1. Introduction
In recent decades, fractional nonlinear systems have developed into effective mathematical tools to describe real-world problems with the rapid development of fractional calculus. Fractional differential equations (FDEs) can describe evolutionary phenomena that depend on both the time instant and time history, and they have been widely used in many fields such as economics and finance [
1,
2], epidemiology of disasters [
3,
4], physics [
5,
6,
7,
8], engineering [
9] and so on. To better model practical problems, different fractional derivatives have been defined and applied, such as Riemann-Liouville [
10,
11], Caputo [
12], modified Riemann-Liouville [
13], Atangana-Baleanu derivative [
14] and so on [
15]. For further information on the fractional derivatives, we refer the readers to [
10,
11,
12] and the cited references.
Since FDEs play an important role in expressing the practical problems mathematically, extraction of exact solutions for these FDEs is imperative. Exact solutions of the governing FDEs can be the benchmark solutions to verify the outcomes and codes of numerical solutions, and even to develop various numerical methods such as their differencing schemes and grid generation skills [
16]. In addition, the effort to find these solutions is significant for the more profound understanding of many physical phenomena, thus they may give more insight into the physical aspects of the problems. For example, the wave distributions observed in fluid dynamics, plasma and elastic media are often described by soliton solutions [
17,
18]. In the past several decades, many effective methods for directly obtaining exact solutions of nonlinear FDEs have been presented, such as Lie symmetry method [
5,
19,
20,
21,
22,
23], fractional sub-equation method [
24,
25,
26],
-expansion method [
27], exp-function method [
28] and many more [
29,
30,
31,
32]. Among those, the fractional complex transform builds a bridge between fractional differential equations, partial differential equations (PDEs) or ordinary differential equations (ODEs) [
6,
29,
30]. Through the fractional complex transform, many fractional differential equations can be transformed into PDEs. Therefore, methods which are used to find exact solutions of PDEs, such as Hirota bilinear method (HBM) [
33,
34], finite symmetry group method (FSGM) [
35,
36], consistent Riccati expansion method (CREM) [
37,
38] and so on can be applied to extract exact solutions of FDEs.
Hirota bilinear method (HBM) was first proposed by Hirota in 1971 [
33,
34]. For almost all nonlinear PDEs, HBM is very effective in exploring explicit soliton excitation solutions, including lump, breather and soliton molecules. These solutions provide a better explanation of the physical problems. Several fractional PDEs have been exactly solved by combining the fractional complex transform and Hirota bilinear method [
39,
40,
41,
42]. The finite symmetry group method (FSGM) was proposed by Lou [
35] in 2005, which is a direct method to find finite symmetry groups of PDEs. By the FSGM, we can get Lie point symmetry group as special case. Particularly, we can get the relationship between known solutions and new solutions. Recently, a consistent Riccati expansion method (CREM) [
37] has been proposed to find soliton or soliton-cnoidal solutions. The CREM has been applied to many PDEs, for example, Korteweg-deVries, Kadomtsev-Petviashvili, sine-Gordon, Sawada-Kotera, Kaup-Kupershmidt, Broer-Kaup, dispersive water wave, and Burgers systems [
37,
38].
To the best of our knowledge, soliton-cnoidal wave solutions for FDEs have not been reported up to now. Furthermore, FSGM has not been applied to derive solutions for FDEs. In this paper, we will apply the HBM, FSGM and CREM to extract new solutions for the two FDEs. The research objects are the (1+1)-dimensional fractional Korteweg-de Vries Sawada-Kotera-Ramani (FKdVSKR) equation
and (2+1)-dimensional FKdVSKR equation
in the sense of the modified Riemann-Liouville derivative, where
is Jumarie’s modified Riemann-Liouville derivative [
13],
,
is a constant,
and
are both constants.
FKdVSKR equations (1) and (2) include a lot of KdV-type equations as their special cases. When
(1) becomes the well-known KdV equation
.
When
(1) becomes the well-known Sawada-Kotera (SK) equation
.
When
(1) is the (1+1)-dimensional KdVSKR equation [
43,
44,
45,
46,
47,
48,
49]
which is also named KdV-SK equation [
46] or extended KdV equation [
48,
49] and is a combination of the KdV equation and the SK equation.
Soliton molecules and asymmetric soliton for (3) are obtained in [
43] by means of the velocity resonance condition. Also, finite symmetry groups of (3) are obtained. Lie symmetry, optimal system, symmetry reductions, power series solutions and N-soliton solution of (3) are derived in [
44]. Exact solitary wave solutions and quasi-periodic travelling wave solutions of (3) are studied in [
45] by a new method. A Kaup-Kupershmidt soliton wave solution has been obtained in [
46]. The authors in [
46] regard (3) as the higher order KdV equation, or KdV-SK equation. Soliton-cnoidal wave interaction solutions of (3) have been obtained by the consistent Riccati expansion method (CREM) [
47]. For the (1+1)-dimensional KdVSKR equation with variable coefficients, rational function solutions, multi-wave rational function solutions and two-soliton rational solutions have been derived by the unified method and its generalized form [
48]. We should point out that all the above work is on (3), and it is the first time to research the FKdVSKR equation (1).
