Startseite Naturwissenschaften Exact solutions of equal-width equation and its conservation laws
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Exact solutions of equal-width equation and its conservation laws

  • Chaudry Masood Khalique EMAIL logo , Karabo Plaatjie und Innocent Simbanefayi
Veröffentlicht/Copyright: 4. Oktober 2019

Abstract

In this work we investigate the equal-width equation, which is used for simulation of (1-D) wave propagation in non-linear medium with dispersion process. Firstly, Lie symmetries are determined and then used to establish an optimal system of one-dimensional subalgebras. Thereafter with its aid we perform symmetry reductions and compute new invariant solutions, which are snoidal and cnoidal waves. Additionally, the conservation laws for the aforementioned equation are established by invoking multiplier method and Noether’s theorem.

1 Introduction

In this work we conduct a study of the third-order equal-width (EW) equation

(1) u t + 2 α u u x β u t x x = 0 , α , β 0 ,

where α is the nonlinearity parameter and β is the dispersion parameter. This equation was first introduced in [1] and represents a model equation that describes nonlinear dispersive waves, e.g., waves created in shallow water channel. Several methods have been used to derive solutions of this equation, for instance the authors of [2] used Petrov-Galerkin method by invoking quadratic B-spline finite element. In [3], authors used least-squares technique to construct numerical solutions of this equation. This equation (1) was recently studied in [4], where the researchers employed simple equation technique and also the exp(−φ(ξ)) expansion technique to construct travelling wave solutions.

In our study, using a different technique, we construct new exact travelling wave solutions, namely cnoidal and snoidal waves solutions of EW equation (1). Moreover, by employing two approaches, viz., the multiplier approach and Noether’s theorem we construct conservation laws of (1).

2 Solutions using optimal system

Firstly, we determine Lie symmetries of (1) and then utilize them to establish an optimal system of one-dimensional subalgebras (OSODS). Thereafter, we use this OSODS to performs symmetry reductions (SRs) and invariant solutions of (1) [5, 6, 7, 8, 9, 10, 11, 12].

2.1 Lie symmetries

The infinitesimal generator

(2) G = τ t + ξ x + η u ,

where the functions (τ, ξ, η) depend on t, x and u, is a Lie symmetry of equation (1) if

(3) G ( 3 ) F | F = 0 = 0 ,

where

F u t + 2 α u u x β u t x x

and G(3) denotes third prolongation [7] of G and is given by

(4) G ( 3 ) = G + ζ 1 u t + ζ 2 u x + ζ 12 u t x + ζ 122 u t x x .

Here the coefficients ζ1, ζ2, ζ12 and ζ122 are determined by

(5) ζ 1 = u t D t ( τ ) u x D t ( ξ ) + D t ( η ) , ζ 2 = u t D x ( τ ) u x D x ( ξ ) + D x ( η ) , ζ 12 = u t t D x ( τ ) u t x D x ( ξ ) + D x ( ζ 1 ) , ζ 122 = u t x x D x ( τ ) u x x x D x ( ξ ) + D x ( ζ 12 ) ,

with

(6) D x = x + u x u + u x x u x + u x t u t + D t = t + u t u + u t t u t + u t x u x + .

We expand equation (3) and since τ, ξ and η do not depend on derivatives of u, equation (3) decomposes into system of linear homogeneous PDEs. Solving these equations one finds values of τ, ξ and η, which lead to three Lie symmetries of (1) given by

G 1 = t , G 2 = x , G 3 = t t u u .

We note that the first two symmetries represent translations in time and space respectively, whereas the third describes a scaling symmetry of the EW equation.

2.2 Optimal system of one-dimensional subalgebras

We now utilize technique of [7] to calculate an OSODS by using Lie symmetries of (1). We first construct the commutator table. Thereafter we compute adjoint representation using

Ad ( exp ( ε G s ) ) G k = n = 0 ε n n ! ( ad G s ) n ( G k ) = G k ε [ G s , G k ] + ε 2 2 ! [ G s , [ G s , G k ] ] ,

where ε ∈ ℜ and commutator [Gs , Gk] is defined as

[ G s , G k ] = G s G k G k G s .

