Symmetry Reductions of ( 2 + 1 )-Dimensional CDGKS Equation and Its Reduced Lax Pairs

With the aid of symbolic computation, we obtain the symmetry transformations of the (2 + 1)-dimensional Caudrey-Dodd-GibbonKotera-Sawada (CDGKS) equation by Lou’s direct method which is based on Lax pairs. Moreover, we use the classical Lie group method to seek the symmetry groups of both the CDGKS equation and its Lax pair and then reduce them by the obtained symmetries. In particular, we consider the reductions of the Lax pair completely. As a result, three reduced (1 + 1)-dimensional equations with their new Lax pairs are presented and some group-invariant solutions of the equation are given.


Introduction
In modern mathematics with ramifications of several fields of mathematics, physics, and other sciences, it is getting more and more popular to study the symmetry analysis of differential equations, especially high-dimensional ones, such as finding symmetries, symmetry groups of transformation, symmetry reductions, and construction group invariant solutions.
Nowadays, there are three basic methods for finding symmetry reductions of the given nonlinear systems [1], namely, the classical Lie group method [2,3], the nonclassical Lie group method [4], and the Clarkson and Kruskal's direct method [5].Then Lou improved the direct method [6], which was based on Lax pairs.With the classical Lie group method, Zhi [7,8] studied symmetry reductions of the Lax pair for the (2+1)-dimensional Konopelchenko-Dubrovsky equation and found that the reduced Lax pairs do not always lead to the reduced KD equations.
In [9], the first two ZS-AKNS members, the coupled nonlinear Schr ö dinger, and the mKdV equations yield special solutions to the KP equation.This means the assembling of (1 + 1)-dimensions into (2 + 1)-dimensions.The technique is applied to the KdV hierarchy.The assembling of the first two KdV equations leads to the (2 + 1)-dimensional CDGKS equation: which is a higher-order generalization of the celebrated Korteweg-de Vries (KdV) equation.Equation ( 1) was first introduced in [10], and its (1 + 1)-dimensional version was studied by Sawada and Kotera [11] and Caudrey et al. [12].The (1 + 1)dimensional CDGKS equation is not a member of the Lax hierarchy of the Korteweg-de Vries equation and has some distinct properties, as reported in [13].In [14], the algebraicgeometric solutions to (1) were obtained.The Lax pair of linear equations of the (2 + 1)-dimensional CDGKS equation ( 1) is as follows: The plan of the present paper is as follows.Section 2 presents the symmetry transformations of the CDGKS equation by means of its Lax pair with Lou's direct method.Section 3 gives the symmetry reductions of the CDGKS equation and its Lax pair, based on the symmetries obtained by the classical Lie group method.A short summary is in Section 4.

Symmetry Transformations by the Direct Method
In this section, we will seek the symmetry transformations of the CDGKS equation and will determine the Lie group of transformations of (1) with the direct method based on the Lax pair due to Lou.By a transformation, (1) is equivalent to the following system: which possesses the Lax pair Let where , , , and  are functions of (, , ) and (, , ) has the same equations as (4).Consider Substitution of ( 5) and ( 6) into (4) leads to a system of differential equations.Comparing the different derivatives of , we get the restricted equations of , , , and .Solving these equations, we obtain and the relations of ,  and w(, , ), ũ(, , ) are as follows: where  1 ,  1 , and  are arbitrary functions of .In this paper, the dots denote differentiation with respect to .
From the above results one can get the following symmetry group theorem for the CDGKS equation.
From Theorem 1, let  1 = (),  1 = (), and  =  + ℎ() in ( 8), with infinitesimal parameter ; we can obtain the Lie point symmetry structure again, w =  + (), ũ =  + ().Furthermore, we have The equivalent vector expression of the symmetries can be expressed as where The associated Lie algebras between any two vector fields become It is easy to show that { 1 ,  2 ,  3 } constructs a Kac-Moody algebra.

Symmetry Reductions of the CDGKS Equation and Its Reduced Lax Pairs
In this section, we will use the classical Lie group method to seek some symmetries of the CDGKS equation and its Lax pair.The Lie point symmetry algebra admitted by its corresponding Lax pair ( 4) is where  1 , where  1 ,  2 are arbitrary constants and , , and ℎ are arbitrary functions of .Also, we can obtain the Lie point symmetry algebra admitted by (3), and we find that the CDGKS equation and its Lax pair admit the same symmetry transformations of the independent variables except the eigenfunction  3 .
After determining the infinitesimals ( 14), the symmetry variables are found by solving the corresponding characteristic equations: While solving the above characteristic equation one has to distinguish between the cases in which some of the functions , , ℎ and the constants  1 ,  2 are identical to zero and cases where they are not.This leads to different relations between the similarity variables ( x, ỹ, , , Ψ) and the original variables (, , , , , Φ).As a result we obtain the following cases.
where  and  are symmetry reduction fields with respect to the group invariants x, ỹ and , , and ℎ are arbitrary functions of .The reduced equation ( 3) turns out to be In the following reductions, we find that  1 ,  2 play the role of spectral parameter in the reduced Lax pair.In this case, the following three cases should be considered; namely, (i)  1 ̸ = 0, (ii)  1 = 0,  2 ̸ = 0, and (iii)  1 =  2 = 0.
From (15), we can obtain the eigenfunction Substituting ( 16) and ( 18) into (4), we obtain the first type of the reduced Lax pair By direct computation, from (19) we can obtain It is easy to check that the reduced (1 + 1)-dimensional equation ( 17) is the compatibility condition of the reduced Lax pair (19).
The eigenfunction is We obtain the second type of the reduced Lax pair Similarly, the reduced equation ( 17) is the compatibility condition of the reduced Lax pair (22).

The eigenfunction is
We obtain the third type of the reduced Lax pair Equation ( 17) is the compatibility condition of the Lax pair (24).
The reduction equation ( 17) is much simpler than the original equation (3).It is easy to obtain the solutions of (17).Consider where  1 ,  2 ,  3 , and  4 are arbitrary constants.
According to (16), we can get the group-invariant solutions of (3).Consider with and , , and ℎ are arbitrary functions of .
We can obtain the eigenfunction Substituting ( 28) and ( 30) into (4), in this case we obtain the first type of the reduced Lax pair: From (31), we obtain It is easy to prove that the reduced (1 + 1)-dimensional equation ( 29) is the compatibility condition of the reduced Lax pair (31).
The eigenfunction is We obtain the second type of the reduced Lax pair  (37) The reduced (1 + 1)-dimensional equation ( 29) is just a subset of (37); then (36) is not the Lax pair of (29).