1. Introduction
Integral equations (InEqs) are highly applicable in modeling various scientific and engineering phenomena. Some of the most important cases of InEqs are the first and second kind of Volterra-type InEqs. Various approaches have been developed for solving InEqs, among which the following methods stand out: successive approximation, series solution, variational iterative, and Adomian decomposition [
1,
2,
3,
4,
5,
6,
7]. In addition, the method of Laplace transformation constitutes an efficient approach to solve the first- and second-order Volterra InEqs if the kernel of these InEqs is a difference kernel, and in such a case, the Laplace transform convolution theorem can be employed [
1,
2]. Symmetry and Laplace transformations play an important role in the computational process and in solving various scientific problems efficiently, where the symmetrical properties and system can simplify the computational process and offer effective solutions. Some results of Laplace transformation sometimes show the symmetrical behavior of the original function, which helps greatly in analysis. One of the essential cases of a singular InEq is called the Abel integral equation (AInEq), which has been applied in radio astronomy, electron emission, atomic scattering, radar ranging, and many other applications. In addition, AInEqs possess a difference kernel, so the method of Laplace transformation represents an effective approach to their solution [
1,
2,
8,
9].
A branch of mathematical analysis to which much relevant research has been devoted is fractional calculus. The notion of a fractional derivative is theoretically established from a non-local conception, such as Riemann–Liouville (RL) and Caputo (C) derivatives. In the past few decades, another local formulation of this concept based on incremental ratios, known as conformable derivative (CoD), has been introduced [
10]. The most relevant properties of these definitions, including their interesting applications, such as fractional InEqs, have been mentioned in [
11,
12,
13,
14,
15].
In recent years, as a result of the challenges associated with both non-local and local fractional derivatives, a new generalization of local definitions has been proposed. In particular, the previously discussed CoD has disadvantages in comparison with the C fractional definition for certain functions [
16]. As a result, a new generalization of non-integer-order derivative has been formulated in [
17], named the Abu-Shady–Kaabar derivative (ASKD), which provides a simple way to analytically solve fractional differential equations, and the results agree exactly with those produced by employing RL and C fractional derivatives.
A combination of both fractional and fractal differentiation constitutes a hybrid differentiation method, which was formulated in [
18], and this approach includes the following properties in its system: fractal geometry, elastic viscosity, heterogeneity, and memory effect.
Very recently, a new generalization of fractal–fractional derivative (FrFrD) of the local type has been defined in [
19], named the Martinez–Kaabar (MK) fractal–fractional derivative. As a result of the MK derivative, all obtained results of this definition were in agreement with the FrFrD’s results in the C sense with the power law, which was proposed in [
18], when this definition was employed for certain elementary functions. In addition, in [
19,
20], this newly proposed calculus (so-called MK calculus) has been investigated, including its main mathematical analysis elements, integration, and InEqs. This included the second kind of Volterra MK InEqs, which was solved via the method of Adomian decomposition.
In comparison with the previous studies on various fractional definitions that have been employed in studying InEqs, our study indicates a high level of novelty by extending the newly proposed MK calculus to Laplace transformation. This generalized technique provides a novel mathematical tool to solving not only InEqs but also a variety of scientific, engineering, and economic problems and modelling scenarios that exhibit a fractal effect.
Motivated by all the above works, this work proposes to extend the MK calculus theory to the integral transformations, with applications in the solutions of certain InEqs. Therefore, this work is outlined as follows:
- (i)
First, we define a new notion of Laplace transformation, named MK Laplace transformation in this work, which involves the MK integral operator. In relation to this new fractal–fractional transformation, we establish fundamental results, such as a sufficient condition that ensures its existence, the transformation of elementary functions, the relation of the transformation of a function and its derivative, the convolution theorem, and some qualitative properties of this transformation, among other results. The notion of inverse MK Laplace transformation is also established, and its main properties are discussed.
- (ii)
Next, the MK Laplace transformation method is constructed in the context of the MK Volterra integral equations of the first and second kind.
- (iii)
Finally, this new technique is employed to find the solutions to MK AInEqs and MK generalized AInEqs.