When
(2) becomes the integrable Sawada-Kotera (SK) equation [
50]
. When
(2) becomes the (2+1)-dimensional KdVSKR equation
which has been proposed recently by generalizing (3) to
direction and can be used to describe the resonances of solitons in shallow water [
51]. In [
51,
52,
53,
54,
55], the authors study soliton excitations or interaction solutions of (4) based on the Hirota bilinear method (HBM). Multi-order lumps, interaction between lump and solitons of (4) are derived by the long wave limit method [
51]. Soliton molecule and multi-breather solutions of (4) are extracted by the velocity resonance mechanism and the complex conjugate relations in the parameters [
52]. In [
53], the fission solution and fusion solution of (4) are studied. The dynamic behaviors of lump molecules and y-type molecules are also illustrated. In [
54], a novel restrictive condition has been given to show the nonlinear superposition between a lump soliton and other nonlinear localized excitations. Interaction solutions of lump solution with hyperbolic cosine function and lump solution with exponential function of (4) are obtained by choosing appropriate function in the bilinear form [
55]. In [
56], Painlevé analysis, Lie point symmetry and symmetry reductions for (4) have been studied. To the best of our knowledge, there is no further work studying (4). Therefore, CREM and FSGM have not been applied to the KdVSKR equation in (2+1)-dimension.
The framework of the rest is in the following. In
Section 2, we construct new exact solutions of the (1+1)-dimensional FKdVSKR equation (1) by the fractional complex transform and the known solutions. In
Section 3, we construct new exact solutions of the (2+1)-dimensional FKdVSKR equation (2) by the fractional complex transform and CREM. In
Section 4, explicit solutions of the (2+1)-dimensional FKdVSKR equation (2) will be studied by the fractional complex transform and FSGM. We will build the relationship of new solutions with the known ones. New interaction solutions will be derived.
Section 5 is devoted to discussion of the results and methods in this paper. In
Section 6, some conclusions and future directions of the paper are presented.
5. Results and Discussion
In this paper, FKdVSKR equation in (1+1)-dimension and (2+1)-dimension have been studied from the point of analytical solutions. The CREM and FSGM are used to study exact solutions of KdVSKR equation (6) and (14). For the (1+1)-dimensional KdVSKR equation (6), the authors in [
48] show that the equation is CRE solvable and a soliton-cnoidal interaction solution is obtained. From the known soliton-cnoidal interaction solution, we obtain a fractional soliton-cnoidal interaction solution (10) for the FKdVSKR equation (1). It is a fractional soliton lying on a fractional cnoidal periodic wave background. It the first time that we have got a fractional soliton-cnoidal interaction solution for a fractional differential system. The FSGM has been applied to (6) in [
43], there aren’t any arbitrary functions in the expression of the finite symmetry group, so we can’t get new solutions from the FSGM. Fortunately, the finite symmetry group of the (2+1)-dimensional KdVSKR system (14) has three arbitrary functions, so that we can get a lot of new solutions making use of the known solutions of (13) and Theorem 2.
From [
55], we know the (2+1)-dimensional KdVSKR equation (13) has the following lump-hyperbolic solution
with
where
and
are constants.
Substituting into (29), a lump-hyperbolic interaction solution for the (2+1)-dimensional FKdVSKR equation (2) is obtained. When taking graphs are as follows.
From graphs (a)-(c) in
Figure 10, we find the impact of fractional order parameter
on the shape of solutions is not obvious.
.
Applying Theorem 2 and the above solution (29), we can get new solutions for (2). In Theorem 2 , we take
a three-wave interaction solution for (13) is as follows.
with
Substituting into (30), an lump-hyperbolic-sine interaction solution for the (2+1)-dimensional FKdVSKR equation (2) is obtained. When taking graphs are as follows.
From graphs (a)-(f) in
Figure 11, we find that as fractional order
increases, the shape of the solution has changed regularly. From
to
, it goes from periodic solution to soliton solution gradually. From
to
, the soliton becomes periodic solution gradually. From
to
, the wave is from disappearing to emerging.
Compared graphs in
Figure 10 with those in
Figure 11, we find the shape of the lump-hyperbolic interaction solution is very different with that of lump-hyperbolic-sine interaction solution. With the change of fractional order
, the three-wave interaction solution changes regularly while two-wave interaction solution changes a little.
Remark 2. We should point out that making use of Theorem 2, lots of new solutions with unexpected dynamic behavior and properties can be obtained. It is necessary to study further.
6. Conclusions
Recently, a new (2+1)-dimensional PDE has been proposed, which is called KdVSKR equation and can model the resonances of solitons in shallow water. The available results show that this equation possesses rich local excitation patterns. Since fractional KdVSKR equation in (1+1) -dimension and (2+1) -dimension have not been studied so far, we perform in-depth study on their exact solutions by means of HBM, CREM and FSGM. For the (1+1)-dimensional FKdVSKR equation, influence of fractional order on the locations of fractional one-soliton, fractional two-soliton and fractional three-soliton have been illustrated by graphs. We find that one-soliton, two-soliton and three-soliton have the same variation tendency: the location of the fractional soliton is more backward when the fractional order becomes larger. In addition, we also get fractional lump-periodic and fractional soliton-cnoidal interaction wave solutions. To our best knowledge, soliton-cnoidal interaction wave solutions of fractional PDE have not been reported before.
For the (2+1)-dimensional KdVSKR equation, we get the finite symmetry group and Theorem 2. From the finite symmetry group, Lie point symmetry can be obtained and is exactly the same with the results in [
56]. Applying Theorem 2, a lot of novel solutions can be derived for the (2+1)-dimensional KdVSKR equation. Making use of the fractional complex transform (12), new types of interaction solutions for the (2+1)-dimensional FKdVSKR equation (2) have been extracted. The solutions include dark soliton with trigonometric sine function interaction solution (26), bright soliton with elliptic function interaction solution (28), and lump-hyperbolic-sine three-wave interaction solution (29) under the condition
. Impact of fractional order
on the shapes of these solutions has been illustrated by plenty of graphs. As far as we know, lump-hyperbolic-sine three-wave interaction solutions for fractional PDEs have not been reported before. We have shown that the combination of the fractional complex transform and FSGM or CREM can be used to obtain new interaction solutions for fractional differential systems. In the future, we will study the fractional higher-order beam equation by applying the research idea in this paper.