk).

Definition 1.2. For Re(θ) > 0, k > 0, and Re(ζ) > 0, the k-beta function is given by

The functions Γ_k and B_k are related by an identity

The k-Mittag-Leffler function given in [36] is defined as follows.

Definition 1.3. Let n ∈ N, k ∈ ℝ⁺, μ, ρ, γ ∈ ℂ, Re(ρ) > 0, and Re(μ) > 0. Then the k-Mittag-Leffler function is defined by

The modified (k, s)-fractional integral operator involving the k-Mittag-Leffler function given in [Samraiz et al., accepted] is defined as follows.

Definition 1.4. Let s ∈ ℝ\{−1}, k ∈ ℝ⁺, μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(γ) > 0, Re(μ) > 0, and Φ ∈ L¹[0, β]. Then the modified (k, s)-fractional integral operator involving the k-Mittag-Leffler function is given by

Definition 1.5 ([Samraiz et al., accepted]). Let s ∈ ℝ\{−1}, k ∈ ℝ⁺, μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(μ) > 0, n=[μk]+1, and Φ ∈ L¹[0, β]. Then the (k, s)-Prabhakar fractional derivative operator with the k-Mittag-Leffler function as its kernel is given by

Definition 1.6 ([Samraiz et al., accepted]). Let s ∈ ℝ\{−1}, k ∈ ℝ⁺, μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(μ) > 0, n=[μk]+1, and Φ ∈ Cⁿ[0, β] with 0 < θ < β < ∞. Then the regularized version of the (k, s)-Prabhakar derivative is

Definition 1.7. Let g ∈ Cⁿ[α, β] such that g′(ζ) > 0 on [α, β]. Then

where Φ[n-1]=(1g′(ζ)ddζ)n-1Φ.

The generalized Laplace transform introduced by Jarad et al. [34] is presented in the following definition.

Definition 1.8. Let Φ and g be real-valued functions on [α, ∞) such that g(ζ) is continuous and g′(ζ) > 0 on [α, ∞). The generalized Laplace transform of Φ is

for all values of u.

Definition 1.9 ([34]). Let Φ and Ψ be two piecewise-continuous functions on each interval [0, T] that are of exponential order. The generalized convolution of Φ and Ψ is given by

Theorem 1.10 ([34]). Let Φ∈Cgn-1[α,T] be such that Φ^[1] is of g-exponential order. Let Φ^[1] be a piecewise-continuous function on the interval [α, T]. Then the generalized Laplace transform of Φ^[1](ζ) exists and

Proposition 1.11. Let s ∈ ℝ\{−1}, k ∈ ℝ⁺, μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(μ) > 0, and β > 0. Then the integral operator 𝔍ks0+;ρ,μω,γ is bounded on C[0, β], i.e.,

where

and

Theorem 1.12. Let s ∈ ℝ\{−1}, k ∈ ℝ⁺, μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(γ) > 0, and Re(μ) > 0. Let Φ ∈ L¹[0, β] be a piecewise-continuous function on each interval [0, θ] that is of g(θ)-exponential order. Then

Theorem 1.13 ([Samraiz et al., accepted]). Let k ∈ ℝ⁺, s ∈ [0, ∞), μ, ρ, ω, γ ∈ ℂ, Re(ρ) > 0, Re(γ) > 0, Re(μ) > 0, and g(θ) = θ^s+1. Let Φ∈ACgn[0,β] and 𝔍ks0+;ρ,nk−m−μω,γ Φ for m = 0, 1, 2, …, n − 1 be of g(θ)-exponential order. Then

with |kω(ku)-ρk|<1.

Theorem 1.14 ([Samraiz et al., accepted]). The generalized Laplace transform of the regularized version of the (k, s)-Prabhakar fractional derivative is

with |kω(ku)-ρk|<1.

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.

Definition 2.1. Let Φ ∈ C¹[0, β], 0 < θ < β < ∞, s ∈ ℝ\{−1}, k, ρ > 0, ω, γ ∈ ℝ, μ ∈ (0, 1), ν ∈ [0, 1], and (Φ∗𝔍ks0+;ρ,(1−ν)(k−μ)ω,−γ(1−ν))(θ)∈AC1[0,β]. The (k, s)-Hilfer-Prabhakar derivative is defined as

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.

Theorem 2.2. For s ∈ ℝ\{−1}, k, ρ > 0, ω, γ ∈ ℝ, μ ∈ (0, 1), ν ∈ [0, 1], and Φ ∈ L¹[0, β], the operator 𝔇ks0+;ρ,ωγ,μ,ν is bounded on C[0, β], i.e.,

where

and

Proof Using the estimates in Proposition 1.11, we get

where C₁ and C₂ are the constants defined by (2.1) and (2.2).

Proposition 2.3. Let s ∈ ℝ\{−1}, k, ρ, λ > 0, ω, γ, σ ∈ ℝ, μ ∈ (0, 1), ν ∈ [0, 1], λ > μ + νk − μν, and Φ ∈ L¹[0, β]. Then

In particular,

Proof. By using Definition 2.1 and the semigroup property of the modified (k, s)-fractional integral operator with the k-Mittag-Leffler function, we obtain

This completes the proof.

Theorem 2.4. Let s ∈ ℝ\{−1}, k, ρ, λ > 0, ω, γ ∈ ℝ, μ ∈ (0, 1), ν ∈ [0, 1], λ > μ + νk − μν, and Φ ∈ L¹[0, β]. Then

Proof. By using Definition 2.1 and Theorem 2 in [Samraiz et al., accepted], we get

and thus the result is proved.

Theorem 2.5. The Laplace transform of the (k, s)-Hilfer-Prabhakar fractional derivative is

Proof. By using Definition 2.1, Theorem 1.12, and Theorem 1.10, we obtain

which proves the result.

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.

Theorem 3.1. The solution of the Cauchy problem

where θ ∈ (0, ∞), f ∈ L¹[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 1.11, we get

which can also be written as

Using the binomial expansion gives

Applying the inverse Laplace transform, we obtain

hence the result.

Remark 3.2. If s = 0, k = 1, γ = ν = 0, ρ = σ = 1, μ → 1, f(θ) = 0, ω = ir, and λ = −iΠp (with r, p ∈ ℝ), then above Cauchy problem reduces to the following free-electron laser equation:

Corollary 3.3. If we take s = 0 and k = 1, then we get the Cauchy problem given in [19]:

where θ ∈ (0, ∞), f ∈ L¹[0, ∞), μ ∈ (0, 1), ν ∈ [0, 1], ω, λ ∈ ℝ, ρ > 0, and γ, σ ≥ 0, and its solution is given by

Lately, numerous papers have been devoted to mathematical analysis of variations of the time-fractional heat equation and its applications in mathematical physics and probability theory [see, for example, [37, 38] and the references therein]. This section focuses on the generalized time-fractional heat equation involving the (k, s)-Hilfer-Prabhakar derivative.

Theorem 4.1. The solution of the Cauchy problem

where s ∈ [0, ∞), μ ∈ (0, 1), ν ∈ [0, 1], ω ∈ ℝ, G, k, ρ > 0, and γ ≥ 0, is given by