The commutators of Lie symmetries of (1) and adjoint symmetry group of (1) are presented in Tables 1 and 2, respectively. Consequently, we utilize Tables 1 and 2 to derive the OSODS for (1).

Table 1

Commutator table of (1)

[ , ] G1 G2 G3
G1 0 0 G1
G2 0 0 0
G3 G1 0 0
Table 2

Adjoint symmetry group of subalgebras

Ad G1 G2 G3
G1 G1 G2 "G1 + G3
G2 G1 G2 G3
G3 eεG1 G2 G3

Thus following [7] and utilizing Table 1 and Table 2 we can obtain an OSODS {G1 + cG2, G3 + aG2, G2}, with c and a are constants.

2.3 Solutions and symmetry reductions

We now utilise the OSODS obtained above and find symmetry reductions and invariant solutions for equation (1).

Consider the first operator G1 + cG2 of the optimal system. This operator has invariants

ξ = x c t , U = u ,

which give the invariant solution U = U(ξ). Using ξ as our new independent variable, equation (1) is reduced into nonlinear ODE

(7) c β U ( ξ ) + 2 α U ( ξ ) U ( ξ ) c U ( ξ ) = 0.

We now construct travelling wave solutions of our PDE (1) by invoking Jacobi elliptic expansion technique [13]. We suppose solutions of ODE (7) are

(8) U ( ξ ) = s = N N A s H ( ξ ) s ,

where positive integer N and As are constants to be determined. The function H is a solution of nonlinear first-order ODE

(9) d H d ξ = ( 1 ω + ω H 2 ) ( 1 H 2 )

or

(10) d H d ξ = ( 1 ω H 2 ) ( 1 H 2 ) .

We recall that Jacobi cosine function

(11) H = cn ( ξ | ω )

solves (9), whereas Jacobi sine function

(12) H = sn ( ξ | ω )

satisfies (10) with 0 ≤ ω ≤ 1 [13, 14].

2.3.1 Cnoidal waves

Nonlinear ODE (7) provides us with N = 2 and so (8) leads to

(13) U = A 2 H 2 + A 1 H 1 + A 0 + A 1 H + A 2 H 2 .