2. Preliminaries
This section includes basic concepts and results, which will be essential for the upcoming results in this research work. First of all, it is interesting to recall the definition introduced in [
18]:
Definition 1. Suppose that is differentiable on , where ; if is a fractal differentiable on of order , then the FrFrD of of order in the context of C with the power lawis written aswhere In the above definition, Γ(⋅) is the Gamma function, the order of FrFrD is controlled by and , and the integration term with the power-law kernel indicates the non-locality and memory-effect properties of fractal systems. In addition, Equation (2) can be updated by including the two-scale fractal derivative for the case of multi-fractal systems, where each of the two scales exhibits different fractal effects, indicating that various processes happen at various scales, as in the cases of turbulence and porous media.
Remark 1. - (i)
in Equation (2) is the fractal derivative of order , with , which was proposed in [
21,
22].
- (ii)
If we assume that and , then Equation (1) can be expressed as
It is also interesting to recall, in relation to the FrFrD of order α defined in Equation (3), that the following two results [
18] are obtained:
Theorem 1. Suppose that , and . Then, we obtain Remark 2. If for every real constant, then .
Theorem 2. Assume that, a function is supposed to be analytic at the origin with McLaurin expansion, written as follows:for
with . Then, we obtain Remark 3. From Theorem 1, Equation (6) can be expressed as From Definition 1 and the above Theorems 1 and 2, the MK derivative of order
is defined [
19] as follows:
Definition 2. A function: , the MK derivative of order , of at is written aswhere
with and .
If is MK -differentiable in some , , and exists, then it is written as Remark 4. Note that if , then Equation (8) is the usual limit-based definition of the derivative.
Theorem 3 ([
19])
. Suppose that and is a MK -differentiable at a point . If, further, is a differentiable function, thenwhere with .
Remark 5. According to [
19]
, all obtained results via the MK derivative are in agreement with the FrFrD’s results of order α in the sense of C with a power law, as in Equation (1). According to Definition 2, we obtain the following [
19]:
Theorem 4. Suppose that , and assume that are MK -differentiable at a point . Then, we obtain
- (i)
, .
- (ii)
constant functions such that .
- (iii)
.
- (iv)
.
Since the MK derivative is local in its nature, results from mathematical analysis and chain rule can be formulated via the help of MK calculus [
19]. This property is presented in the following theorem.
Theorem 5 (Chain Rule). Assume that , is an MK -differentiable at and is differentiable at , then Remark 6. According to Theorem 5, the MK derivative of order of some elementary functions can be expressed as
- (i)
.
- (ii)
.
- (iii)
- (iv)
Remark 7. From the differentiability property of the MK derivative, which is -differentiability, and by assuming that , then Equation (11) can be represented aswhere , and .
The MK
-integral of a function
starting at
, can be recalled as formulated in [
22].
Definition 3. , such that this integral is the well-known Riemann improper integral, , and .
From Definition 3, we obtain the following:
Theorem 6. , for , such that is any continuous function in the domain of .
Theorem 7. Suppose that , , , and is a continuous real-valued function (RVF) on . Let be any RVF with the property: for all . Then Finally, it is also necessary to mention the objectives of our research regarding the notion of MK Volterra InEq, proposed in [
20]. Thus, it is generally given as follows:
where
is the InEq’s kernel,
is an unknown function,
is a perturbation known function,
is non-zero real parameter,
, and .
If
is substituted in Equation (14), we obtain the usual known Volterra InEq [
1,
2,
3]. However, if
, then Equation (14) is expressed as
which is the Volterra InEq of the second kind¸ while if
, then Equation (14) is written as
which is the MK Volterra InEq of the first kind.
3. A New Fractal–Fractional Laplace Transformation
In this section, a new notion of fractal–fractional Laplace transformation is proposed; in this work, it is referred to as Martínez–Kaabar fractal–fractional Laplace transformation (for simplicity, we will use the term MK Laplace transformation in all upcoming results). All its main properties will be established.
Definition 4. Assume that , , and is an RVF. Then, the MK Laplace transformation of order is expressed as Now, let us establish a sufficient condition that confirms the existence of the MK Laplace transformation. To prove this condition, the following definition is needed:
Definition 5 (Generalized Fractal–Fractional Exponential Order (GFrFrEO)). Assume that , , and is an RVF. The function is considered to be of GFrFrEO
if constants , and , such that Remark 8. Furthermore, to establish this existence result of the MK Laplace transformation, we assume the following condition for and (parameters): , with .