Substitution of U from (13) into (7) and utilizing (9) we obtain

4 H 10 α A 2 2 48 β c ω A 2 4 α ω A 2 2 + 2 H 8 α A 1 2 4 H 8 α A 2 2 2 H 8 c A 2 H 7 c A 1 2 H 6 α A 1 2 + 2 H 6 c A 2 + H 5 c A 1 + H 5 c A 1 2 H 4 α A 1 2 + 2 H 4 c A 2 H 3 c A 1 4 H 2 α A 2 2 + 2 H 2 α A 1 2 + 8 H 4 α ω A 2 A 0 2 H 2 c A 2 + 12 H 3 α ω A 2 A 1 2 H 3 α ω A 2 A 1 2 H 3 α ω A 1 A 0 4 H 2 α ω A 2 A 0 6 H ξ α ω A 2 A 1 + 6 H 11 α ω A 1 A 2 + 2 H 9 α ω A 1 A 2 + 2 H 9 α ω A 0 A 1 12 H 9 α ω A 1 A 2 8 H 8 α ω A 0 A 2 2 H 7 α ω A 2 A 1 2 H 7 α ω A 1 A 0 4 H 7 α ω A 1 A 2 4 H 7 α ω A 0 A 1 + 6 H 7 α ω A 1 A 2 4 H 6 α ω A 2 A 0 + 4 H 6 α ω A 0 A 2 6 H 5 α ω A 2 A 1 + 4 H 5 α ω A 2 A 1 + 4 H 10 α ω A 0 A 2 + 4 H 5 α ω A 1 A 0 + 2 H 5 α ω A 1 A 2 + 2 H 5 α ω A 0 A 1 + 24 β c A 2 + 2 H 2 c ω A 2 + 6 H α A 2 A 1 + 6 H β c A 1 + 24 β c ω 2 A 2 + 4 H 12 α ω A 2 2 + 2 H 10 α ω A 1 2 8 H 10 α ω A 2 2 2 H 10 c ω A 2 + 6 H 9 α A 1 A 2 H 9 c ω A 1 4 H 8 α ω A 1 2 + 4 H 8 α ω A 2 2 + 4 H 8 α A 0 A 2 + 4 H 8 c ω A 2 + 2 H 7 α A 1 A 2 + 2 H 7 α A 0 A 1 6 H 7 α A 1 A 2 + H 7 c ω A 1 + 2 H 7 c ω A 1 2 H 6 α ω A 1 2 + 2 H 6 α ω A 1 2 4 H 6 α A 0 A 2 + 2 H 6 c ω A 2 2 H 6 c ω A 2 2 H 5 α A 2 A 1 2 H 5 α A 1 A 0 2 H 5 α A 1 A 2 2 H 5 α A 0 A 1 2 H 5 c ω A 1 H 5 c ω A 1 4 H 4 α ω A 2 2 + 4 H 4 α ω A 1 2 4 H 4 α A 2 A 0 4 H 4 c ω A 2 + 6 H β c ω 2 A 1 + 2 H 3 α A 2 A 1 + 2 H 3 α A 1 A 0 + H 3 c ω A 1 + 8 H 2 α ω A 2 2 12 H β c ω A 1 2 H 2 α ω A 1 2 + 4 H 2 α A 2 A 0 + 4 α A 2 2 8 H 8 β c A 2 H 7 β c A 1 + H 5 β c A 1 + H 5 β c A 1 + 8 H 4 β c A 2 7 H 3 β c A 1 32 H 2 β c A 2 7 H 9 β c ω A 1 56 H 8 β c ω 2 A 2 + 56 H 8 β c ω A 2 2 H 7 β c ω 2 A 1 10 H 7 β c ω 2 A 1 + H 7 β c ω A 1 + 10 H 7 β c ω A 1 16 H 6 β c ω 2 A 2 + 16 H 6 β c ω 2 A 2 + 8 H 6 β c ω A 2 24 H 6 β c ω A 2 + 10 H 5 β c ω 2 A 1 + 8 H 6 β c A 2 + 2 H 5 β c ω 2 A 1 10 H 5 β c ω A 1 3 H 5 β c ω A 1 + 56 H 4 β c ω 2 A 2 56 H 4 β c ω A 2 14 H 3 β c ω 2 A 1 + 21 H 3 β c ω A 1 64 H 2 β c ω 2 A 2 + 96 H 2 β c ω A 2 24 H 12 β c ω 2 A 2 6 H 11 β c ω 2 A 1 + 64 H 10 β c ω 2 A 2 32 H 10 β c ω A 2 + 14 H 9 β c ω 2 A 1 6 H 3 α A 2 A 1 = 0.

The above equation decomposes on similar powers of H and leads to following thirteen algebraic equations:

α ω A 2 2 6 β c ω 2 A 2 = 0 , α ω A 1 A 2 β c ω 2 A 1 = 0 , 6 β c ω 2 A 2 α ω A 2 2 12 β c ω A 2 + α A 2 2 + 6 β c A 2 = 0 , β c ω 2 A 1 α ω A 2 A 1 2 β c ω A 1 + α A 2 A 1 + β c A 1 = 0 , 32 β c ω 2 A 2 + 2 α ω A 0 A 2 + α ω A 1 2 4 α ω A 2 2 16 β c ω A 2 + 2 α A 2 2 c ω A 2 = 0 , 14 β c ω 2 A 1 + 2 α ω A 1 A 2 + 2 α ω A 0 A 1 12 α ω A 1 A 2 7 β c ω A 1 + 6 α A 1 A 2 c ω A 1 = 0 , 28 β c ω 2 A 2 2 α ω A 2 2 + 4 α ω A 2 A 0 + 2 α ω A 1 2 28 β c ω A 2 2 α A 2 A 0 α A 1 2 + 4 β c A 2 2 c ω A 2 + c A 2 = 0 , 4 α ω A 2 2 32 β c ω 2 A 2 2 α ω A 2 A 0 α ω A 1 2 + 48 β c ω A 2 2 α A 2 2 + 2 α A 2 A 0 + α A 1 2 16 β c A 2 + c ω A 2 c A 2 = 0 , α A 1 2 28 β c ω 2 A 2 4 α ω A 0 A 2 2 α ω A 1 2 + 2 α ω A 2 2 + 28 β c ω A 2 + 2 α A 0 A 2 2 α A 2 2 4 β c A 2 + 2 c ω A 2 c A 2 = 0 , 21 β c ω A 1 14 β c ω 2 A 1 + 12 α ω A 2 A 1 2 α ω A 2 A 1 2 α ω A 1 A 0 6 α A 2 A 1 + 2 α A 2 A 1 + 2 α A 1 A 0 7 β c A 1 + c ω A 1 c A 1 = 0 , α ω A 1 2 8 β c ω 2 A 2 + 8 β c ω 2 A 2 2 α ω A 2 A 0 α ω A 1 2 + 2 α ω A 0 A 2 + 4 β c ω A 2 12 β c ω A 2 2 α A 0 A 2 α A 1 2 + 4 β c A 2 + c ω A 2 c ω A 2 + c A 2 = 0 , β c ω A 1 2 β c ω 2 A 1 10 β c ω 2 A 1 2 α ω A 2 A 1 2 α ω A 1 A 0 4 α ω A 1 A 2 4 α ω A 0 A 1 + 6 α ω A 1 A 2 + 10 β c ω A 1 + 2 α A 1 A 2 + 2 α A 0 A 1 6 α A 1 A 2 β c A 1 + c ω A 1 + 2 c ω A 1 c A 1 = 0 , 10 β c ω 2 A 1 + 2 β c ω 2 A 1 6 α ω A 2 A 1 + 4 α ω A 2 A 1 + 4 α ω A 1 A 0 + 2 α ω A 1 A 2 + 2 α ω A 0 A 1 10 β c ω A 1 3 β c ω A 1 2 α A 2 A 1 2 α A 1 A 0 2 α A 1 A 2 2 α A 0 A 1 + β c A 1 + β c A 1 2 c ω A 1 c ω A 1 + c A 1 + c A 1 = 0.

Solving the above system we get two cases, namely

Case 1.

(14) A 2 = 0 , A 1 = 0 , A 0 = c 8 β ω 4 β 1 2 α , A 1 = 0 , A 2 = 6 β c ω α .

Case 2.

(15) A 2 = 6 α β c ω 1 , A 1 = 0 , A 0 = c 2 α 8 β ω 4 β 1 , A 1 = 0 , A 2 = 6 β c ω α .

Thus cnoidal wave solutions of PDE (1) are

u = c 1 + 4 β 8 β ω 2 α + 6 β c ω α cn 2 ξ ω

and

u ( t , x ) = 6 β c ω 1 α nc 2 ξ ω c 8 β ω 4 β 1 2 α + 6 β c ω α cn 2 ξ ω

where nc = 1/cn.

2.3.2 Snoidal waves

In this case, again N = 2 and following the same procedure as above but invoking ODE (10) this time, we obtain