Theorem 8. If is a piece-wise continuous function (PWCF) on and GFrFrEO, then exists for .
Proof. Since
is GFrFrEO,
constants
,
and
, such that
Furthermore,
is a
PWCF on
and hence bounded there as
This implies that a constant
can be selected to be sufficiently large, such that
Letting
, we see that
□
Then, the linearity of the integral operator will be studied, which can obviously be seen from the definition and the MK integral operator’s properties.
Theorem 9. Assume that , with , , are RVFs, and . If and exist, then, Now, we show the main properties of the integral transformation through the following theorem:
Theorem 10. Let , with , . So, we obtain
- (i)
, where is any real number and .
- (ii)
, where the gamma function is represented by , , and .
- (iii)
, where is any real number and .
- (iv)
, with for , where is a real number and .
- (v)
If for , then , for and .
- (vi)
, for and .
- (vii)
, for and .
- (viii)
, for any real number and
- (ix)
, for any real number and
Proof. - (i)
Directly using the definition.
- (ii)
Via the change of variable, the following is obtained:
- (iii)
Consider
, with
. Then,
- (iv)
Suppose
for
. So, we have
- (v)
Suppose
for
. So, through the change of variable
, we obtain
- (vi)
Using the fact that
we can obtain the required result.
- (vii)
Similar to the previous one, using
- (viii)
As
it is easy to obtain the required result.
- (ix)
Then, the connection between the MK Laplace transformation of a function and the MK -derivative is established as follows:
Theorem 11. Suppose that is continuous and is piece-wise continuous for all . Suppose further that is the GFrFrEO. Then, exists for , and moreover, Proof. By using Equation (17), we obtain
Now, by using classical integration by parts, we obtain
If , with , , and given that is GFrFrEO, whenever .
By employing the relevant condition again to the function and its MK -derivative, the MK Laplace transformation of the consecutive MK -derivatives can be written as , which can be formulated via the help of the previously mentioned theorem. We conclude with the following corollary:
Corollary 1. Assume that are continuous, and is PWCF for all . Suppose further that are GFrFrEO. Then, exists for and is given by Hence, means the application of the MK -derivative k times.
It is known that the convolution product of two -transformable functions is considered an essential result of classical Laplace transformation. Therefore, we will construct that result for MK Laplace transformation as follows:
Theorem 12. Let , with , ,
and be RVFs. If and exist, then we obtainwhere Proof. By applying MK Laplace transformation to Equation (23), we have
Now, consider the change of variables,
. So,
which can be written as
□
The MK Laplace transformation of the MK -integral is in connection with the MK Laplace transformation of its integrand, as follows:
Theorem 13. Assume that is a GFrFrEO and continuous for . Then, we obtain Proof. If we take and from Theorem 12, our result follows easily.
□
We now present some qualitative properties of the MK Laplace transformation of PWCFs on and GFrFrEO.
Theorem 14. (Behavior of the MK Laplace Transformation at Infinity). Assume that is PWCF on and GFrFrEO. Then, we obtain Proof. Since
is GFrFrEO,
, such that
for
. Also,
is a PWCF on
, and hence
is bounded, so
such that
for
. Choosing
we have
for
. Now, we obtain
From the previous inequality, it follows easily into Equation (25).
□
Theorem 15 (Initial Value Theorem). Suppose that is MK -differentiable and is PWCF for all . Suppose further that are GFrFrEO. Then, we obtain Proof. From Theorem 11, we write
However, by applying the previous theorem to
, we obtain
and therefore
This concludes our proof. □
The following result relates the MK Laplace transformation to the usual Laplace transformation.
Theorem 16. Let , with , , and is a function such that . Then, we obtainwhere .
Proof. The proof follows easily by setting .
□
Now, let us define the inverse MK Laplace transformation:
Definition 6. For a PWCF on and generalized exponential order whose MK Laplace transformation is , we name the inverse MK Laplace transformation of and express it as . Symbolically, we obtain The inverse GFrFr Laplace transformation satisfies the property of linearity as follows:
Theorem 17. Given two generalized Laplace transformations, and , we obtain Proof. Suppose that
, and
. Since
we have
□