8 H 8 β c A 2 + H 7 β c A 1 8 H 6 β c A 2 H 5 β c A 1 H 5 β c A 1 8 H 4 β c A 2 + 7 H 3 β c A 1 + 32 H 2 β c A 2 + 4 H 2 α ω A 2 2 4 H 2 α A 2 A 0 6 H β c A 1 6 H α A 2 A 1 + 4 H 12 α ω A 2 2 + 2 H 10 α ω A 1 2 4 H 10 α ω A 2 2 2 H 10 c ω A 2 6 H 9 α A 1 A 2 H 9 c ω A 1 2 H 8 α ω A 1 2 4 H 8 α A 0 A 2 + 2 H 8 c ω A 2 2 H 7 α A 1 A 2 2 H 7 α A 0 A 1 + 6 H 7 α A 1 A 2 + H 7 c ω A 1 + H 7 c ω A 1 2 H 6 α ω A 1 2 + 4 H 6 α A 0 A 2 + 2 H 6 c ω A 2 + 2 H 5 α A 2 A 1 + 2 H 5 α A 1 A 0 + 2 H 5 α A 1 A 2 + 2 H 5 α A 0 A 1 H 5 c ω A 1 4 H 4 α ω A 2 2 + 2 H 4 α ω A 1 2 + 4 H 4 α A 2 A 0 2 H 4 c ω A 2 + 6 H 3 α A 2 A 1 2 H 3 α A 2 A 1 2 H 3 α A 1 A 0 4 α A 2 2 4 H 10 α A 2 2 2 H 8 α A 1 2 + 4 H 8 α A 2 2 + 2 H 8 c A 2 + H 7 c A 1 H 5 c A 1 + 2 H 6 α A 1 2 2 H 6 c A 2 H 5 c A 1 + 2 H 4 α A 1 2 2 H 4 c A 2 + 4 H 2 α A 2 2 + H 3 c A 1 2 H 2 α A 1 2 + 2 H 2 c A 2 + 6 H 11 α ω A 1 A 2 + 4 H 10 α ω A 0 A 2 + 2 H 9 α ω A 1 A 2 + 2 H 9 α ω A 0 A 1 6 H 9 α ω A 1 A 2 4 H 8 α ω A 0 A 2 2 H 7 α ω A 2 A 1 2 H 7 α ω A 1 A 0 2 H 7 α ω A 1 A 2 2 H 7 α ω A 0 A 1 4 H 6 α ω A 2 A 0 6 H 5 α ω A 2 A 1 + 2 H 5 α ω A 2 A 1 + 2 H 5 α ω A 1 A 0 + 4 H 4 α ω A 2 A 0 + 6 H 3 α ω A 2 A 1 24 β c A 2 + H 7 β c ω 2 A 1 + H 7 β c ω A 1 + 8 H 7 β c ω A 1 + 8 H 6 β c ω 2 A 2 + 8 H 6 β c ω A 2 8 H 6 β c ω A 2 H 5 β c ω 2 A 1 8 H 5 β c ω A 1 8 H 4 β c ω 2 A 2 40 H 4 β c ω A 2 + 7 H 3 β c ω A 1 H 5 β c ω A 1 + 24 H 12 β c ω 2 A 2 + 32 H 2 β c ω A 2 + 6 H 11 β c ω 2 A 1 32 H 10 β c ω 2 A 2 32 H 10 β c ω A 2 7 H 9 β c ω 2 A 1 7 H 9 β c ω A 1 + 8 H 8 β c ω 2 A 2 + 40 H 8 β c ω A 2 + H 7 β c ω 2 A 1 = 0.

Splitting on similar powers of H yields algebraic equations

α A 2 2 + 6 β c A 2 = 0 , 6 β c ω 2 A 2 + α ω A 2 2 = 0 , α A 2 A 1 + β c A 1 = 0 , β c ω 2 A 1 + α ω A 1 A 2 = 0 , 2 α ω A 2 2 + 16 β c ω A 2 + 2 α A 2 2 2 α A 2 A 0 α A 1 2 + 16 β c A 2 + c A 2 = 0 , 2 α ω A 0 A 2 16 β c ω 2 A 2 + α ω A 1 2 2 α ω A 2 2 16 β c ω A 2 2 α A 2 2 c ω A 2 = 0 , 6 α ω A 2 A 1 + 7 β c ω A 1 + 6 α A 2 A 1 2 α A 2 A 1 2 α A 1 A 0 + 7 β c A 1 + c A 1 = 0 , 2 α ω A 1 A 2 7 β c ω 2 A 1 + 2 α ω A 0 A 1 6 α ω A 1 A 2 7 β c ω A 1 6 α A 1 A 2 c ω A 1 = 0 , 2 α ω A 2 A 0 4 β c ω 2 A 2 2 α ω A 2 2 + α ω A 1 2 20 β c ω A 2 + 2 α A 2 A 0 + α A 1 2 4 β c A 2 c ω A 2 c A 2 = 0 , 4 β c ω 2 A 2 2 α ω A 0 A 2 α ω A 1 2 + 20 β c ω A 2 2 α A 0 A 2 α A 1 2 + 2 α A 2 2 + 4 β c A 2 + c ω A 2 + c A 2 = 0 , 4 β c ω 2 A 2 α ω A 2 A 0 α ω A 1 2 + 2 β c ω A 2 2 β c ω A 2 + α A 0 A 2 + α A 1 2 2 β c A 2 + c ω A 2 c A 2 = 0 , 2 α ω A 2 A 1 β c ω 2 A 1 6 α ω A 2 A 1 + 2 α ω A 1 A 0 8 β c ω A 1 β c ω A 1 + 2 α A 2 A 1 + 2 α A 1 A 0 + 2 α A 1 A 2 + 2 α A 0 A 1 β c A 1 β c A 1 c ω A 1 c A 1 c A 1 = 0 , β c ω 2 A 1 + β c ω 2 A 1 2 α ω A 2 A 1 2 α ω A 1 A 0 2 α ω A 1 A 2 2 α ω A 0 A 1 + β c ω A 1 + 8 β c ω A 1 2 α A 1 A 2 2 α A 0 A 1 + 6 α A 1 A 2 + β c A 1 + c ω A 1 + c ω A 1 + c A 1 = 0.

Solving the above system of equations yields the following two cases:

Case 1.

A 2 = 0 , A 1 = 0 , A 0 = c 2 α 4 β ω + 4 β + 1 , A 1 = 0 , A 2 = 6 β c ω α .

Case 2.

A 2 = 6 β c α , A 1 = 0 , A 0 = c 2 α 4 β ω + 4 β + 1 , A 1 = 0 , A 2 = 6 β c ω α .

Thus we obtain two snoidal wave solutions of the equal-width equation (1) as

(16) u = c 1 + 4 β + 4 β ω ( 1 3 sn 2 ξ ω ) 2 α ,

and

(17) u = 6 β c α ns 2 ξ ω + c 4 β ω + 4 β + 1 2 α 6 β c ω α sn 2 ξ ω

where ns = 1/sn.

We now consider the second operator X3 + aX2 of the optimal system. This symmetry operator yields the two invariants J 1 = e x t a / 2 and J 2 = u t 1 / 2 . Thus J2 = f (J1) provides an invariant solution of (1), viz.,

u = 1 t f ( e x t a / 2 ) .

Substitution of u in PDE (1), gives nonlinear third-order ODE

a β z 3 f ( z ) + β 3 a + 1 z 2 f ( z ) + ( a β a + β ) z f ( z ) + 6 α z f ( z ) 2 f ( z ) f ( z ) = 0 ,

where z = e x t a / 2 .

The third operator X2 provides invariants J1 = t, J2 = u. Thus u = (t) is invariant solution. Substituting this value of u into equation (1) and solving the resultant ODE gives

u = C 1 ,

where C1 is an arbitrary constant.

2.4 Conservation laws

We construct conservations laws [6, 7, 15, 16, 17, 18, 19, 20] of equal-width equation (1) by using two approaches; multiplier approach [21, 22, 23] and Noether’s theorem [24, 25].

2.4.1 Conservation laws by invoking multiplier approach

We seek zeroth-order multiplier Q, which depends on (t, x, u). To determine this multiplier Q we invoke

(18) δ δ u Q u t + 2 α u u x β u t x x = 0 ,

where

δ δ u = u D t u t D x u x D t D x 2 u t x x

and Dt, Dx are as defined in (6). The above equation yields

u t Q u + 2 α u u x Q u β u t x x Q u + 2 α u x Q D t ( Q ) D x ( 2 α u Q ) + D x 2 D t β Q = 0 ,

which on expanding gives

β Q t x x Q t 2 α u Q x + 2 β u x Q t x u + β u x 2 Q t u u + β u x x Q t u + β u t Q x x u + 2 β u t u x Q x u u + 2 β u t x Q x u 2 + β u t u x 2 Q u u + β u t u x x Q u u + 2 β u x u t x Q u u = 0.

Decomposing above equation on derivatives of u, we obtain

Q t u = 0 , Q x u = 0 , Q u u = 0 , Q t + 2 α u Q x β Q t x x = 0 ,

which on solving gives two multipliers

Q 1 ( t , x , u ) = u

and

Q 2 ( t , x , u ) = 1.

Associated with above multipliers, we acquire two conservation laws whose components are

T 1 t = 1 2 β u x 2 + 1 2 u 2 , T 1 x = 2 3 α u 3 β u u t x

and

T 2 t = u , T 2 x = α u 2 β u t x .

2.4.2 Conservation laws using Noether’s theorem

We now invoke Noether’s approach [24, 25] to construct conservation laws for PDE (1). This equation is of third order and as such has no Lagrangian. However, by letting u = vx we can make it variational. Thus (1) now becomes

(19) v t x + 2 α v x v x x β v t x x x = 0 ,

which has a Lagrangian given by

(20) L = 1 2 v t v x 1 3 α v x 3 1 2 β v t x v x x

as δL/δv = 0 on the equation (19) where

δ δ v = v D t v t D x v x + D x 2 v x x + D t D x v t x .

Consider the generator

(21) X = τ t + ξ x + η v ,

where , ξ and η are functions of t, x and v. To determine Noether symmetries X of (19) we insert the value of L from (20) in the determining equation

(22) X [ 2 ] ( L ) + L D t τ + D x ξ = D t B t + D x B x ,

where (Bt , Bx) are gauge terms that are functions of (t, x, v) and X[2] is the second prolongation of X defined as

(23) X [ 2 ] = X + ζ 1 v t + ζ 2 v x + ζ 12 v t x + ζ 22 v x x

with 1, 2, 12 and 22 as defined in [8]. Expansion of equation (22) and decomposing on derivatives of v yields a system of PDEs. Thereafter solving these PDEs we obtain the Noether symmetries together with their gauge functions as

X 1 = t , B t = 0 , B x = 0 , X 2 = x , B t = 0 , B x = 0 , X f = f ( t ) v , B t = 0 , B x = 1 2 v f ( t ) .

Next, we use the above results to compute the conserved vectors of the fourth-order equation (19). The formulae for the conserved vector (Tt , Tx) [26]

T k = L τ k + ( ξ α ψ x j α τ j ) ( L ψ x k α l = 1 k D x l ( L ψ x l x k α ) ) + l = k n ( η l α ψ x l x j α τ j ) L ψ x k x l α f k

yield three conservations laws associated with the three Noether symmetries X1, X2 and Xf . Thus the three conserved vectors are

T 1 t = 1 3 α u 3 , T 1 x = 1 2 u t d x 2 + α u 2 u t d x β u t x u t d x + 1 2 β u t 2 ; T 2 t = 1 2 u 2 + 1 2 β u x 2 , T 2 x = 2 3 α u 3 β u u t x ; T f t = 1 2 f ( t ) u , T f x = β f ( t ) u t x 1 2 f ( t ) u t d x α f ( t ) u 2 + 1 2 f ( t ) u d x .

Remark: It should be noted that because of arbitrary function f (t) we have infinitely many nonlocal conservation laws.

3 Conclusion

In this work we studied the equal-width equation (1) from symmetry standpoint. Firstly, Lie symmetries were computed and were used to construct OSODS. Thereafter, symmetry reductions were performed on (1). On the ODE obtained, Jacobi elliptic function expansion technique was employed which resulted in exact solutions of (1) in form of cnoidal and snoidal waves. Secondly, conservation laws for this equation were established by invoking both the multiplier approach and the celebrated Noether’s theorem.


Department of Mathematics, College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao, Shandong 266590, China

Acknowledgement

The authors would like to thank T Motsepa for fruitful discussions. CMK thanks the North-West University, Mafikeng Campus for its continued support.

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Received: 2019-03-04
Accepted: 2019-05-23
Published Online: 2019-10-04

© 2019 C. Masood Khalique et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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